Boreal forests support an array of insects, including phytophagous (plant-eating) insects, saprophagous (detritus-eating) insects, and their associated parasites, predators, and symbionts. The phytophagous species include folivorous leaf chewers and miners, phloeophagous cambial and sapwood borers, stem gallers, and root feeders. Biological diversity and distribution of insect species exhibit predictable patterns among vegetation types (Werner 1994a). In this chapter, we discuss how phytophagous species of insects differ among plant communities and how various populations of insects react to disturbances that alter forest stand composition and density. The distribution of insects differs among plant communities depending on the ecosystem type and plant height (Table 9.1; Werner 1983, 1994a). Grasses, mosses, small tree seedlings, and other herbaceous plants located on the forest floor have the highest arthropod densities. Shrubs have the lowest densities, and trees are intermediate. The herbaceous layer is inhabited primarily by scavengers, predators, and saprophages but has few defoliators (Werner 1983). Taller shrubs contain more species of phytophagous insects than do herbs, but trees have the most species of phytophagous insects, parasites, and predators (Werner 1981, 1983). Few saprophages and scavengers (carabid beetles), however, occur on shrubs and trees (Werner 1986a). Associations of plants and phytophagous insects in boreal ecosystems are similar to temperate assemblages in that insect species differ in the range of food plants that they utilize (Bernays and Minkenberg 1997, Futuyma et al. 1993, Thorsteinson 1960). Because of low plant diversity, however, many boreal phytophagous insects feed on several species of plants (Werner 1981). For example, the spear-marked black moth, Rheumaptera hastata (L.), feeds primarily on paper birch, but during periods of high populations it also feeds on alder, willow, and rose species but not on poplar (Werner 1977, 1979). When population outbreaks of phytophagous insects deplete their preferred host plants, less desirable species are sometimes consumed or starvation occurs (Werner 1981, 1986a). The biomass of phytophagous insects is greater on broad-leafed than on conifer trees (Werner 1983). Species of Coleoptera, Hemiptera, Homoptera, Hymenoptera, and Lepidoptera are common on broad-leafed trees, whereas only a few taxa of Homoptera, Hymenopera, and Lepidoptera are associated with conifers such as spruce or larch (Table 9.2; Werner 1983, 1994a).
Aerotechnogenic Pollution of Boreal Forests in Northern Europe
