I experimentally evaluated the impact of habitat structural complexity on the interactions between a generalist predator and a benthic macroinvertebrate prey assemblage in a freshwater pond. Benthivorous perch (Perca fluviatilis) were stocked over a range of natural densities (no fish, low, and high densities) into enclosures with or without dense submerged vegetation. The biomass of macroinvertebrate prey decreased over time in the presence of perch and was always higher in enclosures with vegetation present than in enclosures lacking vegetation. The increase in mass of perch was positively related to the abundance of macroinvertebrate prey and negatively related to perch density and the density of vegetation. In the treatments lacking vegetation, the proportion of zooplankton in the diet of perch increased, and the growth rate of perch decreased over time. In the vegetation treatments, the proportion of zooplankton in the diet was low throughout the experiment and the growth rate of perch was constant over time. As a consequence, initial increase in mass was considerably higher in the treatments lacking vegetation than in the vegetation treatments, whereas no such pattern was observed in the second half of the experiment. In the absence of vegetation, perch are apparently able to forage efficiently, but this may reduce the availability of macroinvertebrate prey to the extent that perch are forced to include less profitable zooplankton prey into their diet. In vegetated habitats, the foraging efficiency of perch is reduced, which possibly prevents over-exploitation of macroinvertebrate prey and consequently may allow for a moderate, but relatively constant, consumption of macroinvertebrates by perch. The density-dependence of growth rates in both vegetated and unvegetated habitats can only partly be explained by resource competition, which suggests the presence of an additional mechanism of density-dependence. In natural lake communities, efficient predation from benthivorous fish should keep the biomass of macroinvertebrate prey in structurally simple habitats below the high levels initially present in my experiment. In these communities, submerged vegetation may be an equally profitable habitat for juvenile perch as are open areas. Through its effects on the feeding efficiencies of juvenile perch and other benthivorous fish, submerged vegetation may affect individual growth rates and the size structure of perch populations, which may contribute to explain differences in fish community structure among lakes differing in submerged vegetation cover.
1993. Vol. 67, no 3, 403-414 p.