The number of species in all ecological communities, especially tropical ones, greatly exceeds the number of key ecological processes. We refer to the species that participate in a particular process as a functional group (Vitousek and Hooper 1993). Functional groups are inevitably fuzzy assemblages, but they constitute a useful operational basis for identifying groups of species with potentially similar effects on ecosystem-level processes. If the loss of a species results in a large effect on some functional property of the ecosystem, that species may be called a keystone species (Gilbert 1980; Bond 1993).
Traditionally, ecologists have looked for and identified keystone species by their effects on the species richness and composition of the community in which they live. Here we explore keystone taxa that have major consequences for ecosystem processes, such as primary and secondary production and nutrient cycling. In this context, a keystone species may or may not significantly change the species composition of its community. The existence of keystone species shows that not all members of a functional group are of equal significance for the process in which the group participates.
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