Insect Herbivores on Bracken Do Not Support the Core-Satellite Hypothesis

Abstract

The core-satellite hypothesis predicts bimodality in the regional distribution of a group of taxonomically closely related species. Previous attempts to test the hypothesis have failed to demonstrate either that its assumptions are adequately met or that there is agreement with other predictions of the hypothesis. Here, we test data on the insects feeding on bracken (Pteridium aquilinum) for agreement with the model''s assumptions and with its predictions. Agreement with most of the key assumptions appears reasonably good. However, the long-term stability of species populations is at variance with the behavior of the model, and hence, there is no consistent evidence for bimodality in the frequency of site occupancy by bracken herbivores or for further predictions of the hypothesis. In particular, population data from two well-studied sites provide no evidence of core-satellite switching, in which rare species become common and vice versa. For bracken herbivores at least, the probabilities of colonization and extinction are clearly not entirely deterministic; nor do they appear sufficiently stochastic to conform to the requirements of the model. The insect herbivores of bracken do not support the core-satellite hypothesis of community organization; nor is there much good evidence to suggest that the hypothesis provides a general explanation for the structure of other animal communities.