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Species - abundance and body size relationships in insect herbivores associated with New Guinea forest trees, with particular reference to insect host-specificity

Yves Basset

Abstract

Chewing insects feeding externally on ten species of forest trees were collected by hand collecting, beating, branch clipping, intercept flight traps and pyrethrum knockdown during a one year period at Wau, Papua New Guinea. Insect host specificity was assessed by feeding trials in captivity. The analyses considered 6,130 individuals representing 704 species of adult Coleoptera and larvae of Lepidoptera. The proportion of rare species (defined as being represented by only a single individual in the collections) was high but significantly lower for specialists than for generalists. As a result, the proportion of rare species was significantly lower in the compound community (i.e., the insect community supported by the ten tree species considered together) than in the component communities (i.e., the insect communities supported by each tree species). In other terms, increasing sampling effort to different habitats (tree species) resulted in better estimates of overall insect abundance, because it increased the probability of discovering the "true" abundance of some species, these often being generalists. Species abundance distributions within the component communities were analyzed by fitting regressions through plots of species biomass (i.e., number of individuals x average body weight) against species rank. The number of insect species within the community was related to an estimate of the number of young leaves available year-long on each tree species. The maximum insect biomass which could be achieved on each tree species was also related to an estimate of leaf expansion rates. However, the slopes of the regressions, as well as average insect body size within the community, could not be predicted by either these or other host-tree attributes. Insect biomass, and, presumably, food resources, tended to be more evenly distributed in species-rich than in species-poor communities. This suggests that host attributes influence to some extent the structure of these chewing insect communities but that the way that food resources are shared within these communities may have more to do with intrinsic community characteristics (e.g., vagility of constituent species; resistance to invasion, competition, etc.) or historical factors.