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Towards an empirically-based theory of herbivore demography

Cornell, H.V., Hawkins, B.A., Hochberg, M.E.
Ecological entomology 1998 v.23 no.3 pp. 340-349
insects, herbivores, demography, mortality, population ecology, developmental stages, habitats, life tables
Previous studies have synthesized life-table data from herbivore species to identify general trends in the demography of herbivorous insects. Frequency-based analyses were used to ascertain which of five mortality sources (enemies, plant factors, competition, weather, intrinsic developmental failure) and which of five ecological characteristics of herbivores (feeding biology, invasion status of the herbivore, latitude, cultivation, and successional status of the habitat) had important influences on mortality patterns. Here these results are reinforced with a quantitative analysis that relies on actual numbers of herbivores killed at different developmental stages by each of the five mortality sources in different ecological settings. We also examine the relationship between taxonomic category (Coleoptera, Diptera, Lepidoptera, and Hymenoptera) and mortality. The analysis identified developmental changes of herbivores as having an important influence on sources of mortality; feeding biology, latitude, and cultivation status also influenced the distribution of mortality sources. Other aspects of the herbivores' ecology and taxonomy had limited effects. Natural enemies were identified as the most important mortality source overall, and their importance increased from the early larval stages to the pupal stages. They also kill more exophytic insects than endophytic insects, and kill a higher proportion of insects in cultivated habitats than in natural habitats. Weather kills more temperate-zone immatures than tropical/subtropical immatures. The results of the quantitative analysis generally confirm the earlier frequency-based tests. Several predictions that can serve as the foundation of an empirically-based theory of herbivore demography are offered: (1) natural enemies are the dominant cause of mortality in exophytic herbivore populations and may compete more intensely than on endophytics; (2) plant factors and enemies play a more balanced role in endophytic populations; (3) exophytic species should be particularly susceptible to top-down effects, especially in agroecosystems; (4) plant defences will often have sublethal effects, but when they are lethal they will be most important as the hatchling larva is just getting established on the plant. These predictions should be viewed as a challenge to engage in a broader way of thinking about herbivore demography.