Seasonal changes in community structure along a vertical gradient: patterns and processes in rocky intertidal sessile assemblages

Here we considered two fundamental questions in community ecologyregarding the relationship between seasonalchanges in community structureand environmental gradients: (i) How does the magnitude of seasonalchanges in community structurevary along an environmental gradient? (ii) How do the processes driving seasonalchanges in community structurevary along an environmental gradient? To examine these questions, we investigated intertidal sessile assemblages inhabiting a notable vertical environmental gradientand fitted a transition probability matrix model to decadal time series data gathered at 25 plots along the Pacific coast of eastern Hokkaido, Japan. We found that the magnitude of seasonalchanges in community structurewas the largest at mid shore. The major processes driving seasonalchanges in community structurechanged vertically, reflecting the indirect influence of vertical changes in the physical environment on the vertical distributions of species. An unexpected finding was that the magnitude of seasonalchanges in community structuredid not reflect the strength of seasonalvariation in the physical environment. One explanation may be that sessile organisms living on the high shore have a broad tolerance to environmental stress and are thus less sensitive to the large seasonalvariation in physical stress.