An experimental approach to the effects of predictability on community structure

This paper considers the effects of different levels of resource predictability on niche widths, competition and diversity in a community of three hummingbird species. Three pairs of hypotheses are tested: whether decreasing predictability (a) increases or decreases niche width, (b) increases or decreases competition and (c) increases or decreases diversity. The results show that complementary increases and decreases in niche width occur with decreasing predictability, while competition and diversity decrease, at least with extreme unpredictability. A model, which assumes the species to have similar resource preferences, and its predictions are examined.The dominant species, Lampornis clemenciae, excludes a subordinate species, Archilochus alexandri, from preferred resources. This defense becomes unprofitable with decreasing predictability and Archilochus invades the resources once vigorously defended by Lampornis. When the spectrum of resources is altered in the direction of decreasing suitability for both species, Lampornis becomes even more specialized and sensitive to the effects of unpredictability. A third species, Eugenes fulgens, steals resources undefended by Lampornis. These characteristics seem to be common to a number of communities in very different taxonomic groups, and characteristic of communities in which species share a common preferred resource. The question of how a community organized with distinct resource preferences responds to decreasing predictability remains open. © 1978 American Society of Zoologists.

Duke Scholars

Author Stuart L. Pimm Environmental Sciences and Policy