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 ecology regarding the relationship between seasonal changes in community structure and environmental gradients: (i) How does the magnitude of seasonal changes in community structure vary along an environmental gradient? (ii) How do the processes driving seasonal changes in community structure vary along an environmental gradient? To examine these questions, we investigated intertidal sessile assemblages inhabiting a notable vertical environmental gradient and 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 seasonal changes in community structure was the largest at mid shore. The major processes driving seasonal changes in community structure changed 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 seasonal changes in community structure did not reflect the strength of seasonal variation 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 seasonal variation in physical stress.