This is a very long review paper, covering all aspects of cold aquatic habitats and the adaptations of the insects living in them. For the purposes of this review, “cold” means air temperatures below 0°C and most aquatic habitats are frozen for “at least several months”. This therefore applies to most of Canada and adjacent areas of the United States, as well as northern Europe and Russia, Mongolia, and northern China, as well as alpine areas throughout the world. The sub-antarctic islands, perhaps surprisingly, are not particularly cold, having low annual temperature variability (Danks, 1999).
Cold aquatic environments are a very heterogeneous group. Many physical factors contribute to the abiotic environment experienced by aquatic insects, including latitude, depth, flow, insolation/shading, and even the direction of flow of rivers. Tundra streams may be warmer than boreal streams because of longer days in summer and the lack of shading trees. Deep lakes may remain frozen all year at high-Arctic locations while adjacent shallow ponds achieve summer-long temperatures of 20°C; conversely, deep lakes do not freeze solid the way shallow ponds do. Steep-sided ponds warm more slowly than those with gently sloping shores. Rivers flowing towards the pole experience more disruptive ice break-up in spring than those that flow towards the equator; this is based on phenomena of ice dams and rapid melting occuring in high headwaters.
The biotic environment varies with cold conditions as well. Overall diversity is reduced, and faunal compositions are changed. For example, the Arctic fauna is dominated by Diptera, especially Chironomidae (which may have their center-of-origin in the Arctic). Most saprophagous and detritivorous insects are less specialized than their temperate counterparts, subsisting on a wider range of particle sizes and chemical compositions. Low temperatures strongly reduce microbial and fungal activity, allowing macroinvertebrates to fill the role of basic decomposers. Biting flies may be obligately or facultatively autogenous, meaning they do not require (or cannot obtain) a blood meal for maturing eggs. This is particularly surprising given the strong abiotic-conditions-driven need to complete the life cycle in a very short, often cool summer, but is based on the general absence of vertebrate hosts.
Cold climates are characterized by seasonality, with severe, variable, and unpredictable conditions that may lie close to the abiotic limits for many species. Some entire years may pass without conditions rising to a quality sufficient for many species to conduct normal life-history activities, especially spring emergence from water (persistent ice cover) and adult flying and aerial mating (low air temperatures). Aquatic insects cope with this severity and variability in a number of ways, including voltinism, timing of active and dormant stages, and controls on the emergence of adults and their reproductive behaviours. Rapid development is especially important under conditions of short, cool summers, strongly suggesting that animal genomes may be smaller at high latitudes, not larger as in plants.
Three sentences on page 449 are reminiscent of a group selection argument: “Although species from many zones show patterns of variation that seem designed to cope with unpredictability and variability (Danks 1983), alpine and northern aquatic insects provide some particularly clear examples. On shorter time frames, many species have staggered development that prevents the whole population from being synchronized in a vulnerable stage. The dormant eggs of many species are resistant to adverse temperatures and, unlike the larvae that hatch from them, do not depend upon the availability of food; staggered hatch of such eggs is relatively common in cold and variable habitats, though not confined to them (e.g., Zwick 1996).” I need to read the cited references in that section to determine if Danks is really trying to make a good-of-the-species type of argument.
Winter survival usually depends on adaptations for diapause or resistant life cycle stages. Dehydration and the production of antifreeze, anti-nucleation, and protective chemicals are common among cold-living insects. More is known about terrestrial adaptations, which usually involve a general avoidance of water to help avoid the damaging effects of ice. Aquatic insects are typically surrounded by large amounts of water, which renders this tactic less effective. As mentioned above, large, deep lakes do not freeze to the bottom, though oxygen levels may be severely depleted leading to winter kills. Some insects, including dytiscid beetles, move onto the land in fall to overwinter in the soil or vegetation litter. This may help them to avoid the injuries caused by mechanical expansion of ice, by sheltering them from the injuring forces. Flowing water freezes in complex ways, including under some circumstances the formation of near-0°C “anchor ice” that can seal off the benthos from further disturbance and is not itself cold enough to freeze intracellular water.
In spring, warming temperatures (and daily cycles of temperatures) may reconfigure intercellular ice crystals, causing injury. Externally, ice from break-up can scour river bottoms, cause ice-dams and widespread flooding, and cause other disturbances in habitats. Early spring emergence is critical for many species to compensate for short, cool summers.
Summer activities such as larval feeding and adult mating are strongly temperature dependent. Thermodynamically, small body size may be favoured because smaller animals require less heat to achieve operating temperatures. This also argues in favour of smaller genomes at high latitudes, though less strongly so than the developmental constraints.
There are several adaptations of aquatic insects in cold climates that suggest the faunal composition of cold climates is driven largely by selection for particular combinations of physiology, habitat, and habits that are present in a few key groups of aquatic insects. Diptera have already been mentioned, others include Dytiscidae, and the Stoneflies, Caddisflies, and Mayflies, especially the stonefly family Nemouridae, which includes winter-active adults in Alaskan streams (Oswood 1989). Alpine taxa at least do not appear to be highly isolated, suggesting strong dispersal and gene-flow capabilities and further suggesting that adaptations of cold-climate insects may be useful for more than coping with low temperatures.
The general trends of cold aquatic climates are
1. A great diversity of available habitats with different conditions, this drives a wide diversity of faunal compositions, though some common patterns (Diptera) emerge.
2. Aquatic and terrestrial habitats are closely linked, both abiotically (temperatures grade together near shore, and aquatic habitats expand and contract) and biotically (terrestrial predators of aquatics, aquatics movement to terrestrial after emergence).
3. Most cold aquatic systems are characterized by low overall productivity, yet have high ecological complexity.
In my on-harddrive version of this annotated bibliography entry, I copied in the five tables included in this paper that summarize responses of insects to food limitation, to winter conditions, to spring conditions, and to summer conditions in cold climates. I think posting those tables here may constitute copyright violation.
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