Species densities of macrobenthos associated with seagrass: A field experimental study of prédation

Abstract

Caging experiments in a wide variety of marine environments over a period of almost fifty years have produced dramatic density increases of macrobenthos in the absence of predators. This study utilized predator exclusion cages in seagrass areas of a subtropical estuary in east central Florida to determine if predicted increases in density would result. Cages were erected at three sites in the Indian River estuary. At each of the three sites, four replicate box cores were taken, inside and outside the cages, at four sampling times over a period of five months. The 33 numerically most abundant taxa were selected for testing of differences in species densities by two-way multivariate analyses of variance with interaction. The analyses showed that the effect of caging differed at the three sites. At two of the sites, species densities increased inside the cages, consistent with our expectations, while at the third site (St. Lucie) densities of species decreased. The presence of carnivorous decapod crustaceans inside cages in larger numbers at the St. Lucie site is advanced as an explanation for the un- predicted decrease in species densities at that location. The use of caging in this experiment separated the predatory impact of fishes from that of the decapods, further clarifying the structure of the food web in this seagrass habitat.