This study examined the latitudinal gradient of species diversity of rocky intertidal sessile assemblages on the slopes of rocks along the Northwestern Pacific coast of Japan, located between 31°Na n d4 3°N, by explicitly incorporating an hierarchical spatial scale into the monitoring design. The specific questions were to examine: (1) whether there is a latitudinal gradient of regional diversity, (2) how spatial components of the regional diversity (local diversity and turnover diversity) vary with latitude depending on spatial scale, and (3) whether the latitudinal gradient differs between different measures of species diversity, i.e. species richness and Simpson's diversity index. We measured coverage and the presence or absence of all sessile organisms in a total of 150 census plots established at five shores in each of six regions. The results showed that there were clear latitudinal gradients in regional species richness and in species turnover among shores. However, these patterns were not reflected in smaller-scale local species richness. For Simpson's diversity index, there was no evidence of latitudinal clines either in regional diversity or in spatial components. These results suggest that relative abundance of common species does not vary along latitude, while the number of rare species increases with decreasing latitude.
It has been debated whether the community structure of an open system is more dependent on environmental processes associated with niche explanations, or on spatial processes related to dispersal. Their relative importance may differ among taxonomic groups with properties of the community such as ecological characteristics (e.g., dispersal ability and life history) and habitat type. We examined the relative importance of environmental and spatial processes on community structure for three taxonomic groups with different ecological characteristics (macroalgae, sessile invertebrates, and mobile molluscs) in rocky intertidal shores of Sanriku Coast, Japan. To evaluate the relative contribution of the two processes in determining community structure, we conducted variation partitioning to reveal the degree of variation of community structure (i.e., β-diversity) explained by environmental heterogeneity and spatial arrangement of local communities. The results of our analyses indicated that β-diversity was significantly explained by both environmental factors (macroalgae, 29.3% of community variation: sessile animal, 40.7%: mobile molluscs, 16.7%) and spatial factors (macroalgae, 19.9%: sessile animal, 3.6%: mobile molluscs, 6.6%) in all taxonomic groups. These results imply that although some taxonomic groups live in the same ecosystem, share common resources, and interact with each other, the mechanisms determining their community structure change depending on ecological characteristics such as dispersal ability and life history.
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