Analyzing the functional significance of biodiversity is a difficult task because there are no widely accepted schemes for classifying functional groups, and no single classification can aggregate organisms appropriately for more than one major ecosystem process. In our analysis of tropical forests we use two major ecosystem processes - energy flow and materials cycling - as the primary basis for establishing junctional groups. We analyze these processes by examining interfaces at which most of the energy or materials are exchanged. At each of these interfaces there is a discontinuity of resource availability that is used by groups of species as an energy or nutrient source (Table 9.1).
Because energy is consumed and not recycled, flows along most energy pathways are unidirectional, and much energy is lost as heat at each transfer. Flow of materials at interfaces is typically bidirectional, but transfer rates are not necessarily equal. Indeed, because changes in relative rates of transfer of materials at interfaces may trigger major changes in ecosystem functioning, species that influence transfer rates are likely to be keystone species.
We emphasize that the amount of energy flow or materia! transfer at different interfaces may be a poor indicator of the significance of an interface for the ecosystem processes that are affected. For example, the transport of a small amount of energy and material by a pollinator may catalyze large investments by plants in fruit and seed production, with subsequent effects on population sizes and dynamics of frugivores, and, on longer time frames, on recruitment of plants (Terborgh 1986b). Similarly, the quantities of nutrients transferred may be an inadequate measure of the importance of an interface for ecosystem processes. Mobilization or immobilization of modest amounts of nutrients may also trigger large responses on the parts of organisms, with cascading effects in the ecosystem.
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