Spatial relationships and some structuring processes in benthic intertidal animal communities

Abstract

Species diversity and the occupation of primary and secondary space was examined in a benthic intertidal habitat. Replicate climax communities on experimental blocks (over 3 yr old) were compared with short-term (3-month) collections of settling larvae. Rather than occupying free primary space, perennial species occupied secondary (epizooic) space by overgrowth (in colonial bryozoans) and by referential settlement (studied in the solitary tubeworm Spirorbis). The clumped distributions of animals and the occupation of secondary space while free primary space remained suggests that primary space was not limiting during all periods of the year, as is frequently assumed for sessile communities. Differences in patterns of diversity were observed for colonial and solitary animals. Although there were comparable numbers of colonial bryozoan species on each climax block, there was little overlap in species composition among blocks. While solitary animals were consistently present and showed reduced diversity, high diversity was maintained in colonial forms with similar species apparently substituting for each other on different blocks. These collections of animals are suggestive of multiple climax states and were identified as different communities. Those species initially occupying free space seemed to affect secondary colonization of the area, presumably by attracting or excluding settling larvae or spreading species. Seasonally abundant species, competitively superior to non-seasonal species, may produce selection pressures favouring the occupation of secondary space by perennial animals.