Nonlinear effects of an aquatic consumer: causes and consequences

The assumption that per capita consumer effects on prey density are independent of consumer and prey density is examined with a large-scale manipulation of an aquatic herbivore (Daphnia). A gradient of consumer removal was maintained long enough to allow the abundances of both consumer and prey (phytoplankton) to equilibrate to the manipulation. Strong and unequivocal nonlinearities were found in the effect of Daphnia on total phytoplankton abundance and the abundance of most of the common phytoplankton species. Daphnia's suppression of phytoplankton was strong between 0 and approximately 400 microg Daphnia L(-1) but essentially nil from approximately 400 to 900 microg Daphnia L(-1). The sharp deceleration in Daphnia's effect was not caused by a shift within the phytoplankton community toward consumption-resistant forms. The most likely explanation for the deceleration was a reduction in Daphnia's filtering effort at low phytoplankton abundance, that is, a Type III functional response. A review of experimental literature suggested that decelerating effects of consumers are the norm in aquatic systems. Nonlinear effects present problems for the estimation of interaction strength and the building of community interaction models from the results of predator manipulations. It is suggested that the role of field experiments in community ecology should be to test rather than to parameterize models.