Spatial patterns of species diversity along horizontal and vertical environmental stress gradients in intertidal rocky shore communities

Environmental stress is a major factor structuring communities. An environmental stress model predicts that species richness and diversity are unimodal trends along full environmental stress gradients within the same biogeographic region. In this study, we investigated the relationships between vertical (elevation intertidal height) and horizontal (wave-exposure, expressed as wave fetch) environmental stress gradients with species diversity, richness, and evenness of sessile macrobenthic invertebrates in five rocky shores of southern Chile. At each site, we estimated species percentage covers at high, medium, and low intertidal zones. Linear regression analyses revealed a significant linear wave-exposure gradient between sites. General additive model (GAM) showed that, in comparison with wave fetch, the vertical stress gradient accounted for the highest amount of spatial variation in diversity, richness, and evenness. Richness and evenness showed the strongest fits with the environmental factors. Therefore, our study suggests that different state variables respond differently to environmental stress gradients and account for different community patterns. The strong and pervasive vertical environmental stress gradients in intertidal rocky shores can have stronger effects on species diversity in comparison to the high wave exposure that characterise the Chilean coasts.