Community Responses to Variable Predation: Field Studies with Sunfish and Freshwater Macroinvertebrates

I studied the impact of variable predation by bluegill sunfish on macroinvertebrate prey in a North Florida lake. Underwater time-lapse cinematography and censuses of bluegill abundances in shallow, middepth, and deep habitats permitted estimation of predation intensity and variability within and among lake habitats. I then incorporated predation rates typical of the middepth zone in caging experiments where predation fluctuated in one treatment and remained constant in another. Prey community structure was subsequently monitored for 1 yr under variable, constant, ambient, and no predation regimes. Patchy, temporally variable predation characterized middepth and deep lake habitats, whereas in the shallow zone predation was relatively constant and homogeneous. Predation varied significantly every 2-4 wk in the middepth zone, but varied little between consecutive weeks or days. Caging experiments revealed that variable predation altered prey community composition and increased the mean size and size range of some prey (e.g., Odonata) as compared to the constant predation treatment. Prey abundances also appeared more heterogeneous among cages (patches) and varied more temporally under a variable predation regime. However, total prey abundance, species abundance, and within-patch spatial heterogeneity did not differ among predator treatments. In general, the macroinvertebrate community exposed to variable predation more closely approximated the natural middepth zone community than that from the constant predation regime. Previous studies of fish predation on macroinvertebrate communities have concentrated on shallow littoral habitats, but these results suggest that conclusions drawn from shallow habitats may not be representative of all lake zones. Variable predation may occur in many systems, and may contribute substantially to the spatial heterogeneity, temporal inconstancy, and species composition of prey communities.