Effect of habitat complexity on predation success: re-evaluating the current paradigm in seagrass beds

The complex structure of seagrass beds provides increased protection for prey compared to unvegetated substrates. Additionally, most studies support the notion that predation intensity is negatively correlated with seagrass density. However, virtually every study to previously test this idea has used the same number of predators and prey across a gradient of seagrass density treatments. This design ignores another well-established fact: invertebrate and fish abundance increase with increasing seagrass density. Therefore, it makes sense to carry out predation experiments that increase the number of both predators and prey as seagrass density increases. Outdoor tank experiments (7 density treatments: 0 to 3000 leaves m -2 ) were carried out using artificial seagrass units modeled after turtlegrass Thalassia testudinum and field mesocosm experiments (4 density treatments: 0 to 3000 leaves m -2 ) in living T. testudinum in Big Lagoon (Florida, USA). In these experiments the absolute number of predators (the pinfish Lagodon rhomboides) and prey (grass shrimp Palaemonetes pugio) were increased with increasing seagrass density. Both experiments yielded similar results: the only significant difference in predation rate was between the unvegetated treatment and the seagrass treatments as a group. These results suggest that dense seagrass does not necessarily provide increased protection for prey organisms.