Proximity to reef influences density of small predatory fishes, while type of seagrass influences intensity of their predation on crabs
Computing, Health and Science
The magnitude of the influence that predators exert on their prey is partly determined by factors that influence the density and behaviour of predators, and partly by factors that influence the survival of prey in the presence of predators. We tested whether the presence of reefs influences the density of small predatory fishes inhabiting seagrass meadows, and the intensity of predation by these fishes on tethered crabs, by taking measurements at increasing distances from 10 rocky reefs at 2 locations separated by several hundred km. We also tested whether these variables were influenced by type of seagrass by comparing patterns yielded in meadows dominated by Amphibolis spp. and Posidonia sinuosa, 2 seagrasses with different morphological and architectural characteristics. The densities of small predatory fishes declined rapidly with increasing distance from reefs, with significantly lower densities 30 m from reefs than immediately adjacent to reefs. Densities >300 m from reefs were generally not significantly different to densities 30 m from reefs. However, the survival of tethered crabs showed no consistent trend associated with distance from reefs. Type of seagrass was not a significant influence on the density of small predatory fishes, but did influence the survival of tethered crabs: those in Amphibolis spp. meadows were 3.1× more likely to be eaten than those in P. sinuosa meadows. These patterns were consistent between locations. Our results suggest that over landscape scales (hundreds of metres) reefs are an important influence on abundance of small predatory fishes, but not on the intensity of predation on crabs. Differences in the intensity of predation between the 2 seagrass species are likely due to their dissimilar morphological and architectural characteristics.