Olsson TI. 1981. Overwintering of benthic macroinvertebrates in ice and frozen sediment in a North Swedish river. Holarctic Ecology 4: 161-166.
This author examined freezing tolerance and freezing resistance in some river-dwelling invertebrates in the Arctic. The study river is one of the few in northern Sweden that has not been dammed for hydroelectric purposes, allowing water levels to fluctuate through a wide range. Ice thickness in winter can exceed 50 cm, and the shallow littoral zone of the river freezes several centimetres into the sediment. Water level is lowest in winter, freezing sediments that are under as much as 4m of flowing water in summer. Spring thaw may occur bottom-to-top in shallow areas, as sunlight penetrates ice and heats underlying sediment, which thaws under a layer of ice; this slow thawing in sediments may be important for winter and spring survival of invertebrates and plants.
Ice and sediment cores taken from the river edge in winter included a range of frozen invertebrates. These animals were returned to the lab and allowed to thaw, to estimate winter survival. Most animals had very high survivorship; one major exception was the isopod Asellus aquaticus, found in a single aggregration of nearly 500 individuals, most of whom were dead upon thawing.
Winter survival was also estimated by freezing some animals in the lab, maintaining them frozen for several months, and thawing. Mechanical damage was inferred to be more severe in the lab than under field conditions as animals without shells or hard cases (e.g. gastropods, trichoptera larvae) such as oligochaetes suffered very high mortalities in the lab, but high survivorship in the field. This author is careful to note that lab freezing conditions included natural sediments and plants, as it has previously been shown that simple freezing of open water (e.g. in a bucket) is lethal to even the most cold-tolerant species, probably due to the mechanical damage incurred by expanding ice crystals that can be avoided by shelter among sediments or plant tissues.
Several cold and freezing putative adaptations were discovered, including the formation of epiphragms in some gastropods, a thin closure of the shell apeture not previously observed in aquatic snails, but common among dessication-resistant land snails. Some trichopteran larvae were found to have blocked their cases, though they were not pupal or prepupal. This blockage may have served to prevent ice formation and associated mechanical damage inside the cases. Some species were found in summer collections but were absent from frozen cores, including gammarid amphipods, suggesting winter migration to unfrozen deeper portions of the river.
Wednesday, April 30, 2008
Milner 1994
Milner AM. 1994. Colonization and succession of invertebrate communities in a new stream in Glacier Bay National Park, Alaska. Freshwater Biology 32: 387-400.
This author describes long term monitoring of colonization and succession in a stream recently formed from a retreating glacier in south-eastern Alaska. The glacier filled its bay around 1700 AD, and has been retreating since, forming new streams and lakes, and novel habitats similar to what is thought to have occurred across northern North America and Eurasia at the end of the last ice age. There are few previous studies of stream systems that completely lack an upstream source of drift-colonizing organisms; this author describes this work as unique regarding its long time frame (12 years), spatial extent (kilometres), and primary succession characteristics.
Of the possible routes for colonization of the study stream by invertebrates, only aerial oviposition is possible as the other routes rely on suitable habitat upstream or downstream of the study site. The study site is bounded by the ocean below, and a new proglacial lake and associated ice field above. The first organisms present in the stream were chironomids, of species known to be exceptionally tolerant of cold water (~2°C). Species richness increased through the study period, with the addition of one species of Ephemeroptera, one species of Plecoptera, and a turnover in chironomid species and relative abundances.
A portion of the discussion section describes the distinction between fugitive species, good dispersers but poor competitors with habitat refugia in extreme environments, and opportunistic species, good dispersers that are also good competitors in their local microhabitats, maintained by disturbance. The first few chironomid species found in the stream are considered fugitive species because their population abundances were severely reduced in later years as other species, including a predatory stonefly, became established. A later portion discusses deterministic and stochastic processes in succession, arguing that water temperature and flow characteristics have been strong deterministic drivers of this stream system, in contrast to the strong role argued for stochastic processes (‘first come first served’) in other, more temperate streams studied by other authors.
The distinctions between fugitives and opportunists, and between stochastic and deterministic, would not be possible without species-level identification of chironomid larvae. Several species are described as Genus sp. A or similar, but nonetheless the ability to discriminate between closely related species with different ecological characteristics is clearly applied, allowing levels of analysis not normally seen in stream-succession studies.
This paper is part of a special issue of the journal Freshwater Biology, devoted to alpine and polar freshwater environments, and seems to be slightly lower in scientific rigour compared to normal papers in this journal. Many of the citations in this paper are of the author’s own previous unpublished data, and key blocks of data such as particular field collection seasons, have already been described in previous publications; this paper apparently serves primarily to integrate across the long time frame of repeated sampling.
This author describes long term monitoring of colonization and succession in a stream recently formed from a retreating glacier in south-eastern Alaska. The glacier filled its bay around 1700 AD, and has been retreating since, forming new streams and lakes, and novel habitats similar to what is thought to have occurred across northern North America and Eurasia at the end of the last ice age. There are few previous studies of stream systems that completely lack an upstream source of drift-colonizing organisms; this author describes this work as unique regarding its long time frame (12 years), spatial extent (kilometres), and primary succession characteristics.
Of the possible routes for colonization of the study stream by invertebrates, only aerial oviposition is possible as the other routes rely on suitable habitat upstream or downstream of the study site. The study site is bounded by the ocean below, and a new proglacial lake and associated ice field above. The first organisms present in the stream were chironomids, of species known to be exceptionally tolerant of cold water (~2°C). Species richness increased through the study period, with the addition of one species of Ephemeroptera, one species of Plecoptera, and a turnover in chironomid species and relative abundances.
A portion of the discussion section describes the distinction between fugitive species, good dispersers but poor competitors with habitat refugia in extreme environments, and opportunistic species, good dispersers that are also good competitors in their local microhabitats, maintained by disturbance. The first few chironomid species found in the stream are considered fugitive species because their population abundances were severely reduced in later years as other species, including a predatory stonefly, became established. A later portion discusses deterministic and stochastic processes in succession, arguing that water temperature and flow characteristics have been strong deterministic drivers of this stream system, in contrast to the strong role argued for stochastic processes (‘first come first served’) in other, more temperate streams studied by other authors.
The distinctions between fugitives and opportunists, and between stochastic and deterministic, would not be possible without species-level identification of chironomid larvae. Several species are described as Genus sp. A or similar, but nonetheless the ability to discriminate between closely related species with different ecological characteristics is clearly applied, allowing levels of analysis not normally seen in stream-succession studies.
This paper is part of a special issue of the journal Freshwater Biology, devoted to alpine and polar freshwater environments, and seems to be slightly lower in scientific rigour compared to normal papers in this journal. Many of the citations in this paper are of the author’s own previous unpublished data, and key blocks of data such as particular field collection seasons, have already been described in previous publications; this paper apparently serves primarily to integrate across the long time frame of repeated sampling.
Tuesday, April 22, 2008
Murkin et al. 1983
Murkin HR, Abbott PG, Kadlec JA. 1983. A comparison of activity traps and sweep nets for sampling nektonic invertebrates in wetlands. Freshwater Invertebrate Biology 2: 99-106.
These authors compared a specific activity trap design to a specific sweep net technique for sampling nektonic animals in small ponds in a wetland in Manitoba, in the context of evaluating incorporation of these techniques into long-term wetlands ecology monitoring programs. The activity trap consists of a 3.8 L glass bottle with a plast funnel inserted in the opening, held together with wire and elastic bands, and suspended in the water column from a stake driven at an angle into the sediment. The sweep net technique avoids benthic organisms and most benthic debris and vegetation by sweeping vertically upwards from resting flat on the substrate.
The fauna collected by the two methods was correlated when measured across variables of water temperature and water depth, suggesting that for at least total diversity, the two methods are collecting similar samples. Differences emerged when fish and predatory invertebrates were present in the traps, possibly attracted to the traps by the presence of prey species, including Hyalella azteca, which may have been subsequently consummed. Fish and other fast-moving animals were also rarely collected by the sweep nets. Activity traps appeared to select for the most mobile size- and age-classes of gastropods such as lymnaeids, which were absent from most sweep net samples.
The activity traps provide quantitative samples of only some taxa, primarily those that were classified as “herbivore-detritivores” in the absence of predators inside the traps. Predatory taxa were probably overrepresented in the traps, while some apparent prey taxa were underrepresented in traps when predators were present.
The activity traps provided several important advantages compared to sweep nets. There is reduced inter-operator variation, traps were easier to use among vegetation, they collected even fast-moving animals such as fish and large predatory invertebrates, and they integrated the nekton over 24 hours, unlike the time-of-day specific sweep nets.
These authors compared a specific activity trap design to a specific sweep net technique for sampling nektonic animals in small ponds in a wetland in Manitoba, in the context of evaluating incorporation of these techniques into long-term wetlands ecology monitoring programs. The activity trap consists of a 3.8 L glass bottle with a plast funnel inserted in the opening, held together with wire and elastic bands, and suspended in the water column from a stake driven at an angle into the sediment. The sweep net technique avoids benthic organisms and most benthic debris and vegetation by sweeping vertically upwards from resting flat on the substrate.
The fauna collected by the two methods was correlated when measured across variables of water temperature and water depth, suggesting that for at least total diversity, the two methods are collecting similar samples. Differences emerged when fish and predatory invertebrates were present in the traps, possibly attracted to the traps by the presence of prey species, including Hyalella azteca, which may have been subsequently consummed. Fish and other fast-moving animals were also rarely collected by the sweep nets. Activity traps appeared to select for the most mobile size- and age-classes of gastropods such as lymnaeids, which were absent from most sweep net samples.
The activity traps provide quantitative samples of only some taxa, primarily those that were classified as “herbivore-detritivores” in the absence of predators inside the traps. Predatory taxa were probably overrepresented in the traps, while some apparent prey taxa were underrepresented in traps when predators were present.
The activity traps provided several important advantages compared to sweep nets. There is reduced inter-operator variation, traps were easier to use among vegetation, they collected even fast-moving animals such as fish and large predatory invertebrates, and they integrated the nekton over 24 hours, unlike the time-of-day specific sweep nets.
Billington et al. 1989
Billington N, Boileau MG, Hebert PDN. 1989. Range extension of the fairy shrimp Polyartemiella hazeni (Murdoch, 1884) (Crustacea: Anostraca) to the eastern Canadian Arctic, with notes on the distribution of other eastern Arctic Anostraca. The Canadian Field-Naturalist 103: 404-405.
These authors present a short note that describes the capture of several species of Anostracans from the western shore of Hudson Bay, in what was then the Northwest Territories. For Polyartemiella hazeni, collections from Rankin Inlet and Eskimo Point represent a range expansion of about 1300 km eastwards; for the other species records here and from Igloolik provide similar range expansion estimates. The ability of one species, Branchinecta paludosa, to facultatively achieve two generations per year when conditions are good suggests circumstantial evidence in favour of a link between dessication and cold tolerance in freshwater animals.
These authors present a short note that describes the capture of several species of Anostracans from the western shore of Hudson Bay, in what was then the Northwest Territories. For Polyartemiella hazeni, collections from Rankin Inlet and Eskimo Point represent a range expansion of about 1300 km eastwards; for the other species records here and from Igloolik provide similar range expansion estimates. The ability of one species, Branchinecta paludosa, to facultatively achieve two generations per year when conditions are good suggests circumstantial evidence in favour of a link between dessication and cold tolerance in freshwater animals.
Whiteside and Lindegaard 1980
Whiteside MC, Lindegaard C. 1980. Complementary procedures for sampling small benthic invertebrates. Oikos 35: 317-320.
These authors recommend the use of two techniques simultaneously for comprehensive sampling of benthic invertebrates in freshwater, soft-bottom habitats. Cores approximately 20 cm in diameter work well for sampling burrowing and non-swimming taxa such as oligochaetes and gastropods, while a funnel trap worked well for sampling taxa that migrate vertically, especially very small forms that would otherwise be lost in benthic samples during seiving.
The funnel trap consists of a plastic funnel fit into a 300 mL glass jar, placed open-side-down on the substrate, being careful not to accidentally trap any planktonic animals during installation. Only individuals prone to vertical movement were collected in funnel traps; all coleoptera in the traps were adults, while the majority of coleoptera collected in cores were larvae. One major advantage of the funnels is the lack of debris or vegetation to sort through during specimen processing. The difference in sampled taxa suggests that both methods would be useful in any strategy of freshwater-benthos sampling.
Saturday, April 12, 2008
Stevens 1989
Stevens GC. 1989. The latitudinal gradient in geographical range: how so many species coexist in the tropics. The American Naturalist 133: 240-256.
This paper is an essay that provides an overview of the evidence suggesting a relationship between Rapoport’s rule (larger species geographic ranges at higher latitudes) and the global latitude biodiversity gradient. Both phenomena show coincident exceptions of taxa, indicating a common underlying cause. This is also the paper that coins the term “Rapoport’s rule”.
The original explanatory mechanism for Rapoport’s rule invokes the range of temperatures or other climatic conditions experienced by individuals during their lifetimes at different latitudes. High latitude locations have wider annual ranges of temperature, for example a spruce tree in an Alaskan forest may experience lows below -50°C and highs above 30°C in a single year, while no tropical sites below mountain tops experience that range. Within the geographic range of a species in the tropics, such ranges of environmental variation may occur, but few or no individuals would experience the full range. In addition, bands of climate (such as between annual mean temperatures) that lie along mountain slopes are narrower in the tropics, providing less space for meso-habitat adapted populations to establish and persist, such that population sizes are smaller and long term persistence and local adaptation are less likely.
Narrower tolerances for abiotic conditions in the tropics results in smaller geographic ranges and higher diversities because areas of diversity measurement will encompass a larger number of distinct climate zones than comparable measurements made in temperate or polar latitudes. Further increasing diversity estimates is a predicted strong-in-the-tropics “rescue effect” (Brown and Kodric-Brown, 1977) that produces sink populations in areas a species is poorly adapted to, maintained by immigration from nearby populations in more suitable habitats.
I find these arguments convincing, but some of the exceptions described here are problematic. One key exception that Stevens (1989) mentions more than once is the hymenopteran family Ichneumonidae. Ichneumonid species richness peaks at temperate, not tropical latitudes (Owen & Owen, 1974). Stevens (1989) explains this exception to the general latitude-diversity pattern by invoking the summer-only activity pattern of these parasitoid wasps. Stevens (1989) claims that because they are inactive during all but the warmest part of the year, ichneumonids “… in sense, live in the tropics, no matter what latitude they call home.” I see two critical problems with this argument as applied to Ichneumonidae as an informative exception to Rapoport’s rule and the latitude-diversity pattern. First, many other organisms, plants and animals and presumably other kingdoms and phyla, show severely reduced winter activity in temperate regions. Why do they not similarly show exceptional species-richness patterns? Second, “inactive” is not a synonym with “immune to environmental factors”. Overwintering in places that have winter (i.e. distinctly lower temperatures in one season compared to other times of the year) requires adaptations, sometimes extreme adaptations. The large body of literature concerning overwintering strategies of animals, and the various adaptations that constitute “freeze-tolerant” and “freeze-resistant” forms, comes readily to mind. Overwintering ichneumonids must survive the conditions of winter, even if they do not move around or show high metabolic rates during winter. As a further consideration, another section of this paper clarifies that Rapoport’s rule does not describe a pattern of increased species numbers per genus, it describes changes in the geographical distribution of functional groups of organisms, and includes the example of willows. Willows do not have higher diversity in the tropics; they are absent from tropical regions. But, willows are not an exception to either pattern because they are replaced by several other genera of morphologically and ecologically similar plants at lower latitudes. Why does Stevens (1989) not apply this functional-group replacement criterion to the example of the Ichneumonidae? There are other families of parasitoids, surely one or more of those taxa have high tropical diversity.
This paper is an essay that provides an overview of the evidence suggesting a relationship between Rapoport’s rule (larger species geographic ranges at higher latitudes) and the global latitude biodiversity gradient. Both phenomena show coincident exceptions of taxa, indicating a common underlying cause. This is also the paper that coins the term “Rapoport’s rule”.
The original explanatory mechanism for Rapoport’s rule invokes the range of temperatures or other climatic conditions experienced by individuals during their lifetimes at different latitudes. High latitude locations have wider annual ranges of temperature, for example a spruce tree in an Alaskan forest may experience lows below -50°C and highs above 30°C in a single year, while no tropical sites below mountain tops experience that range. Within the geographic range of a species in the tropics, such ranges of environmental variation may occur, but few or no individuals would experience the full range. In addition, bands of climate (such as between annual mean temperatures) that lie along mountain slopes are narrower in the tropics, providing less space for meso-habitat adapted populations to establish and persist, such that population sizes are smaller and long term persistence and local adaptation are less likely.
Narrower tolerances for abiotic conditions in the tropics results in smaller geographic ranges and higher diversities because areas of diversity measurement will encompass a larger number of distinct climate zones than comparable measurements made in temperate or polar latitudes. Further increasing diversity estimates is a predicted strong-in-the-tropics “rescue effect” (Brown and Kodric-Brown, 1977) that produces sink populations in areas a species is poorly adapted to, maintained by immigration from nearby populations in more suitable habitats.
I find these arguments convincing, but some of the exceptions described here are problematic. One key exception that Stevens (1989) mentions more than once is the hymenopteran family Ichneumonidae. Ichneumonid species richness peaks at temperate, not tropical latitudes (Owen & Owen, 1974). Stevens (1989) explains this exception to the general latitude-diversity pattern by invoking the summer-only activity pattern of these parasitoid wasps. Stevens (1989) claims that because they are inactive during all but the warmest part of the year, ichneumonids “… in sense, live in the tropics, no matter what latitude they call home.” I see two critical problems with this argument as applied to Ichneumonidae as an informative exception to Rapoport’s rule and the latitude-diversity pattern. First, many other organisms, plants and animals and presumably other kingdoms and phyla, show severely reduced winter activity in temperate regions. Why do they not similarly show exceptional species-richness patterns? Second, “inactive” is not a synonym with “immune to environmental factors”. Overwintering in places that have winter (i.e. distinctly lower temperatures in one season compared to other times of the year) requires adaptations, sometimes extreme adaptations. The large body of literature concerning overwintering strategies of animals, and the various adaptations that constitute “freeze-tolerant” and “freeze-resistant” forms, comes readily to mind. Overwintering ichneumonids must survive the conditions of winter, even if they do not move around or show high metabolic rates during winter. As a further consideration, another section of this paper clarifies that Rapoport’s rule does not describe a pattern of increased species numbers per genus, it describes changes in the geographical distribution of functional groups of organisms, and includes the example of willows. Willows do not have higher diversity in the tropics; they are absent from tropical regions. But, willows are not an exception to either pattern because they are replaced by several other genera of morphologically and ecologically similar plants at lower latitudes. Why does Stevens (1989) not apply this functional-group replacement criterion to the example of the Ichneumonidae? There are other families of parasitoids, surely one or more of those taxa have high tropical diversity.
Friday, April 11, 2008
Matthews 1979
Matthews JV Jr. 1979. Late Tertiary carabid fossils from Alaska and the Canadian archipelago. In: Carabid Beetles: Their Evolution, Natural History, and Classification (Erwin TL, Ball GE, Whitehead DR, Halpern AL eds.). Dr. W Junk bv Publishers, The Hague, Netherlands.
For this special symposium, this author summarizes recently discovered and analysed beetle fossils dating from the late Tertiary, and compares them with similar fossils from the Pleistocene. Most Tertiary fossils of insects are either casts or impressions, and beetle fossils tend to be crushed and scattered too severely for good identification and analysis. However, fossils dating from the late Miocene and early Pliocene (roughly 5 million years ago) were discovered at several sites across the western islands of the Canadian Archipelago and a site in western Alaska that resemble Pleistocene fossils in their quality of preservation. The western Alaska site is particularly valuable because the fossil-bearing layer is overlain by a layer of basalt flow, possibly from a volcanic eruption, that can be dated without recourse to biological materials. There was apparently a narrow connection between Alaska and Siberia at the time these fossils were produced.
In general, smaller-bodied beetles are better preserved than large, but many specimens of numerous genera were discovered. Matthews (1977) includes a complete list of all fossil Coleoptera found, including the carabids described here. Several examples of species-diagnostic features were found, including highly detailed elytra and parts of male genitalia.
The first part of the discussion of this paper is a critique of the strictly-Hennigian methods of Phylogenetic Systematics, which disallows phyletic evolution, i.e. changes in species phenotypes through time without associated lineage splitting. Later parts of the discussion describe the probable Taiga ecosystem present at very high latitudes in the late Miocene. The author ends the paper with optimism that similar high-quality Tertiary fossils may soon be found in other high latitude areas around the world.
For this special symposium, this author summarizes recently discovered and analysed beetle fossils dating from the late Tertiary, and compares them with similar fossils from the Pleistocene. Most Tertiary fossils of insects are either casts or impressions, and beetle fossils tend to be crushed and scattered too severely for good identification and analysis. However, fossils dating from the late Miocene and early Pliocene (roughly 5 million years ago) were discovered at several sites across the western islands of the Canadian Archipelago and a site in western Alaska that resemble Pleistocene fossils in their quality of preservation. The western Alaska site is particularly valuable because the fossil-bearing layer is overlain by a layer of basalt flow, possibly from a volcanic eruption, that can be dated without recourse to biological materials. There was apparently a narrow connection between Alaska and Siberia at the time these fossils were produced.
In general, smaller-bodied beetles are better preserved than large, but many specimens of numerous genera were discovered. Matthews (1977) includes a complete list of all fossil Coleoptera found, including the carabids described here. Several examples of species-diagnostic features were found, including highly detailed elytra and parts of male genitalia.
The first part of the discussion of this paper is a critique of the strictly-Hennigian methods of Phylogenetic Systematics, which disallows phyletic evolution, i.e. changes in species phenotypes through time without associated lineage splitting. Later parts of the discussion describe the probable Taiga ecosystem present at very high latitudes in the late Miocene. The author ends the paper with optimism that similar high-quality Tertiary fossils may soon be found in other high latitude areas around the world.
DeBruyn & Ring 1999
DeBruyn AMH, Ring RA. 1999. Comparative ecology of two species of Hydroporus (Coleoptera: Dytiscidae) in a high arctic oasis. The Canadian Entomologist 131: 405-420.
These authors examined the beetles living in two ponds in a polar oasis on the east coast of Ellesmere Island, during the summers of 1992 and 1993. The two ponds are different from each other in a number of important respects, including sediment and vegetations characteristics, shoreline structure, temperature profile, and duration. Pond A is smaller than Pond B, has a greater diversity of aquatic plants and benthic sediments, and never dries completely, while Pond B dries up in August and has benthic sediment composed mainly of bare rocks with occasional patches of silt and sand. These features may explain why both species of Hydroporus were found in Pond A but only one species was found in Pond B.
While high arctic conditions are generally considered extreme for organisms, because abiotic conditions approach the physical limits for life (e.g. Downes, 1964), aquatic habitats are considered relatively benign, because the large mass of water and winter ice cover buffer temperature changes compared to adjacent terrestrial habitats. However, other authors such as Danks (2007) have noted that smaller ponds freeze completely in winter and may respond to winter temperature variations in much the same way as terrestrial habitats. Interestingly, Danks (1987) does consider aquatic habitats to be more favourable to organisms, and invokes this effect to explain the higher ratios of species richness of aquatic versus terrestrial insects at high latitudes.
Polar oases are described more fully in a book edited by Svoboda and Freedman (1994); briefly, these are locations of high biodiversity and mild conditions. Alexandra Fiord’s lowland (78°53” N) is such an oasis because of the gentle slope near sea level and the surrounding landscape providing good exposure to summer sunlight while restricting exposure to chilling winds. The two ponds examined in this study had exceptionally high temperature profiles in summer, with some microhabitats rising to 37.5°C on one particularly sunny day in July of 1993.
The discussion of how these species of beetles are able to persist at such a high latitude site is somewhat confusing. Both species overwinter as adults, one in microhabitats that dry to at least some extent in winter while the other in microhabitats that remain wet and consequently probably freeze solid. These authors state that the dry-winter species must resist both desiccation and low temperatures, but do not consider that desiccation is a strategy employed by freeze-resistant insects, especially in terrestrial habitats. This is very confusing considering that the second author has published extensively about cold adaptations in insects.
Low temperatures have been invoked to explain the frequent pattern of longer life cycles among Arctic insects compared temperate conspecifics and congeners (e.g. Danks, 1981). These authors do not dismiss this possibility, but point out that the longer development time of one species may explain its absence from the temporary pond, rather than temperature per se. Other potentially explanatory variables, such as various aspects of water chemistry, are dismissed as unlikely, given the other places these species have been found.
These authors examined the beetles living in two ponds in a polar oasis on the east coast of Ellesmere Island, during the summers of 1992 and 1993. The two ponds are different from each other in a number of important respects, including sediment and vegetations characteristics, shoreline structure, temperature profile, and duration. Pond A is smaller than Pond B, has a greater diversity of aquatic plants and benthic sediments, and never dries completely, while Pond B dries up in August and has benthic sediment composed mainly of bare rocks with occasional patches of silt and sand. These features may explain why both species of Hydroporus were found in Pond A but only one species was found in Pond B.
While high arctic conditions are generally considered extreme for organisms, because abiotic conditions approach the physical limits for life (e.g. Downes, 1964), aquatic habitats are considered relatively benign, because the large mass of water and winter ice cover buffer temperature changes compared to adjacent terrestrial habitats. However, other authors such as Danks (2007) have noted that smaller ponds freeze completely in winter and may respond to winter temperature variations in much the same way as terrestrial habitats. Interestingly, Danks (1987) does consider aquatic habitats to be more favourable to organisms, and invokes this effect to explain the higher ratios of species richness of aquatic versus terrestrial insects at high latitudes.
Polar oases are described more fully in a book edited by Svoboda and Freedman (1994); briefly, these are locations of high biodiversity and mild conditions. Alexandra Fiord’s lowland (78°53” N) is such an oasis because of the gentle slope near sea level and the surrounding landscape providing good exposure to summer sunlight while restricting exposure to chilling winds. The two ponds examined in this study had exceptionally high temperature profiles in summer, with some microhabitats rising to 37.5°C on one particularly sunny day in July of 1993.
The discussion of how these species of beetles are able to persist at such a high latitude site is somewhat confusing. Both species overwinter as adults, one in microhabitats that dry to at least some extent in winter while the other in microhabitats that remain wet and consequently probably freeze solid. These authors state that the dry-winter species must resist both desiccation and low temperatures, but do not consider that desiccation is a strategy employed by freeze-resistant insects, especially in terrestrial habitats. This is very confusing considering that the second author has published extensively about cold adaptations in insects.
Low temperatures have been invoked to explain the frequent pattern of longer life cycles among Arctic insects compared temperate conspecifics and congeners (e.g. Danks, 1981). These authors do not dismiss this possibility, but point out that the longer development time of one species may explain its absence from the temporary pond, rather than temperature per se. Other potentially explanatory variables, such as various aspects of water chemistry, are dismissed as unlikely, given the other places these species have been found.
Wednesday, April 9, 2008
Krasnov et al. 2008
Krasnov BR, Shenbrot GI, Khokhlova IS, Mouillot D, Poulin R. 2008. Latitudinal gradients in niche breadth: empirical evidence from haematophagous ectoparasites. Journal of Biogeography 35: 592-601.
This paper examined Rapoport’s rule in fleas that use small mammals in the Palaearctic as hosts, applying phylogenetic independent contrasts (PIC) to questions of geographic range size, latitude, and niche breadth. These authors consider Rapoport’s rule, of increased geographic range sizes with higher latitudes, to be a special case of a more general pattern of niche-breadth expansion (less specialization) with higher latitudes. Under this explanation, specialized species have narrow tolerances of abiotic conditions, use a small range of resources, and / or tolerant of a very limited set of competitors, predators, parasites, and diseases.
There is controversy in the current biogeographic literature about the extent of application of Rapoport’s rule or Rapoport’s effects. Some authors consider it a global phenomenon, driven by global mechanisms such orbital dynamics (Dynesius & Jansson, 2000) or habitat stability (MacArthur 1955; 1972) differences across the globe. Other authors, most notably Rhode (1996; 1999) consider Rapoport’s effects to be localized to only some latitudes or taxonomic groups.
Recently, Vazquez and Stevens (2004) presented a hypothesis for a mechanism underlying a global Rapoport’s rule. Briefly, they proposed that a positive relationship between niche breadth and latitude will occur if 1) there is a latitudinal gradient in species richness and 2) the species interaction network is an asymmetrically specialized interaction network such that specialists tend to interact with generalists. This hypotheses appears to involve the opposite direction of causality compared to hypotheses relating high tropical species richness to narrow species niches (i.e. high specialization) via character displacement and competitive exclusion.
The measure of niche breadth used in this study includes an estimate of the taxonomic distinctiveness of the fleas’ hosts. A flea species with a broad niche will use hosts that are more distantly related to each other than will a flea species with a narrow niche, even if both flea species use the same number of species of host. The authors describe this as the STD index, in which high values indicate more taxonomically distinct hosts such as hosts in different orders. In their analysis, these authors excluded all extreme specialist flea species, those found in only one geographic site or on only one mammal host species.
Analyses included regressions of two dependent variables (host specificity and geographic range size) against the independent variable (geographic range position) using both conventional statistics and PIC. Geographic range position was taken as the midpoint latitude of a species geographic range, which was computed by constructing minimum surface polygons and other techniques more fully described in their methods section.
Species level phylogenies were constructed for each family of fleas, based on a family-level phylogeny previously produced by Medvedev (1995) and morphological and taxonomic characters. Polytomies were considered “soft” i.e. they assumed no knowledge of hidden branching patterns. The PIC conducted by these authors included a range of sophisticated statistical controls, again more fully detailed in their methods section.
The results of this study demonstrate 1. fleas follow Rapoport’s rule, at least in the Palaearctic and 2. host specificity in fleas declines at higher latitudes. Thus the positive relationship in fleas of niche breadth and latitude holds for both abiotic (geographic range size) and biotic (diversity of hosts) components of their niches. There were some exceptions, but the overall trend is clear. These authors propose a mechanism underlying this trend that is much in line with Vazquez and Stevens (2004), involving interactions between niche breadth and geographic range and latitude and niche breadth.
This paper examined Rapoport’s rule in fleas that use small mammals in the Palaearctic as hosts, applying phylogenetic independent contrasts (PIC) to questions of geographic range size, latitude, and niche breadth. These authors consider Rapoport’s rule, of increased geographic range sizes with higher latitudes, to be a special case of a more general pattern of niche-breadth expansion (less specialization) with higher latitudes. Under this explanation, specialized species have narrow tolerances of abiotic conditions, use a small range of resources, and / or tolerant of a very limited set of competitors, predators, parasites, and diseases.
There is controversy in the current biogeographic literature about the extent of application of Rapoport’s rule or Rapoport’s effects. Some authors consider it a global phenomenon, driven by global mechanisms such orbital dynamics (Dynesius & Jansson, 2000) or habitat stability (MacArthur 1955; 1972) differences across the globe. Other authors, most notably Rhode (1996; 1999) consider Rapoport’s effects to be localized to only some latitudes or taxonomic groups.
Recently, Vazquez and Stevens (2004) presented a hypothesis for a mechanism underlying a global Rapoport’s rule. Briefly, they proposed that a positive relationship between niche breadth and latitude will occur if 1) there is a latitudinal gradient in species richness and 2) the species interaction network is an asymmetrically specialized interaction network such that specialists tend to interact with generalists. This hypotheses appears to involve the opposite direction of causality compared to hypotheses relating high tropical species richness to narrow species niches (i.e. high specialization) via character displacement and competitive exclusion.
The measure of niche breadth used in this study includes an estimate of the taxonomic distinctiveness of the fleas’ hosts. A flea species with a broad niche will use hosts that are more distantly related to each other than will a flea species with a narrow niche, even if both flea species use the same number of species of host. The authors describe this as the STD index, in which high values indicate more taxonomically distinct hosts such as hosts in different orders. In their analysis, these authors excluded all extreme specialist flea species, those found in only one geographic site or on only one mammal host species.
Analyses included regressions of two dependent variables (host specificity and geographic range size) against the independent variable (geographic range position) using both conventional statistics and PIC. Geographic range position was taken as the midpoint latitude of a species geographic range, which was computed by constructing minimum surface polygons and other techniques more fully described in their methods section.
Species level phylogenies were constructed for each family of fleas, based on a family-level phylogeny previously produced by Medvedev (1995) and morphological and taxonomic characters. Polytomies were considered “soft” i.e. they assumed no knowledge of hidden branching patterns. The PIC conducted by these authors included a range of sophisticated statistical controls, again more fully detailed in their methods section.
The results of this study demonstrate 1. fleas follow Rapoport’s rule, at least in the Palaearctic and 2. host specificity in fleas declines at higher latitudes. Thus the positive relationship in fleas of niche breadth and latitude holds for both abiotic (geographic range size) and biotic (diversity of hosts) components of their niches. There were some exceptions, but the overall trend is clear. These authors propose a mechanism underlying this trend that is much in line with Vazquez and Stevens (2004), involving interactions between niche breadth and geographic range and latitude and niche breadth.
Witt and Hebert 2000
Witt JDS, Hebert PDN. 2000. Cryptic species diversity and evolution in the amphipod genus Hyalella within central glaciated North America: a molecular phylogenetic approach. Canadian Journal of Fisheries and Aquatic Sciences 57: 687-698.
These authors examined mtDNA and nuclear (allozyme) markers in a species of amphipod that is a strong candidate for cryptic diversification. Hyalella azteca has a vast geographic range, stretching from Panama to north of the Arctic circle and from the Atlantic to Pacific coasts of North America. It is found in a wide range of different freshwater habitats, including streams, rivers, ponds and lakes up to and including the Laurentian Great Lakes. These freshwater habitats are also highly disjunct in geographic distribution. All of these factors together present the large number of populations of this species with a huge range of selective pressures and long times since many populations last were in contact.
The goals of this study were first to extend earlier work that was suggestive of diversity within this putative species, and second to determine the ages of lineages with Hyalella azteca and compare those ages with major geological events such as the Pleistocene glaciations and the formation of the Isthmus of Panama. The subgenus, Hyalella, to which H. azteca belongs was previously thought to have originated after the formation of the Isthmus, by colonization from the genus’ center of origin in South America.
Seven mtDNA lineages were detected that differed from each other by between 9 and 28% at the gene COI. This exceeds many divergence values between congeneric or occasionally confamilial species. These authors recommend that these seven lineages be considered cryptic species, based on this and some of the biogeographic distribution evidence, and furthermore suggest that Clade 7, apparently divergent also in morphology, be included in the species Hyalella inermis, a taxon that was included into H. azteca early in the 20th century but should be resurrected.
Patterns of allozyme differences within Clade 5 and Clade 6 were consistent with a scenario of divergence during isolation in distinct glacial refugia during the Pleistocene. Other patterns of divergence strongly suggest the subgenus originated long before the Isthmus of Panama formed. These authors estimated a divergence date for the subgenus of 11 million years ago, while the Isthmus formed approximately 3 million years ago.
From a practical point of view, this paper is very important to me. First, it provides an example of a comparison of phylogeographic patterns with major geological events and inferred patterns of dispersal and colonization. Second, many of the collection locations described in this paper are near my proposed route of collection and travel from Guelph to Winnipeg, planned for the summer of 2008. As I intend to collect specimens of Hyalella azteca (or, indeed, of species within this apparent species complex), recollections from some of the same locations as this study are desirable.
These authors examined mtDNA and nuclear (allozyme) markers in a species of amphipod that is a strong candidate for cryptic diversification. Hyalella azteca has a vast geographic range, stretching from Panama to north of the Arctic circle and from the Atlantic to Pacific coasts of North America. It is found in a wide range of different freshwater habitats, including streams, rivers, ponds and lakes up to and including the Laurentian Great Lakes. These freshwater habitats are also highly disjunct in geographic distribution. All of these factors together present the large number of populations of this species with a huge range of selective pressures and long times since many populations last were in contact.
The goals of this study were first to extend earlier work that was suggestive of diversity within this putative species, and second to determine the ages of lineages with Hyalella azteca and compare those ages with major geological events such as the Pleistocene glaciations and the formation of the Isthmus of Panama. The subgenus, Hyalella, to which H. azteca belongs was previously thought to have originated after the formation of the Isthmus, by colonization from the genus’ center of origin in South America.
Seven mtDNA lineages were detected that differed from each other by between 9 and 28% at the gene COI. This exceeds many divergence values between congeneric or occasionally confamilial species. These authors recommend that these seven lineages be considered cryptic species, based on this and some of the biogeographic distribution evidence, and furthermore suggest that Clade 7, apparently divergent also in morphology, be included in the species Hyalella inermis, a taxon that was included into H. azteca early in the 20th century but should be resurrected.
Patterns of allozyme differences within Clade 5 and Clade 6 were consistent with a scenario of divergence during isolation in distinct glacial refugia during the Pleistocene. Other patterns of divergence strongly suggest the subgenus originated long before the Isthmus of Panama formed. These authors estimated a divergence date for the subgenus of 11 million years ago, while the Isthmus formed approximately 3 million years ago.
From a practical point of view, this paper is very important to me. First, it provides an example of a comparison of phylogeographic patterns with major geological events and inferred patterns of dispersal and colonization. Second, many of the collection locations described in this paper are near my proposed route of collection and travel from Guelph to Winnipeg, planned for the summer of 2008. As I intend to collect specimens of Hyalella azteca (or, indeed, of species within this apparent species complex), recollections from some of the same locations as this study are desirable.
Stork 2007
Stork NE. 2007. World of insects. Nature 448: 657-658.
This is a “News & Views” article in the August 9 2007 issue of Nature, summarizing and contrasting papers in that issue by Novotny et al. (2007) and Dyer et al. (2007).
Novotny et al. (2007) found low beta diversity, few barriers to dispersal, and low host specificity in tropical insects (primarily caterpillars) in Papua New Guinea. Dyer et al. (2007) found higher host specificity of caterpillars in tropical than in temperate South and North America. These apparently contradictory results are described by this author as requiring explanation derived from further large-scale, highly-cooperative studies in a similar vein as these described studies.
While I agree with this broad conclusion, I found this article somewhat annoying. An early section claims, without references, that up to 95% of insect species remain undescribed. However, Novotny et al. (2007) extensively discuss what they view as widely-publicized overestimates of global insect species richness, and suggest that total estimates should be revised downward from their currently-popular level of 10 million. A later section in Stork (2007) is little more than a rambling call for greater sampling effort of insects, other invertebrates, and other eukaryotes, particularly fungi, ending with a mention of the mid-domain “theory” (Colwell and Hartt, 1994); I was under the impression it was the mid-domain “hypothesis”, and was a useful null model for testing some large-scale biogeographic patterns.
This is a “News & Views” article in the August 9 2007 issue of Nature, summarizing and contrasting papers in that issue by Novotny et al. (2007) and Dyer et al. (2007).
Novotny et al. (2007) found low beta diversity, few barriers to dispersal, and low host specificity in tropical insects (primarily caterpillars) in Papua New Guinea. Dyer et al. (2007) found higher host specificity of caterpillars in tropical than in temperate South and North America. These apparently contradictory results are described by this author as requiring explanation derived from further large-scale, highly-cooperative studies in a similar vein as these described studies.
While I agree with this broad conclusion, I found this article somewhat annoying. An early section claims, without references, that up to 95% of insect species remain undescribed. However, Novotny et al. (2007) extensively discuss what they view as widely-publicized overestimates of global insect species richness, and suggest that total estimates should be revised downward from their currently-popular level of 10 million. A later section in Stork (2007) is little more than a rambling call for greater sampling effort of insects, other invertebrates, and other eukaryotes, particularly fungi, ending with a mention of the mid-domain “theory” (Colwell and Hartt, 1994); I was under the impression it was the mid-domain “hypothesis”, and was a useful null model for testing some large-scale biogeographic patterns.
Dyer et al. 2007
Dyer LA, Singer MS, Lill JT, Stireman JO, Gentry GL, Marquis RJ, Ricklefs RE, Greeney HF, Wagner DL, Morais HC, Diniz IR, Kursar TA, Coley PD. 2007. Host specificity of Lepidoptera in tropical and temperate forests. Nature 448: 696-700.
These authors examined thousands of species of caterpillars and host plants in the New World from southern Canada to Brazil. They were testing the hypothesis that greater niche specialization accounts for higher species richness in tropical regions. They measured ecological specialization along a latitudinal gradient by quantifying diet breadth in caterpillars and beta diversity of caterpillars on widespread focal tree species. Host plant specificity of forest caterpillars decreased with increasing latitude. They were able to control for varying sampling area and sampling effort, but not for phylogeny.
Possible explanations for this trend include the probability that tropical plants are chemically “nastier”, prompting stronger selection among herbivores for specialization in diet, under the assumption (not stated in this paper) that tradeoffs exist among strategies for dealing with plant chemical defences. The trend of increased tropical specialization could be strengthened if, as is likely, widespread and generalized tropical species are actually assemblies of cryptic specialists (i.e. conserved morphology but reproductive isolation and local specialization for diet). Additionally, temperate species of both Lepidoptera and trees are generally better-studied than their tropical counterparts, suggesting that many more species of highly specialized and possibly cryptic herbivores await description in the tropics.
This paper appears in an issue of Nature with a paper by Novotny et al. (2007). Both papers are summarized in a “News & Views” by Stork (2007), as they found apparently contradictory results.
These authors examined thousands of species of caterpillars and host plants in the New World from southern Canada to Brazil. They were testing the hypothesis that greater niche specialization accounts for higher species richness in tropical regions. They measured ecological specialization along a latitudinal gradient by quantifying diet breadth in caterpillars and beta diversity of caterpillars on widespread focal tree species. Host plant specificity of forest caterpillars decreased with increasing latitude. They were able to control for varying sampling area and sampling effort, but not for phylogeny.
Possible explanations for this trend include the probability that tropical plants are chemically “nastier”, prompting stronger selection among herbivores for specialization in diet, under the assumption (not stated in this paper) that tradeoffs exist among strategies for dealing with plant chemical defences. The trend of increased tropical specialization could be strengthened if, as is likely, widespread and generalized tropical species are actually assemblies of cryptic specialists (i.e. conserved morphology but reproductive isolation and local specialization for diet). Additionally, temperate species of both Lepidoptera and trees are generally better-studied than their tropical counterparts, suggesting that many more species of highly specialized and possibly cryptic herbivores await description in the tropics.
This paper appears in an issue of Nature with a paper by Novotny et al. (2007). Both papers are summarized in a “News & Views” by Stork (2007), as they found apparently contradictory results.
Novotny et al. 2007
Novotny V, Miller SE, Hulcr J, Drew RAI, Basset Y, Janda M, Setliff GP, Darrow K, Stewart AJA, Auga J, Isua B, Molem K, Manumbor M, Tamtiai E, Mogia M, Weiblen GD. 2007. Low beta diversity of herbivorous insects in tropical forests. Nature 448: 692-697.
These authors examined 370 species of caterpillars and about 130 species of ambrosia beetles (Scolytinae and Platypodinae) and fruitflies (Tephritidae) in a large contiguous patch of rainforest in Papua New Guinea. While total species richness was high, as expected for the tropics, beta diversity or turnover in species composition through space, was surprisingly low. The authors attribute this surprising finding primarily to the genus-specific rather than species-specific diets of many of the Lepidoptera. '
Beta diversity can be overestimated by inadequate sampling of many rare species, or underestimated by relying on taxonomically known species that tend to be the most widespread and abundant species. In this study, these issues were addressed by studying relatively well-described insect taxa on a limited subset of the available plant hosts, in this case four genera of trees including 175 species in the study area. Study sites were evenly distributed across a large area that was nearly homogeneous for altitude, climate, soil, vegetation, and other factors. In general, herbivore diversity tracked the diversity (alpha and beta) of their plant hosts, which in itself is a less than surprising finding. Other studies that have found high beta diversity of tropical insects have followed topological or climatic gradients, where plant host beta diversity is also high.'
This paper appears in the same issue of Nature as Dyer et al. (2007), and a “News & Views” commentary by Stork (2007). Dyer et al. (2007) found high beta diversity of tropical insects, but their study is different in many ways from that of Novotny et al. (2007).
These authors examined 370 species of caterpillars and about 130 species of ambrosia beetles (Scolytinae and Platypodinae) and fruitflies (Tephritidae) in a large contiguous patch of rainforest in Papua New Guinea. While total species richness was high, as expected for the tropics, beta diversity or turnover in species composition through space, was surprisingly low. The authors attribute this surprising finding primarily to the genus-specific rather than species-specific diets of many of the Lepidoptera. '
Beta diversity can be overestimated by inadequate sampling of many rare species, or underestimated by relying on taxonomically known species that tend to be the most widespread and abundant species. In this study, these issues were addressed by studying relatively well-described insect taxa on a limited subset of the available plant hosts, in this case four genera of trees including 175 species in the study area. Study sites were evenly distributed across a large area that was nearly homogeneous for altitude, climate, soil, vegetation, and other factors. In general, herbivore diversity tracked the diversity (alpha and beta) of their plant hosts, which in itself is a less than surprising finding. Other studies that have found high beta diversity of tropical insects have followed topological or climatic gradients, where plant host beta diversity is also high.'
This paper appears in the same issue of Nature as Dyer et al. (2007), and a “News & Views” commentary by Stork (2007). Dyer et al. (2007) found high beta diversity of tropical insects, but their study is different in many ways from that of Novotny et al. (2007).
Tuesday, April 8, 2008
Stewart and Dillon 2004
Stewart TW, Dillon RT Jr. 2004. Species composition and geographic distribution of Virginia’s freshwater gastropod fauna: a review using historical records. American Malacological Bulletin 19: 79-91.
These authors reviewed museum collections, electronic databases, and published and unpublished reports of freshwater gastropod species occurring in Virginia. More than 50 species were found in the state, with a few species records only from many decades previously suggesting possible extinction or extirpation. The habitats and some tolerances of some species were also described, especially for particularly abundant species and a few rare species restricted to small areas such as cave systems. This constitutes the first comprehensive review of Virginia’s freshwater Gastropoda in 30 years. The authors end the paper by suggesting it be used to further conservation efforts in the state.
These authors reviewed museum collections, electronic databases, and published and unpublished reports of freshwater gastropod species occurring in Virginia. More than 50 species were found in the state, with a few species records only from many decades previously suggesting possible extinction or extirpation. The habitats and some tolerances of some species were also described, especially for particularly abundant species and a few rare species restricted to small areas such as cave systems. This constitutes the first comprehensive review of Virginia’s freshwater Gastropoda in 30 years. The authors end the paper by suggesting it be used to further conservation efforts in the state.
Monday, April 7, 2008
Rhode 1996
Rhode K. 1996. Rapoport’s Rule is a local phenomenon and cannot explain latitudinal gradients in species diversity. Biodiversity Letters 3: 10-13.
This author summarizes current evidence for Rapoport’s Rule, here defined as a gradient of increasing species latitudinal ranges with increasing latitude. The studies cited fall into one of two categories: either they support the existence of Rapoport’s Rule, but only in the northern hemisphere, in terrestrial or freshwater environments north of approximately 40°N, or they did not find evidence in support of Rapoport’s Rule and studied organisms living in the tropics, the southern hemisphere, and/or marine environments. The landmasses north of 40°N are roughly coincident with the extent of the glaciation of the Pleistocene, suggesting that Rapoport’s Rule may be more a result of historical processes than current ecological conditions.
Much of the paper is a direct critique of Stevens (1989, 1996), who (in 1996) attempts to extend the generality of Rapoport’s Rule to marine teleosts and depth. According to Rhode (1996), Stevens (1989, 1996) neglects to consider two important potentially confounding effects. First, sampling bias alone (as described by Colwell & Hurtt, 1994) can lead to an apparent Rapoport’s Effect as per-species sampling effort will decrease with increasing species richness if total sampling effort is held constant, and the species in the more species rich region will appear to have smaller ranges. Second, Stevens (1989, 1996) used means of species ranges (latitudinal, depth, or altitude), but the mean is a poor measure of central tendency in this context (as pointed out by Roy et al., 1994) because of the strongly non-normal distributions of species ranges. Additionally, Rhode (1996) states that high-latitude marine fishes experience less temperature variation with depth than do tropical species, so cannot be considered to have broader temperature tolerances.
This paper clarifies some of the debate apparent in the literature extending to 2008 (e.g. Krasnov et al. 2008), but I am left with some suspicions about Rhode’s (1996) fact-checking. Besides some odd typos in the literature cited section, Rhode (1996) describes France (1992) as finding a Rapoport’s Rule pattern in North American freshwater crayfish and amphipods north of approximately 40°N. However, France (1992) includes data about crayfish and amphipod species ranges from approximately 30°N, which I do not consider equivalent to “approximately 40°N”. France (1992) also includes relevant data about other groups, molluscs, mammals, “herps”, and “fishes” derived from Stevens (1989). In short, this paper opens as many questions about the status of the debate in the biogeographical literature about Rapoport’s Rule as it answers.
This author summarizes current evidence for Rapoport’s Rule, here defined as a gradient of increasing species latitudinal ranges with increasing latitude. The studies cited fall into one of two categories: either they support the existence of Rapoport’s Rule, but only in the northern hemisphere, in terrestrial or freshwater environments north of approximately 40°N, or they did not find evidence in support of Rapoport’s Rule and studied organisms living in the tropics, the southern hemisphere, and/or marine environments. The landmasses north of 40°N are roughly coincident with the extent of the glaciation of the Pleistocene, suggesting that Rapoport’s Rule may be more a result of historical processes than current ecological conditions.
Much of the paper is a direct critique of Stevens (1989, 1996), who (in 1996) attempts to extend the generality of Rapoport’s Rule to marine teleosts and depth. According to Rhode (1996), Stevens (1989, 1996) neglects to consider two important potentially confounding effects. First, sampling bias alone (as described by Colwell & Hurtt, 1994) can lead to an apparent Rapoport’s Effect as per-species sampling effort will decrease with increasing species richness if total sampling effort is held constant, and the species in the more species rich region will appear to have smaller ranges. Second, Stevens (1989, 1996) used means of species ranges (latitudinal, depth, or altitude), but the mean is a poor measure of central tendency in this context (as pointed out by Roy et al., 1994) because of the strongly non-normal distributions of species ranges. Additionally, Rhode (1996) states that high-latitude marine fishes experience less temperature variation with depth than do tropical species, so cannot be considered to have broader temperature tolerances.
This paper clarifies some of the debate apparent in the literature extending to 2008 (e.g. Krasnov et al. 2008), but I am left with some suspicions about Rhode’s (1996) fact-checking. Besides some odd typos in the literature cited section, Rhode (1996) describes France (1992) as finding a Rapoport’s Rule pattern in North American freshwater crayfish and amphipods north of approximately 40°N. However, France (1992) includes data about crayfish and amphipod species ranges from approximately 30°N, which I do not consider equivalent to “approximately 40°N”. France (1992) also includes relevant data about other groups, molluscs, mammals, “herps”, and “fishes” derived from Stevens (1989). In short, this paper opens as many questions about the status of the debate in the biogeographical literature about Rapoport’s Rule as it answers.
Chapin and Körner 1994
Chapin FS III, Körner C. 1994. Arctic and alpine biodiversity: patterns, causes and ecosystem consequences. Trends in Ecology and Evolution 9: 45-47.
These authors summarize the major points discussed at a meeting in Norway of researchers studying Arctic and alpine ecosystems, in the context of climate change and 14 major biomes. No published works are cited in this paper, rather several prominent researchers are mentioned as contributing various components of the meeting.
Arctic and alpine ecosystems were grouped together and described as “critical” for five reasons: 1. High latitudes are expected to experience the most change in climate; 2. The ecological consequences of warming will be most severe in cold regions; 3. High altitudes with low atmospheric pressures are expected to be most limiting for CO2 and consequently will respond strongly to changes in CO2 concentrations; 4. Arctic systems include large pools of frozen carbon and methane, and will thus generate important feedback effects during warming; 5. Arctic and alpine ecosystems are relatively simple systems and may show clear effects on species of ecosystem processes. Point 5 is probably most directly applicable to my own work, in that it reinforces the utility of low-species-richness and extreme-climate environments for examinations of interactions between abiotic factors and evolutionary processes.
Much of the discussion centres on the Arctic and alpine flora, which show patterns of diversity strongly associated with historical forces such as the Pleistocene glaciations. Arctic floras tend to be broadly distributed, often holarctic, while most alpine systems are more specific to small areas. In general, stable Arctic and alpine systems show diversity curves that fit the geometric model, suggesting that competitive interactions for limiting resources best explain patterns of diversity, rather than abiotic factors.
In contrast, animal diversity shows a clear latitudinal and altitudinal gradient, with associated patterns of taxonomic replacement. For example, coleopteran species richness declines with latitude while dipteran species richness increases.
The processes that humans are most interested in for economic and other reasons are those most sensitive to species composition, such as pollination and trophic dynamics. Other processes are much less sensitive to species composition within functional groups, for example many biogeochemistry processes.
In summary, the described conference demonstrated that a great deal is known about patterns of biodiversity in Arctic and alpine ecosystems as well as globally. These patterns appear to have important consequences for ecosystem function, and further research is urged in refining knowledge of species diversity patterns to better detect changes due to climate, experimental manipulations simulating changes in climate and CO2, and simulation modelling of long-term and large-scale processes.
These authors summarize the major points discussed at a meeting in Norway of researchers studying Arctic and alpine ecosystems, in the context of climate change and 14 major biomes. No published works are cited in this paper, rather several prominent researchers are mentioned as contributing various components of the meeting.
Arctic and alpine ecosystems were grouped together and described as “critical” for five reasons: 1. High latitudes are expected to experience the most change in climate; 2. The ecological consequences of warming will be most severe in cold regions; 3. High altitudes with low atmospheric pressures are expected to be most limiting for CO2 and consequently will respond strongly to changes in CO2 concentrations; 4. Arctic systems include large pools of frozen carbon and methane, and will thus generate important feedback effects during warming; 5. Arctic and alpine ecosystems are relatively simple systems and may show clear effects on species of ecosystem processes. Point 5 is probably most directly applicable to my own work, in that it reinforces the utility of low-species-richness and extreme-climate environments for examinations of interactions between abiotic factors and evolutionary processes.
Much of the discussion centres on the Arctic and alpine flora, which show patterns of diversity strongly associated with historical forces such as the Pleistocene glaciations. Arctic floras tend to be broadly distributed, often holarctic, while most alpine systems are more specific to small areas. In general, stable Arctic and alpine systems show diversity curves that fit the geometric model, suggesting that competitive interactions for limiting resources best explain patterns of diversity, rather than abiotic factors.
In contrast, animal diversity shows a clear latitudinal and altitudinal gradient, with associated patterns of taxonomic replacement. For example, coleopteran species richness declines with latitude while dipteran species richness increases.
The processes that humans are most interested in for economic and other reasons are those most sensitive to species composition, such as pollination and trophic dynamics. Other processes are much less sensitive to species composition within functional groups, for example many biogeochemistry processes.
In summary, the described conference demonstrated that a great deal is known about patterns of biodiversity in Arctic and alpine ecosystems as well as globally. These patterns appear to have important consequences for ecosystem function, and further research is urged in refining knowledge of species diversity patterns to better detect changes due to climate, experimental manipulations simulating changes in climate and CO2, and simulation modelling of long-term and large-scale processes.
Friday, April 4, 2008
D'Amico et al. 2002
D’Amico S, Claverie P, Collins T, Georlette D, Gratia E, Hoyoux A, Meuwis M-A, Feller G, Gerday C. 2002. Molecular basis of cold adaptation. Proceedings of the Royal Society of London B 257: 917-925.
These authors review the relationships between enzyme parameters and temperature. A trade-off exists between enzyme activity at low temperatures and thermal stability. This trade-off is driven by enzyme flexibility: more flexible enzymes are more active (lower activation energy) at low temperatures, but become denatured at lower temperatures than less flexible, more stable enzymes. Enzyme flexibility is related to the strengths and frequencies of bonds that hold enzymes in their three-dimensional shapes; greater flexibility, with weaker and/or fewer such bonds can interact with their substrates more readily, especially at low temperatures, but are denatured easily by increasing temperatures.
The flexible portion of a cold-adapted enzyme has always been found in the domain of the active site. This argues strongly against some proposed explanations for enzymes structures and temperature that rely on relaxed selection pressure and the predominance of drift at low temperatures.
High enzyme flexibility may also allow reversible denaturation, though this point is barely explored by these authors. This suggests to me that some cold-adapted organisms may be able to tolerate temporary high temperatures more readily than “mesothermic” organisms could tolerate an increase of a similar magnitude.
Enzymes that break the trade-off and have high stability, high flexibility, and high low-temperature activity may be possible in the laboratory, especially when using particular types of artificial substrates, but have so far never been found in nature.
These authors review the relationships between enzyme parameters and temperature. A trade-off exists between enzyme activity at low temperatures and thermal stability. This trade-off is driven by enzyme flexibility: more flexible enzymes are more active (lower activation energy) at low temperatures, but become denatured at lower temperatures than less flexible, more stable enzymes. Enzyme flexibility is related to the strengths and frequencies of bonds that hold enzymes in their three-dimensional shapes; greater flexibility, with weaker and/or fewer such bonds can interact with their substrates more readily, especially at low temperatures, but are denatured easily by increasing temperatures.
The flexible portion of a cold-adapted enzyme has always been found in the domain of the active site. This argues strongly against some proposed explanations for enzymes structures and temperature that rely on relaxed selection pressure and the predominance of drift at low temperatures.
High enzyme flexibility may also allow reversible denaturation, though this point is barely explored by these authors. This suggests to me that some cold-adapted organisms may be able to tolerate temporary high temperatures more readily than “mesothermic” organisms could tolerate an increase of a similar magnitude.
Enzymes that break the trade-off and have high stability, high flexibility, and high low-temperature activity may be possible in the laboratory, especially when using particular types of artificial substrates, but have so far never been found in nature.
Bennett et al. 2005
Bennett VA, Sformo T, Walters K, Toien O, Jeannet K, Hochstrasser R, Pan Q, Serianni AS, Barnes BM, Duman JG. 2005. Comparative overwintering physiology of Alaska and Indiana populations of the beetle Cucujus clavipes (Fabricius): roles of antifreeze proteins, polyols, dehydration and daipause. Journal of Experimental Biology 208: 4467-4477.
These authors measured various aspects of winter survival in the larvae of a bark-dwelling beetle, in two locations (Alaska and Indiana) over three years. These larvae produce antifreeze proteins (AFP) and polyols such as glycerol that lower their supercooling points considerably. Dehydration in Alaskan insects increases the haemolymph concentration of these AFPs and further depresses the supercooling point of body tissues. Previous reports of this species’ overwintering abilities suggested they were freeze tolerant, but this paper strongly suggests they are freeze avoiding, and may be capable of body water vitrification under some extreme circumstances.
To examine the distinction between freeze tolerance and freeze avoidance, these authors measured supercooling points, thermal hysteresis activity, body water content, polyols content, and respiration rates in larvae. Supercooling points were taken as the temperature at which exotherms were recorded, when the heat of fusion of water was released by ice formation. All larvae that produced exotherms died, while all larvae that were cooled to just above expected supercooling points but were not frozen survived. No larvae produced exotherms colder than -58°C, and about half of those cooled lower than this survived, which was taken by these authors as indirect evidence of body water vitrification. Further analyses would be necessary to confirm this suggestion.
The differences in survivability of winter temperatures shown by Alaskan and Indianan larvae may relate primarily to the timing of AFP production. Alaskan larvae synthesize AFPs much earlier in the season than do Indianan, and are capable of survival in Indiana while Indianan larvae all died when overwintered in Alaska. The extreme dehydration of Alaskan larvae prevented measurement of body water content and AFP concentration, thus the relative importance of amounts produced, timing of production, and duration and severity of winter could not be determined.
In summary, there appears to be a genetic component to differences in larval overwintering capabilities across this species’ very broad latitudinal range. The physiological mechanism of this difference, either timing, magnitude, or composition of the production of antifreeze molecules, is not clear.
These authors measured various aspects of winter survival in the larvae of a bark-dwelling beetle, in two locations (Alaska and Indiana) over three years. These larvae produce antifreeze proteins (AFP) and polyols such as glycerol that lower their supercooling points considerably. Dehydration in Alaskan insects increases the haemolymph concentration of these AFPs and further depresses the supercooling point of body tissues. Previous reports of this species’ overwintering abilities suggested they were freeze tolerant, but this paper strongly suggests they are freeze avoiding, and may be capable of body water vitrification under some extreme circumstances.
To examine the distinction between freeze tolerance and freeze avoidance, these authors measured supercooling points, thermal hysteresis activity, body water content, polyols content, and respiration rates in larvae. Supercooling points were taken as the temperature at which exotherms were recorded, when the heat of fusion of water was released by ice formation. All larvae that produced exotherms died, while all larvae that were cooled to just above expected supercooling points but were not frozen survived. No larvae produced exotherms colder than -58°C, and about half of those cooled lower than this survived, which was taken by these authors as indirect evidence of body water vitrification. Further analyses would be necessary to confirm this suggestion.
The differences in survivability of winter temperatures shown by Alaskan and Indianan larvae may relate primarily to the timing of AFP production. Alaskan larvae synthesize AFPs much earlier in the season than do Indianan, and are capable of survival in Indiana while Indianan larvae all died when overwintered in Alaska. The extreme dehydration of Alaskan larvae prevented measurement of body water content and AFP concentration, thus the relative importance of amounts produced, timing of production, and duration and severity of winter could not be determined.
In summary, there appears to be a genetic component to differences in larval overwintering capabilities across this species’ very broad latitudinal range. The physiological mechanism of this difference, either timing, magnitude, or composition of the production of antifreeze molecules, is not clear.
Rigler 1975
Rigler FH. 1975. The Char Lake project, an introduction to limnology in the Canadian arctic. In: Energy Flow – Its Biological Dimensions (Eds. Cameron TWMC and Billingsley LW). Royal Society of Canada, Ottawa.
This book was published to provide educated laypeople with information about the International Biological Programme (IBP) as it was conducted in Canada. Chapter 10, by this author, provides an overview of the work conducted at Char Lake and adjacent areas, and some of the results obtained.
Arctic lakes are much simpler ecosystems than are temperate lakes, with fewer species and a less structured physical environment. Unlike often-stratified and occasionally anoxic temperate lakes, most Arctic lakes including Char Lake are remarkably homogeneous, with well mixed waters varying in temperature between 0°C and 4°C. Most are never anoxic, due to no summer thermal stratification, high levels of winter mixing driven by temperature differences through the lake, and oxygen supply to the water by freeze-out from the usually large mass of winter ice.
Char Lake sits in a catchment basin about 8 times larger in area than the lake itself. The lake and its basin are very unproductive, with biomass accumulation rates a small fraction of temperate or even tundra ecosystems. The plankton is apparently nearly homogeneous across the lake at all depths, driven by well mixed water and the very low species richness. Only one species of zooplankter was reported, a copepod. This author divides the benthos into four regions. The first, comprising the shallow fringe of the lake to about 4m depth, freezes solid in winter and is composed of rocks grading into silt, with diatoms lying on the surface of the sediment. The second zone is silty and extends down to about 15m. This zone is dominated by abundant terrestrial mosses and an epiphytic community of algae and small grazers. The third zone is comprised of bare patches of silt among the mosses, centered on depressions likely generated by down-pushing winter ice. This author hypothesizes that these depressions become anoxic in winter due to barriers to water flow, but are gradually recolonized by mosses from the shallow edge. In summer these depressions attract relatively high densities of animals. The last zone is the deepest part of the lake, too deep for sufficient sunlight to penetrate for the mosses. This is a flat, silty plain of oxygenated sediments a few centimetres deep overlying older strata that reflect the lake’s history as a raised feature of the bottom of the Arctic Ocean.
Char Lake appears to be stuck in a post-glacial stage that most temperate lakes passed through rapidly. About 5300 years ago, Char Lake was a bay on the coast of Cornwallis Island that was isolated from the sea by isostatic rebound. This lead to a period of perhaps a few thousand years in which the lake had a deep benthos occupied by seawater and no organisms, which was gradually flushed out by freshwater flows through the system.
This book was published to provide educated laypeople with information about the International Biological Programme (IBP) as it was conducted in Canada. Chapter 10, by this author, provides an overview of the work conducted at Char Lake and adjacent areas, and some of the results obtained.
Arctic lakes are much simpler ecosystems than are temperate lakes, with fewer species and a less structured physical environment. Unlike often-stratified and occasionally anoxic temperate lakes, most Arctic lakes including Char Lake are remarkably homogeneous, with well mixed waters varying in temperature between 0°C and 4°C. Most are never anoxic, due to no summer thermal stratification, high levels of winter mixing driven by temperature differences through the lake, and oxygen supply to the water by freeze-out from the usually large mass of winter ice.
Char Lake sits in a catchment basin about 8 times larger in area than the lake itself. The lake and its basin are very unproductive, with biomass accumulation rates a small fraction of temperate or even tundra ecosystems. The plankton is apparently nearly homogeneous across the lake at all depths, driven by well mixed water and the very low species richness. Only one species of zooplankter was reported, a copepod. This author divides the benthos into four regions. The first, comprising the shallow fringe of the lake to about 4m depth, freezes solid in winter and is composed of rocks grading into silt, with diatoms lying on the surface of the sediment. The second zone is silty and extends down to about 15m. This zone is dominated by abundant terrestrial mosses and an epiphytic community of algae and small grazers. The third zone is comprised of bare patches of silt among the mosses, centered on depressions likely generated by down-pushing winter ice. This author hypothesizes that these depressions become anoxic in winter due to barriers to water flow, but are gradually recolonized by mosses from the shallow edge. In summer these depressions attract relatively high densities of animals. The last zone is the deepest part of the lake, too deep for sufficient sunlight to penetrate for the mosses. This is a flat, silty plain of oxygenated sediments a few centimetres deep overlying older strata that reflect the lake’s history as a raised feature of the bottom of the Arctic Ocean.
Char Lake appears to be stuck in a post-glacial stage that most temperate lakes passed through rapidly. About 5300 years ago, Char Lake was a bay on the coast of Cornwallis Island that was isolated from the sea by isostatic rebound. This lead to a period of perhaps a few thousand years in which the lake had a deep benthos occupied by seawater and no organisms, which was gradually flushed out by freshwater flows through the system.
Lasenby and Langford 1972
Lasenby DC, Langford RR. 1972. Growth, life history, and respiration of Mysis relicta in an arctic and temperate lake. Journal of the Fisheries Research Board of Canada 29: 1701-1708.
These authors studied the small “glacial relict” freshwater crustacean Mysis relicta from Char Lake, NWT (now Nunavut) and from Stony Lake, southern Ontario, two populations separated by about 30 degrees of latitude. They were attempting to test the hypothesis that poikilotherms show metabolic compensation for changes in environmental temperatures.
This study did not find evidence of such metabolic compensation; if they were compensating for temperature, similar activity levels would be expected in different populations, but the Char Lake animals were “more lethargic” and consumed significantly less oxygen than the Stony Lake animals. Char Lake mysids did not migrate vertically, with most animals found at 10m depth. Stony Lake mysids did show daily vertical migration, covering more than 20m depth ranges.
Previous authors (Vernberg and Vernberg, 1970) suggested that northern species may be restricted in southern distribution by abiotic factors, primarily increased metabolic rates associated with higher temperatures and a resulting inability to consume sufficient food. However, these authors point out that temperate lakes are usually much more productive than arctic lakes, so food supply is probably not limiting. Stony Lake mysids appear to be compensating for their warm environment by feeding in the productive surface waters but spending much of their time in cooler bottom waters.
The conclusions of this study are very interesting. Char Lake female mysids took two years and consumed about 200 calories to achieve maturity, and Stony Lake females took one year and consumed about 200 calories to achieve maturity. This possibility that a species uses the same total energy for maturity regardless of environmental conditions “should perhaps be further investigated”; I agree.
These authors studied the small “glacial relict” freshwater crustacean Mysis relicta from Char Lake, NWT (now Nunavut) and from Stony Lake, southern Ontario, two populations separated by about 30 degrees of latitude. They were attempting to test the hypothesis that poikilotherms show metabolic compensation for changes in environmental temperatures.
This study did not find evidence of such metabolic compensation; if they were compensating for temperature, similar activity levels would be expected in different populations, but the Char Lake animals were “more lethargic” and consumed significantly less oxygen than the Stony Lake animals. Char Lake mysids did not migrate vertically, with most animals found at 10m depth. Stony Lake mysids did show daily vertical migration, covering more than 20m depth ranges.
Previous authors (Vernberg and Vernberg, 1970) suggested that northern species may be restricted in southern distribution by abiotic factors, primarily increased metabolic rates associated with higher temperatures and a resulting inability to consume sufficient food. However, these authors point out that temperate lakes are usually much more productive than arctic lakes, so food supply is probably not limiting. Stony Lake mysids appear to be compensating for their warm environment by feeding in the productive surface waters but spending much of their time in cooler bottom waters.
The conclusions of this study are very interesting. Char Lake female mysids took two years and consumed about 200 calories to achieve maturity, and Stony Lake females took one year and consumed about 200 calories to achieve maturity. This possibility that a species uses the same total energy for maturity regardless of environmental conditions “should perhaps be further investigated”; I agree.
Thursday, April 3, 2008
Cheal et al. 1993
Cheal F, Davis JA, Growns JE, Bradley JS, Whittles FH. 1993. The influence of sampling method on the classification of wetland macroinvertebrate communities. Hydrobiologia 257: 47-56.
These authors compared three inexpensive and simple sampling methods for freshwater macroinvertebrates in five lakes near Perth, Australia. The five lakes had been previously well studied for chemical and other factors, and all were quite shallow with soft sediment bottoms. Plankton tows, sweeps with a D-net, and benthic cores were collected at several sites at each lake, and macroinvertebrate species richness measured.
These authors were primarily interested in the ability of collected samples to discriminate between lakes. In this context, the sweep net was most efficient, collecting active nektonic species that differed between the lakes. The sweep net was also most effective at simple species richness, collecting more species than the plankton tow at four of the five lakes and more species than the cores at all lakes. Some taxa, for example oligochaetes and gastropods, were primarily collected in the cores, though a few individuals were found in some sweeps. The authors conclude that the ability of the sweep nets to sample both water column and benthic / epibenthic fauna simultaneously makes them very cost-effective for monitoring biodiversity in wetland habitats, where time spent collecting and sorting invertebrates may be limiting.
These authors compared three inexpensive and simple sampling methods for freshwater macroinvertebrates in five lakes near Perth, Australia. The five lakes had been previously well studied for chemical and other factors, and all were quite shallow with soft sediment bottoms. Plankton tows, sweeps with a D-net, and benthic cores were collected at several sites at each lake, and macroinvertebrate species richness measured.
These authors were primarily interested in the ability of collected samples to discriminate between lakes. In this context, the sweep net was most efficient, collecting active nektonic species that differed between the lakes. The sweep net was also most effective at simple species richness, collecting more species than the plankton tow at four of the five lakes and more species than the cores at all lakes. Some taxa, for example oligochaetes and gastropods, were primarily collected in the cores, though a few individuals were found in some sweeps. The authors conclude that the ability of the sweep nets to sample both water column and benthic / epibenthic fauna simultaneously makes them very cost-effective for monitoring biodiversity in wetland habitats, where time spent collecting and sorting invertebrates may be limiting.
Wednesday, April 2, 2008
Piepenburg 2005
Piepenburg D. 2005. Recent research on Arctic benthos: common notions need to be revised. Polar Biology 28: 733-755.
This author reviews the broad scale ecology of Arctic marine benthic environments, with emphasis on energy and matter flows and biodiversity patterns. There is an early summary of what was known or supposed about Arctic benthic biodiversity in the late 1980s and early 1990s, before major shifts in research took place. With the breakup of the Soviet Union and concommitent opening to Western researchers of the Siberian Arctic, and an increased public awareness of polar issues, the mid-to-late 1990s saw a large increase in scientific attention on the Arctic benthos.
No single clear definition of “Arctic” for the marine realm is recognized, but this author follows the work of Zenkevitch (1963; original in Russian 1955), which broadly defines the Arctic marine as the Arctic Ocean proper, plus its adjacent seas including the northern shorelines of Eurasia and North America, as well as the continental shelves of Greenland, Baffin Bay, and everything north of the “Polar Front” of the Barents and Bering seas. A map of these areas is provided in figure 1.
While the Arctic Ocean proper is almost entirely covered by permanent sea-ice, the adjacent Arctic seas are primarily characterized by very low but relatively constant water temperatures, long-lasting seasonal ice cover, and very pronounced seasonal fluctuations in insolation and, hence, primary production. There are broad continental shelves underlying several Arctic seas such as the Laptev Sea of central Siberia, and these shelves receive a large amount of input from rivers, approximately 10% of the total global river outflow. The broad width of several of these shelves prevents movement of this terrestrial input into the deeper basins of the central Arctic ocean.
The Arctic as a low-temperature ecosystem is much younger than that of Antarctica. While a cold circumpolar current formed around Antarctica perhaps 23mya, the Arctic ocean was temperate until a drastic fall in sea temperatures in the Pliocene about 4mya. During that recent 4my, many areas of Arctic shelf have been either dry due to sea level falls, or covered by glaciers, effectively removing the benthic faunas. Many Antarctic shelf areas also probably experienced glaciation, sometimes to surprisingly deep depths of hundreds of meters. This pattern of repeated extermination and recolonization may imply that Arctic organisms are particularly resilient in the face of environmental change.
Work since the increase in Arctic research has generally supported but modified pre-existing views of Arctic ecology. Both the Arctic and Antarctic benthos can be considered to harbor intermediate biodiversity, though the Antarctic still appears to be slightly more diverse. Disturbances once thought to be unique to the Arctic (such as iceberg bottom-scouring) have been found at significant frequencies in the Antarctic. Most Arctic organisms appear to be wide-spread boreal-Arctic species rather than endemic, while much more of the Antarctic biota is endemic to areas south of the Antarctic convergence.
The major faunal groups of the Arctic benthos are:
On fine sand and mud, bivalves and polychaetes.
On coarse grained sediments, gammaridean amphipods.
On many shelf and slope habitats, brittle stars, which can reach carpet-like population densities.
In the Barents Sea, sea urchins
In the Laptev Sea, sea cucumbers and bivalves
In the Bearing and Chukchi Seas, sea stars and crustaceans, including dense populations of ampeliscid amphipods in some areas
In the deep ocean basins of the Arctic Ocean proper, deposit feeding polychaetes, crustaceans, and bivalves.
“Pelago-benthic coupling” is a blanket term coined by Hargrave (1973) to refer to the downward flux of matter and energy from the water column to the seabed, and related processes of upwelling and other mixing effects. This coupling seems especially strong and important in the Arctic ocean, with meso-scale variations (10-100km) driven by local bottom topology, currents, ice cover dynamics (including polynyas), et cetera. The stronger coupling in the Arctic appears to be driven by a slower response of zooplankton to seasonal changes in phytoplankton, allowing more “fresh” captured carbon to reach the sea floor.
The deep Arctic Ocean basins are apparently relatively uncoupled from adjacent shelf areas, with most productivity in the basins driven by very strong pelago-benthic coupling. Nonetheless, the Arctic Ocean supports productivity about an order of magnitude higher than previously suspected, in part due to the role of ice-bound algae and stochastic large inputs of food.
This is a useful review paper that introduces many of the issues of Arctic marine biology at an ecosystem scale. There are useful links here to other community ecology considerations and to deep ocean research programs.
This author reviews the broad scale ecology of Arctic marine benthic environments, with emphasis on energy and matter flows and biodiversity patterns. There is an early summary of what was known or supposed about Arctic benthic biodiversity in the late 1980s and early 1990s, before major shifts in research took place. With the breakup of the Soviet Union and concommitent opening to Western researchers of the Siberian Arctic, and an increased public awareness of polar issues, the mid-to-late 1990s saw a large increase in scientific attention on the Arctic benthos.
No single clear definition of “Arctic” for the marine realm is recognized, but this author follows the work of Zenkevitch (1963; original in Russian 1955), which broadly defines the Arctic marine as the Arctic Ocean proper, plus its adjacent seas including the northern shorelines of Eurasia and North America, as well as the continental shelves of Greenland, Baffin Bay, and everything north of the “Polar Front” of the Barents and Bering seas. A map of these areas is provided in figure 1.
While the Arctic Ocean proper is almost entirely covered by permanent sea-ice, the adjacent Arctic seas are primarily characterized by very low but relatively constant water temperatures, long-lasting seasonal ice cover, and very pronounced seasonal fluctuations in insolation and, hence, primary production. There are broad continental shelves underlying several Arctic seas such as the Laptev Sea of central Siberia, and these shelves receive a large amount of input from rivers, approximately 10% of the total global river outflow. The broad width of several of these shelves prevents movement of this terrestrial input into the deeper basins of the central Arctic ocean.
The Arctic as a low-temperature ecosystem is much younger than that of Antarctica. While a cold circumpolar current formed around Antarctica perhaps 23mya, the Arctic ocean was temperate until a drastic fall in sea temperatures in the Pliocene about 4mya. During that recent 4my, many areas of Arctic shelf have been either dry due to sea level falls, or covered by glaciers, effectively removing the benthic faunas. Many Antarctic shelf areas also probably experienced glaciation, sometimes to surprisingly deep depths of hundreds of meters. This pattern of repeated extermination and recolonization may imply that Arctic organisms are particularly resilient in the face of environmental change.
Work since the increase in Arctic research has generally supported but modified pre-existing views of Arctic ecology. Both the Arctic and Antarctic benthos can be considered to harbor intermediate biodiversity, though the Antarctic still appears to be slightly more diverse. Disturbances once thought to be unique to the Arctic (such as iceberg bottom-scouring) have been found at significant frequencies in the Antarctic. Most Arctic organisms appear to be wide-spread boreal-Arctic species rather than endemic, while much more of the Antarctic biota is endemic to areas south of the Antarctic convergence.
The major faunal groups of the Arctic benthos are:
On fine sand and mud, bivalves and polychaetes.
On coarse grained sediments, gammaridean amphipods.
On many shelf and slope habitats, brittle stars, which can reach carpet-like population densities.
In the Barents Sea, sea urchins
In the Laptev Sea, sea cucumbers and bivalves
In the Bearing and Chukchi Seas, sea stars and crustaceans, including dense populations of ampeliscid amphipods in some areas
In the deep ocean basins of the Arctic Ocean proper, deposit feeding polychaetes, crustaceans, and bivalves.
“Pelago-benthic coupling” is a blanket term coined by Hargrave (1973) to refer to the downward flux of matter and energy from the water column to the seabed, and related processes of upwelling and other mixing effects. This coupling seems especially strong and important in the Arctic ocean, with meso-scale variations (10-100km) driven by local bottom topology, currents, ice cover dynamics (including polynyas), et cetera. The stronger coupling in the Arctic appears to be driven by a slower response of zooplankton to seasonal changes in phytoplankton, allowing more “fresh” captured carbon to reach the sea floor.
The deep Arctic Ocean basins are apparently relatively uncoupled from adjacent shelf areas, with most productivity in the basins driven by very strong pelago-benthic coupling. Nonetheless, the Arctic Ocean supports productivity about an order of magnitude higher than previously suspected, in part due to the role of ice-bound algae and stochastic large inputs of food.
This is a useful review paper that introduces many of the issues of Arctic marine biology at an ecosystem scale. There are useful links here to other community ecology considerations and to deep ocean research programs.
Weider and Hobæk 2000
Weider LJ, Hobæk A. 2000. Phylogeography and arctic biodiversity: a review. Annales Zoologici Fennici 37: 217-231.
These authors review a variety of phylogeographic studies, to examine the importance of dispersal, vicariance, and selection in shaping the current distributions of Arctic organisms. Much of the focus is on closely-related organisms such as species complexes and subspecies. They provide justification for studying the species-poor Arctic in three ways: 1. the Arctic consists of relatively simple ecosystems, with reduced trophic interactions; 2. Arctic ecosystems are relatively fragile, where minor perturbations have immediate and long-lasting effects; 3. the Arctic has been shaped by a recent geological history of waxing and waning of glaciers, providing a series of vicariance events and recolonization opportunities to organisms.
These authors briefly review a debate in the recent literature, discussing modes of speciation in the Arctic. Some authors have proposed the importance of vicariance and speciation of isolated small populations, while others have emphasized the long periods necessary for the evolution of reproductive isolation when populations re-contact. Both factors are probably important, with hybridization in interglacial periods playing a strong role. The botanical literature has also debated the competing hypotheses of “Nunataks” (Dahl 1987) versus “Tabula rasa” (Nordal 1987).
Most of the reviewed literature has been studies of endothermic vertebrates, with consistent evidence across examined species of episodes of population isolation followed by recontact and often hybridization. Population and subspecies structures are typically consistent with past periods of glaciation, often between 300k and 500k years ago.
The summary findings of this paper are that 1. speciation has occurred or appears to be progressing by vicariance and hybridization in the Arctic; 2. dispersal, founder effects, range expansions, secondary contacts, and other processes have structured population genetics; 3. life-cycle and breeding system optimizations have been very important as adaptations to abiotic factors.
The literature cited section is full of papers that I need to read, especially Klicka and Zink (1997) and Freckman & Virginia (1997).
These authors review a variety of phylogeographic studies, to examine the importance of dispersal, vicariance, and selection in shaping the current distributions of Arctic organisms. Much of the focus is on closely-related organisms such as species complexes and subspecies. They provide justification for studying the species-poor Arctic in three ways: 1. the Arctic consists of relatively simple ecosystems, with reduced trophic interactions; 2. Arctic ecosystems are relatively fragile, where minor perturbations have immediate and long-lasting effects; 3. the Arctic has been shaped by a recent geological history of waxing and waning of glaciers, providing a series of vicariance events and recolonization opportunities to organisms.
These authors briefly review a debate in the recent literature, discussing modes of speciation in the Arctic. Some authors have proposed the importance of vicariance and speciation of isolated small populations, while others have emphasized the long periods necessary for the evolution of reproductive isolation when populations re-contact. Both factors are probably important, with hybridization in interglacial periods playing a strong role. The botanical literature has also debated the competing hypotheses of “Nunataks” (Dahl 1987) versus “Tabula rasa” (Nordal 1987).
Most of the reviewed literature has been studies of endothermic vertebrates, with consistent evidence across examined species of episodes of population isolation followed by recontact and often hybridization. Population and subspecies structures are typically consistent with past periods of glaciation, often between 300k and 500k years ago.
The summary findings of this paper are that 1. speciation has occurred or appears to be progressing by vicariance and hybridization in the Arctic; 2. dispersal, founder effects, range expansions, secondary contacts, and other processes have structured population genetics; 3. life-cycle and breeding system optimizations have been very important as adaptations to abiotic factors.
The literature cited section is full of papers that I need to read, especially Klicka and Zink (1997) and Freckman & Virginia (1997).
Tuesday, April 1, 2008
Blenckner 2005
Blenckner T. 2005. A conceptual model of climate-related effects on lake ecosystems. Hydrobiologia 533: 1-14.
This author presents a verbal model of factors that may influence how lake ecosystems react to climate change and climate variation. This verbal model is presented as a hierarchical conceptual framework of lake characteristics that control how a lake might react to a “climate signal”. This paper is too long and highly repetitive. There are three major sections, but the first two (“present the approach” and “apply published findings to the approach”) should have been condensed into one, and the third section is very short and consists mainly of suggestions for future research and some rather weak arguments for water resource conservation by political bodies.
Essentially, the conceptual framework is that a climate signal passes through two filters in influencing a lake ecosystem. The first filter is here named the “landscape filter”, and is composed of the geographical position, catchment characteristics, and morphology of a lake, the second filter is the “internal lake filter”, consisting of the lake’s history and the abiotic and biotic interactions within the lake. These points are made at least three times in this paper, without major differences in the descriptions. There is little, for example, in the way of elaboration on these essential points, and indeed some phrases appear to be repeated through this paper.
While the conceptual framework appears to have been constructed with existing data in mind, the papers that provide strongest support for major suppositions do not get cited until the second section. The first section merely outlines the framework, then the second section fills in a few details with citations, though words like “might” and “may” appear far too often for any of the conclusions to be taken very seriously.
No surprising findings are presented in this paper. This conceptual framework, while perhaps useful for encouraging greater interdisciplinary cooperation in the field of climate change research, does not appear to supply any great novel insights into ecosystems and climates. Some discussions seem contradictory or non-applicable, for example a discussion of winter climate on lake productivity that first describes Lake Constance in Germany and the rarity of ice cover on that lake, followed by a description of the effects of ice and snow cover on phytoplankton, ending with a description of how winter climate (presumably involving little or no ice) did not strongly influence variation in phytoplankton in Lake Constance. Additionally, this paper contains several apparent mistakes, such as the misuse of the word “successional”, “i.e.” when “e.g.” is probably implied, and twice referring to the United States of America as “the US”. This paper is labelled “Opinion”; perhaps Hydrobiologia’s peer-review policies for opinion pieces are not very stringent.
This author presents a verbal model of factors that may influence how lake ecosystems react to climate change and climate variation. This verbal model is presented as a hierarchical conceptual framework of lake characteristics that control how a lake might react to a “climate signal”. This paper is too long and highly repetitive. There are three major sections, but the first two (“present the approach” and “apply published findings to the approach”) should have been condensed into one, and the third section is very short and consists mainly of suggestions for future research and some rather weak arguments for water resource conservation by political bodies.
Essentially, the conceptual framework is that a climate signal passes through two filters in influencing a lake ecosystem. The first filter is here named the “landscape filter”, and is composed of the geographical position, catchment characteristics, and morphology of a lake, the second filter is the “internal lake filter”, consisting of the lake’s history and the abiotic and biotic interactions within the lake. These points are made at least three times in this paper, without major differences in the descriptions. There is little, for example, in the way of elaboration on these essential points, and indeed some phrases appear to be repeated through this paper.
While the conceptual framework appears to have been constructed with existing data in mind, the papers that provide strongest support for major suppositions do not get cited until the second section. The first section merely outlines the framework, then the second section fills in a few details with citations, though words like “might” and “may” appear far too often for any of the conclusions to be taken very seriously.
No surprising findings are presented in this paper. This conceptual framework, while perhaps useful for encouraging greater interdisciplinary cooperation in the field of climate change research, does not appear to supply any great novel insights into ecosystems and climates. Some discussions seem contradictory or non-applicable, for example a discussion of winter climate on lake productivity that first describes Lake Constance in Germany and the rarity of ice cover on that lake, followed by a description of the effects of ice and snow cover on phytoplankton, ending with a description of how winter climate (presumably involving little or no ice) did not strongly influence variation in phytoplankton in Lake Constance. Additionally, this paper contains several apparent mistakes, such as the misuse of the word “successional”, “i.e.” when “e.g.” is probably implied, and twice referring to the United States of America as “the US”. This paper is labelled “Opinion”; perhaps Hydrobiologia’s peer-review policies for opinion pieces are not very stringent.
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