The boreal zone of Canada extends across the continent over millions of square kilometres. Characterizing the insects of this cool temperate zone is especially important for an understanding of the Canadian fauna in general, and for northern forestry. The boreal zone supports simple coniferous forests with some deciduous trees and an understory of a few common vascular plants, mosses, and lichens. This enormous area of apparently similar vegetation nevertheless shows very great spatial and temporal heterogeneity, with a wide range of subhabitats. Aquatic habitats of many different kinds are especially well represented in the zone. The diversity of terrestrial habitats is maintained chiefly by disturbance, especially fire and seasonal flooding, and contributes greatly to faunal diversity.
About 22 000 insect species are estimated to occur in the zone, far fewer than in more southern zones. Northern taxa, notably Diptera, are relatively well represented. The distributions and patterns of variation of the species are summarized. About half have transcontinental ranges, and many occur also in forested habitats in the western mountains south of the boreal zone. About 8% of the species appear to be holarctic. Few species that occur in the boreal zone are strictly confined to it, however. Generalist species in fresh water and other widespread habitats are conspicuous. Many species are centred farther south, and extend northward into the zone to varying degrees.
Adaptations to northern conditions have been reported in many boreal insects. For example, the short growing season is reflected by the prevalence of univoltine species. Insects survive the long cold winters by cold-hardiness and dormancies. Species from disturbed habitats disperse widely. The limited diversity of resources is confirmed by the fact that the food range of some groups of herbivores is wider than in their southern relatives.
Boreal ecosystem relationships are complex, especially relative to the arctic. Numerous associations among insects, and between insects and other organisms, have been demonstrated. However, some evidence suggests that the structure of northern biotic communities might depend more on the tolerances of individual species than on interactions among the species.
The population dynamics of boreal forest insects, notably the spruce budworm, are discussed. In view of the spatial and temporal complexity of the boreal zone, the diversity of interactions with abiotic and biotic factors, and the prevalence of data that correlate with rather than explain population changes, our current failure to understand “outbreaks” of certain boreal insects is not surprising, because many factors probably combine to determine the population of a given species at a given lime and place.
Information on most aspects of the composition and biology of the boreal fauna is incomplete. Data on boreal species have been collected chiefly in southern transitional ecosystems adjacent to boreal zones, rather than in truly boreal systems. Basic taxonomic information on several important taxa, as well as detailed taxonomic and morphometric information about individual species, is especially scanty. However, substantial and coordinated studies of boreal faunas will yield information of great interest and value. Some approaches relevant to further work in taxonomic and ecological arenas are suggested.