The effects of pumpkinseed predation on Physella grazing and the resulting algal communities.

Abstract

The snail genus of Physella is a known periphyton grazer. These snails often graze depending upon the abundance of a predator or the predation pressure they experience in a habitat (Turner, 1996). These snails are known to move to safer and more covered areas when in the presence of predators (Turner, 1997). It has been observed that snails will react to the presence of predators by detecting the presence of crushed shells in the water and interpreting it as increased predation. This has been shown to influence the periphyton community composition (Turner, 1997). The reduced grazing due to the induced reaction of the snails increases with predation to a point where the snails need for food outweighs this reaction. Some people think of fish as being the controlling factor in the algal community by limiting the snail populations and therefore controlling grazing (Lodge, et al., 1994). Snails with larger body size are less vulnerable to predation due to the thicker shell and therefore do not react as readily to an increase in predation pressure (Turner, 1997). Studies have been done which support the theory that the snails do not react to the presence of fish but rather the presence of crushed snails in the water (Turner, 1996). If this is ture one would get the same results using fish feeding on snails as they would using manually crushed snails being put directly into the tanks. I hypothesize that tanks containing snails and a fish cue will produce a larger and more abundant periphyton community than a tank with no fish cue. The fish cue will act as a natural inhibitor to snail feeding on certain surfaces that are not in cover and will therefore produce a more abundant community than surfaces that are grazed freely.