Intertidal Community Structure: Space-Time Interactions in the Northern Gulf of California

Long-term studies are required for an understanding of how temporal variation and space-time interactions affect the structure of communities. Here we report on a long-term study of the independent effects of, and the interactions among, two sources of temporal variation (seasonal and annual) and two sources of spatial variation for a rocky intertidal community in the northern Gulf of California. The sources of spatial variation were: (1) microspatial effects due to the foraging patterns of a common predatory snail (Acanthina angelica) and (2) macrospatial effects due to differences among sites. The results from semiannual samples of 100-cm^2 quadrats showed highly significant temporal and spatial effects for all members of the sessile community (barnacles, mussles, algae) and for limpets over the 8-yr study period. There were also highly significant season X space interactions for all sessile members of the community, which probably resulted from seasonal settlement by the sessile members of the community, and aestivation by the predator. Finally, we observed highly significant year X space effects as well year X season X space effects for most species. These latter interactions can be understood as an amplification of seasonal and spatial effects due to the largely unpredictable differences among years. An Analysis of the variance components showed that most of the variation in percentage cover of barnacles and a brown encrusting alga was due to microspatial effects, while most of the variation in mussels, limpets, and a green alga was due to year and season effects. This combination of results suggests that competition and predation by Acanthina are relatively more important in controlling the distributions and local abundances of barnacles and encrusting algae, and that unpredictable differences among years in settlement are more important in controlling the local population densities of mussels and limpets. The importance of these differences are discussed in relation to interpretation of short-term experimental studies in population and community studies.