An Experimental Approach to the Effects of Predictability on Community Structure

This paper considers the effects of different levels of resourcepredictability on niche widths, competition and diversity ina community of three hummingbird species. Three pairs of hypothesesare tested: whether decreasing predictability (a) increasesor decreases niche width, (b) increases or decreases competitionand (c) increases or decreases diversity. The results show thatcomplementary increases and decreases in niche width occur withdecreasing predictability, while competition and diversity decrease,at least with extreme unpredictability. A model, which assumesthe species to have similar resource preferences, and its predictionsare examined. The dominant species, Lampornis clemenciae, excludes a subordinatespecies, Archilochus alexandri, from preferred resources. Thisdefense becomes unprofitable with decreasing predictabilityand Archilochus invades the resources once vigorously defendedby Lampornis. When the spectrum of resources is altered in thedirection of decreasing suitability for both species, Lampornisbecomes even more specialized and sensitive to the effects ofunpredictability. A third species, Eugenes fulgens, steals resourcesundefended by Lampornis. These characteristics seem to be commonto a number of communities in very different taxonomic groups,and characteristic of communities in which species share a commonpreferred resource. The question of how a community organizedwith distinct resource preferences responds to decreasing predictabilityremains open.