PREDATOR IMPACTS ON STREAM BENTHIC PREY

The impact that predators have on benthic, macroinvertebrate prey dens ity in streams is unclear. While some studies show a strong effect of predators on prey density, others show little or no effect. Two factor s appear to influence the detection of predator impact on prey density in streams. First, many field studies have small sample sizes and thu s might be unable to detect treatment effects. Second, streams contain two broad classes of predators, invertebrates and vertebrates, which might have different impacts on prey density for a variety of reasons, including availability of refuge for prey and prey emigration respons es to the two types of predators. In addition, predatory vertebrates h ave more complex prey communities than predatory invertebrates; this c omplexity might reduce the impact that predatory vertebrates have on p rey because of indirect effects. I conducted a meta-analysis on the re sults of field studies that manipulate predator density in enclosures to determine (1) if predators have a significant impact on benthic pre y density in streams, (2) if the impacts that predatory invertebrates and vertebrates have differ, and (3) if predatory vertebrates have dif ferent impacts on predatory prey versus herbivorous prey. The results of the meta-analysis suggest that on average predators have a signific ant negative effect on prey density, predatory invertebrates have a si gnificantly stronger impact than predatory vertebrates, and predatory vertebrates do not differ in their impact on predatory versus herbivor ous invertebrate prey. Three methodological variables (mesh size of en closures, size of enclosures, and experimental duration) were examined to determine if cross correlations exist that may explain the differe nces in impact between predatory invertebrates and vertebrates. No cor relation exists between mesh size and predator impact. Over all predat ors, no correlation exists between experimental duration and predator impact; however, within predatory invertebrates a correlation does exi st between these variables. Also, a correlation was found between encl osure size and predator impact. This correlation potentially explains the difference in impact between predatory invertebrates and predatory vertebrates. Results of the meta-analysis suggest two important areas for future research: (1) manipulate both types of predators within th e same system, and (2) examine their impacts on the same spatial scale .