Danks 1993

Danks HV. 1993. Patterns of diversity in the Canadian insect fauna. Memoirs of the Entomological Society of Canada 165: 51-74.

This author reviews primarily occurrence data for the insects and a few other arthropods of Canada, with a focus on patterns associated with continent-scale variation, especially climate. There are a few deficiencies in the available dataset, including the absence of full taxonomic treatments for many groups, the inability of collections to distinguish between residents and visitors in regions, and varying levels of collecting effort across regions.

The author analyses geographic patterns of reported species in association with estimates and methods of estimating undescribed species abundance. Collection effort in northern Canada is considered probably adequate, despite the logistical difficulties, primarily due to the Northern Insect Survey (reviewed by Danks, 1981). Overall, Canada’s insect fauna is well enough described to permit analysis of regional variation and comparisons between regions.

At the broadest latitudinal and taxonomic scales, some insect orders are overrepresented in the North, while others are underrepresented. In particular, diversity of Orthoptera, Hemiptera, and Coleoptera are reduced in the North, and diversity of Diptera is increased. There are probably more species of Diptera and of Hymenoptera in Canada than species of Coleoptera. These observations lead to questions about why some groups, at multiple taxonomic levels, are overrepresented in the Arctic.

Biogeographic zones in this paper are based on vegetation types. In the arctic, this includes the presence or absence of prostrate shrubs; High Arctic islands such as Bathurst lack these shrubs. While the majority of insect species show fairly consistent boundaries of their geographic ranges, there are many exceptions, and the lines drawn on a map (e.g. Fig. 2) should not be considered as hard and precise as they may appear. Many factors, both biotic and abiotic, change gradually across the landscape, and the occurrence or absence of a species may be a reflection of where its individual requirements are or are not met; these individual requirements probably vary considerably between species, thus absent sharp boundaries (e.g. ocean) many species distributions probably do not overlap completely.

Some Arctic species of insects do not occur on the mainland, suggesting a role for theories of Island Biogeography in structuring the biotas of Canada’s Arctic archipelago, even against the very strong pattern of decreasing diversity with latitude. Diversity is generally higher in the West of Canada; this is especially true north of 60°N. Local diversity may be strongly driven by small patches of favourable habitat, especially in the polar deserts, where these patches may be represented by rare clumps of vegetation surrounded by bare sediment. Most Arctic and Boreal insect species appear to respond to habitat variation individually, not as communities.

The biota of Canada is strongly influenced by the recent history of Pleistocene glaciations. The retreat of the glaciers was not a simple northward progression (Matthews, 1979). The expansion of populations out of glacial refugia varied, particularly with latitude. While Beringian forms were able to expand Eastward (partly driven by prevailing winds), southern populations appear to have been extirpated as local conditions warmed but the areal extent of the ice sheets did not decrease immediately. Much of the evidence for this pattern comes from fossils and sub-fossils of Pleistocene beetles (Schwert and Ashworth, 1988).

The concept of “distributional equilibrium” was introduced by Clarke (1973) in describing and analysing patterns of freshwater mollusc distributions in the Canadian Interior Basin. In that study, approximately 26% of species had achieved full realization of their estimated geographic range. This author suggests that insufficient data are available for a similar analysis of insects, though there are some interesting patterns in insect diversity.

Northern ecosystems may be incomplete and relatively less integrated compared to more southerly systems. This author defines an incomplete ecosystem as one where “…litter accumulates because it is too cold for complete decomposition, where many plants are not seriously attacked by insect herbivores because herbivores are rare, where many sites have not yet been colonized following disturbance (of various scales and frequencies), where large segments of the year and even of the day are unsuitable for activity, and so on.” (Danks, 1993, pg 71). There is apparently insufficient evidence available to state that any particular region qualifies as “incomplete” under these loose criteria, but several studies are suggestive in this direction. This author does not discuss the possibility of ecological release for (some) Arctic vascular plants, though tables 9 and 10 suggest that Arctic herbivorous insects may be under twin constraints. They seem to be more strongly impacted by climate than their plant hosts (table 9), and under relatively strong pressure from parasitoid hymenoptera (table 10).

There is also an indirect suggestion in this paper in favour of the “exclusion” hypothesis of my PhD thesis: overrepresented groups of insects in the Arctic appear to be exapted for Arctic life by family- or subfamily-level traits. For example, butterflies are relatively more diverse in the Arctic than in temperate zones (esp. superfamily Papilionoidea), and have day-active adults and desiccation resistant large-bodied larvae. Both of these traits are especially valuable in the Arctic because of the absence of suitable night-time conditions (cool nights or 24-hour days), and the large areas of dry polar desert.

The author concludes with recommendations that future surveys of the fauna of Canada focus on example regions such that the hypothesis of incomplete ecosystems may be tested, and that the relative roles of biotic vs. abiotic factors in structuring biodiversity may be examined.