Diversity, distribution and abundance of species in rocky intertidal assemblages

Multiple sources of variation, including biogeographic processes, upwelling and El Nino, have differential effects on rocky intertidal communities over the north-central coast of Chile, and they could generate a high variability in diversity, particularly at a regional scale. In this context, species richness, dominance, evenness and composition were assessed in 17 sessile assemblages located between 18 and 34 S, along with the abundance and distribution of their 68 constituent species, for determining their patterns of variation at different spatial scales. Different analyses showed, among other results, that: (a) excepting species composition, the spatial variation of all diversity attributes at the assemblage level was explained almost entirely (83-100%) by local factors; (b) the variation in species richness at the scale of the sampling unit (0.25 m(2)) explained more than 61% of the variance, reflecting an apparent pattern of saturation; (c) at a geographical scale, rarity in both distribution and abundance was the prevailing trait, and these two variables were positively correlated at all spatial scales; (d) common (core) species showed a low spatial variability in abundance and tended to be the same in all assemblages; and (e) taxonomic composition was the only attribute showing regional differences, related with a higher beta diversity and a higher level of rarity in northern Chile, which also appeared to be completely included within the Peruvian biogeographic Province. The results suggest that local and small-scale processes could be important determinants of diversity in these assemblages.