Spatial-scale dependency in diversity of biological community and its determination mechanisms : A broad-scale comparative experiment at rocky intertidal shore

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This study aims to examine the effects of spatial scale on structure, dynamics and diversity of biological communities using rocky intertidal communities as a model system. We established a total of 150 census plots along the Pacific coast of Japan according to nested hierarchical sampling design, and conducted census of community, experiment on succession and monitoring of environmental conditions. Major findings are summarized as follows :1.We found clear latitudinal gradients of regional species richness and species turnover among shores. However, these patterns were not reflected at a smaller-scale. On the other hand, there was no evidence of latitudinal clines in Simpson's diversity index in regional diversity or in its 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.2.We analyzed how similarities of rocky intertidal assemblages vary at different spatial scales. The assemblages were categorized into distinct regional groups except for the two southern regions, whereas they did not separate clearly from each shore. Similarity was negatively correlated with geographic distance at the regional level, but not at the shore and the rock levels. The analyses highlighted the importance of broad-scale abiotic/biotic factors in determining observed patterns in similarity of rocky intertidal assemblages.3.We determined habitat specificity, abundance, and geographic range for algal species, and classified them into the 'seven forms of rarity' using Rabinowitz's classification scheme. The largest category was the category of rarity with "large geographic range, broad habitat specificity and low local density". The number and proportion of the rare species were positively correlated with algal species richness at shore level, suggesting that shore-level algal species richness can be s useful yardstick for deciding conservation areas.