Increased habitat structure does not always provide increased refuge from predation

Abstract

Submerged aquatic vegetation (SAV) usually hosts higher numbers of both inverte- brates and fish than do unvegetated substrates. In addition, based on prior laboratory experiments predation risk is thought to decrease with increasing SAV biomass or stem/shoot density, resulting in higher abundance and diversity of potential prey species in dense vegetation. However, all previous tests of the effects of vegetation on prey capture have been similar: constant numbers of predators and prey have been tested at different vegetation densities. Because sampling has repeatedly shown that the abundance of both predators and prey increases with increasing SAV density, an experiment that tests the effects of increasing SAV density on prey capture would mirror reality more closely than would the design of prior experiments and would also allow predator and prey numbers to increase. Thus, in laboratory trials we increased the number of predators (pinfish Lagodon rhomboides (Linnaeus)) and prey (grass shrimp Palaemonetes pugio Hulthuis) in proportion to increases in SAV shoot density to re-evaluate whether increasing SAV density altered prey survival rates under these more realistic conditions. Treatments included an unvegetated substrate and 3 different densities (400, 2000 and 4000 leaves m -2 of artificial SAV (mimicking turtlegrass Thalassia testudinum Banks and Sol). Our results conflicted with those of prior experiments and showed no significant differences in grass shrimp survival among the different SAV densities (although there was greater grass shrimp survival in SAV than on unvegetated substrate), and indicated that increasing vegetation density will not consistently lead to proportionally greater prey survival rates.