Species Compensation and Complementarity in Ecosystem Function

Abstract

Functional complementarity occurs when ecosystem processes are maintained at constant levels despite stresses that induce shifts in the populations driving those processes. Understanding when such complementarity occurs depends on an integration of perspectives from population and ecosystem ecology. Here, we examine the extent to which functional complementarity is linked with compensatory dynamics among species that carry out a particular ecosystem function. Our approach combines analyses of long-term zooplankton data from a whole-lake acidification experiment with a theoretical treatment of species compensation. Results from the acidification of Little Rock Lake, WI indicated that the biomass of cladocerans, copepods, rotifers, and total zoo-plankton remained at high levels despite the loss of component species from each group. Compensatory increases by other taxa were responsible for this complementarity of function. Theoretical considerations indicated that the degree of compensation occurring among species in response to environmental change increased in response to two different factors: the functional similarity of interacting species and the degree to which an environmental change acts nonuniformly on their interactions. Finally, we tested the extent to which compensation in unperturbed systems could predict functional complementarity. We analyzed a 7-year record from the reference basin of Little Rock Lake and found that substantial compensation occurred only in the natural dynamics of rotifers and cladocerans during some seasons. Complementarity in response to acidification was evident, however, among copepods and total zooplankton in addition to rotifers and cladocerans, and could not have been predicted solely on the basis of compensation prior to stress. Taken together, our analyses reveal how functional complementarity is linked to a variety of compensatory interactions among species responding to stress.