Spatial structure of a multi-species guild: the dominant biofilm-grazing microgastropods of seagrass

Abstract

Although all-too-often ignored ecologically, gastropods < 5 mm in largest dimension may be dominant aquatic assemblage components. In Moreton Bay, Australia, for example, intertidal seagrass supports a guild of 24 such biofilm-grazing species at mean density one-third that of the whole macrobenthic seagrass assemblage. Detailed spatial patterns of the 14 guild members at one c. 1 ha locality were investigated via a 256-station lattice. Relative importance of the guild within the macrobenthic assemblage as a whole was spatially uniform but its abundance was significantly patchy at all spatial scales—a few stations supporting up to 170 times background levels of some species—and guild patchiness showed a relatively constant magnitude across those scales. Patches of individual species were independently distributed, showing no evidence of negative inter-specific interactions, the few significant spatial correlations between species being very weak and positive. Levels of syntopy (up to six species within a 0.0054 m2 area) did not differ from those expected under null models of independent assortment. Although diverse, overall guild abundance was low, appearing well below potential carrying capacity, and dominated by few species. Power–law relationships suggested temporal stability of these patterns. Possible causes of such guild structure are discussed.