Patch shape and orientation influences on seagrass epifauna are mediated by dispersal abilities

While there have been theoretical arguments supporting the importance of the shape and orientation of habitat patches for determining species abundances, there have been few empirical demonstrations that these processes actually operate. Instead, most field studies have focussed on the importance of patch area, isolation and edge effects. I demonstrate that passively dispersed seagrass epifauna respond to the shape and orientation of artificial seagrass patches when currents, the dispersal mechanism, are strong, but not when they are weak. Orientation is important because animals dispersing via tidally induced water currents move predominantly in a single direction, and thus patches oriented across the current intercept more potential colonists than do those patches oriented perpendicular to the current. Currents less affect taxa that actively disperse, or that are relatively sedentary. Fish species that tend to use intertidal areas at high tide, however, were more abundant in patches perpendicular to shore (and parallel to the current), presumably because these patches offer the greatest amount of edge to animals undergoing tidal migrations.