TOWARDS AN EMPIRICALLY-BASED THEORY OF HERBIVORE DEMOGRAPHY

Previous studies have synthesized life-table data from herbivore speci es to identify general trends in the demography of herbivorous insects . Frequency-based analyses were used to ascertain which of five mortal ity sources (enemies, plant factors, competition, weather, intrinsic d evelopmental failure) and which of five ecological characteristics of herbivores (feeding biology, invasion status of the herbivore, latitud e, cultivation, and successional status of the habitat) had important influences on mortality patterns. Here these results are reinforced wi th a quantitative analysis that relies on actual numbers of herbivores killed at different developmental stages by each of the five mortalit y sources in different ecological settings. We also examine the relati onship between taxonomic category (Coleoptera, Diptera, Lepidoptera, a nd Hymenoptera) and mortality. The analysis identified developmental c hanges of herbivores as having an important influence on sources of mo rtality; feeding biology, latitude, and cultivation status also influe nced the distribution of mortality sources. Other aspects of the herbi vores' ecology and taxonomy had limited effects. Natural enemies were identified as the most important mortality source overall, and their i mportance increased from the early larval stages to the pupal stages. They also kill more exophytic insects than endophytic insects, and kil l a higher proportion of insects in cultivated habitats than in natura l habitats.Weather kills more temperate-zone immatures than tropical/s ubtropical immatures. The results of the quantitative analysis general ly confirm the earlier frequency-based tests. Several predictions that can serve as the foundation of an empirically-based theory of herbivo re demography are offered: (1) natural enemies are the dominant cause of mortality in exophytic herbivore populations and may compete more i ntensely than on endophytics; (2) plant factors and enemies play a mor e balanced role in endophytic populations; (3) exophytic species shoul d be particularly susceptible to top-down effects, especially in agroe cosystems; (4) plant defences will often have sublethal effects, but w hen 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 he rbivore demography.