Invertebrate predation and lotic prey communities: Evaluation of in situ enclosure/exclosure experiments

Abstract

The influence of the stonefly Kogotus nonus on prey communities was assessed by varying the predator density within small enclosures in a first-order stream in southwestern Alberta. Experimental containers with natural densities of prey, standardized substrate and detritus, and zero, one, two, or three Kogotus were buried in a rifflefor 10 days-during July 1981, June 1982, and July 1982. The extent to which prey densities were depressed was found to be dependent on predator density within the enclosures: increased predator densities resulted in lower prey densities. An experimental design in which containers that excluded predators were compared with controls accessible to predators was used to determine if results would be similar to results from the enclosure experiments. It was concluded that observation of predator effects in field manipulative experiments may be strongly dependent on the selection of an experimental design in which predator effects are not confounded with container effects.