Stream flow and predation effects on the spatial dynamics of benthic invertebrates

Field experiments were carried out to determine the influence of predation and prey movements on the accumulation of prey in enclosures. Experimental enclosures permitted exchange of prey with the benthos, but not of the large, predatory larvae of the caddisfly, Plectrocnemia conspersa (Curtis). Unseasonally heavy rainfalls during the experiment resulted in high flows and enabled us to examine the effects of a major, abiotic disturbance on invertebrate spatial dynamics. Prey colonization rates of cages without predators were determined in nine 24 h periods. Colonization rates increased exponentially with flow and were species-specific, depending on dispersal behaviour. Prey accumulation and predator impacts were measured in cages, with and without P. conspersa larvae, placed in the stream for 1, 2 or 3 weeks. Prey densities in cages increased with exposure time, but increases were not gradual and depended on flow regime. Flow was reduced within cages and they accumulated large numbers of invertebrates during high discharge. Analogous, naturally occurring refugia in the stream channel could be important for the recovery of lotic communities after major disturbances. Overall, prey densities were lowest in cages with predators. For fast colonizers, predation effects were detectable early in the experiment, but quickly obscured thereafter by continuous exchange of prey. For slow colonists, predation effects were detectable later, but persisted longer. Consumption rates for P. conspersa varied with prey density and flow regime. We suggest that the spatial dynamics of benthic invertebrates, especially as they are influenced by stochastic events, are important in understanding and detecting predation effects in stream communities.