The influence of seagrass structure on the distribution and abundance of mobile epifauna: pattern and process in a Western Australian Amphibolis bed

Abstract

Patches within a mixed bed of the seagrasses Amphibolis antarctica and Amphibolis griffithii were manipulated in three ways: by the removal of epiphytes, by the removal of leaves, and by altering seagrass density. 1 month after the manipulation, large differences remained between the faunas inhabiting the variously treated patches. Leaf removal and epiphyte removal decreased total abundances by 64 and 31%, respectively, with the majority of the common species being negatively affected by one or the other of these treatments but not both. Three species significantly increased in abundance in the leaf removal treatment and were not affected by the removal of epiphytes. The fauna inhabiting Amphibolis beds therefore can be subdivided into relatively independent leaf-associated and epiphyte-associated assemblages, with a small additional assemblage of animals, possibly associated with plant stems, which was negatively affected by the presence of leaves. Four species of mollusc required both epiphyte and leaf habitats, thus providing some overlap between the two major assemblages. When seagrass plant density was reduced by half, major changes in the three faunal assemblages occurred which were almost identical to those caused by the leaf removal manipulation. The faunal densities (g −1 seagrass) of all species negatively affected by leaf removal decreased in reduced density reduction treatment while the densities of species in the epiphyte-associated assemblage showed no significant differences when compared with control patches. Species which increased in abundance in the leaf removal treatment were more abundant in the reduced density seagrass patches than in the control patches. Field experiments carried out subsequent to the seagrass manipulation experiment showed that a change in the leaf-associated faunal assemblage similar to that identified in the seagrass density reduction treatment occurred in <4 d when seagrass plants were placed in the open, and that the settlement rates of animals onto isolated seagrass plants were generally similar to the settlement rates amongst dense Amphibolis. Caging experiments further showed that a reduction in faunal densities occurred on seagrass plants in the open even when plants were enclosed in 10-mm mesh cages, hence predation by fish or decapod predators was unlikely to be the proximate cause for the faunal decline on open seagrasses. Epifauna associated with seagrass leaves at our shallow Seven Mile Beach site appear to actively select dense seagrass habitats, possibly because of evolutionary selection to minimize predation or to avoid high levels of solar radiation.