Functional uniformity underlies the common spatial structure of macrofaunal assemblages in intertidal seagrass beds

Previous work has shown that the intertidal seagrass macrobenthos at three geographically and ecologically disparate localities (in the north-east Atlantic, south-west Indian and south-west Pacific Oceans) possess similar relative species occurrence distributions and uniform species densities. These common features are here demonstrated to be related to the presence in those assemblages of: (1) similar functional diversities and evennesses, (2) the same set of dominant component functional groups, and (3) similar ranked relative occurrence distributions both of those groups and of the component genera within each of the larger groups. The two lower-latitude systems were particularly similar in all these respects. Although sharing the same subset of individual functional groups, however, the relative importance of members of that subset varied from locality to locality and even within a single locality, whilst still maintaining the same ranked relative functional-group occurrence distribution. Therefore the broad structure of available macrobenthic functional roles and the relative occurrences of the component taxa in intertidal seagrass beds (and hence, granted stochastic assembly, the total numbers of taxa supported by unit area) are likely to be linked causally, although the form of the relationship is unclear.