Contrasting the species abundance, species density and diversity of seaweed assemblages in alternative states: Urchin density as a driver of biotic homogenization

- A spatial approach was used to compare urchin barrens and upright seaweed beds.
- Spatial patterns in assemblage structure were different in both habitats.
- We demonstrate that urchin densities reduces the spatial variability of assemblages.
- Potential patterns of assemblage structure could only be observed in pristine areas.

Differences in seaweed assemblages' structure (species abundance, species density and diversity) were examined in two habitats, urchin barrens and upright seaweed beds on the Canarian Archipelago (eastern Atlantic Ocean) to demonstrate the key role of extreme density of the sea urchin Diadema africana in the homogenization of assemblages in shallow rocky reefs. Univariate and multivariate analyses were used to test for differences in seaweed the assemblages at multiple spatial scales, from sites (< 10 km apart) to islands (25-450 km apart), based on samples collected from six islands. Distance-based linear model routine (DistLM) and distance-based redundancy analysis (dbRDA) were also applied to analyze and model relationships between seaweed assemblages and environmental variables in each habitat. The patterns of spatial variation in assemblage structure were different in urchin barrens compared to upright seaweed beds. In urchin barrens, spatial variation of seaweed assemblages differed between sites only, whereas in upright seaweed beds were observed differences between sites and islands. Sea urchin density and substrate roughness were the two factors determining assemblage structure in urchin barrens. In contrast, in upright seaweed beds, the major factors influencing assemblages were wave exposure, temperature and productivity. We conclude that potential biogeographic patterns of assemblage structure induced by oceanographic conditions were observed only in pristine areas with low urchin density.