Beyond the classical nurse species effect: diversity assembly in a Mediterranean semi‐arid dwarf shrubland

Questions Shrublands often exhibit very high levels of species richness. We ask which mechanisms underlie plant species co‐existence in a hyper‐diverse Mediterranean shrubland, and the scales at which individual species exert their effects during organization of diversity in these communities. Location Brea de Tajo, Madrid, Spain. Methods We computed the individual species–area relationships ( ISAR s) in a completely mapped Mediterranean dwarf shrubland. Using appropriate species‐specific null models, we evaluated how diversity was organized around individual species, covering a wide array of spatial scales. Results We mapped 7858 individuals from 48 perennial species. Our analyses showed that most species (80%) acted as accumulators and exhibited more diversity than expected, whereas 20% were neutral at fine scales. At larger spatial scales, most species exhibited no significant effects in terms of diversity accumulation. Conclusions In contrast to other species‐rich ecosystems such as tropical forests, our results suggest that positive interactions and niche complementarities are the main drivers of community assembly in these communities. The most important finding was the existence of non‐hierarchical multi‐specific clumping that was independent of the fine‐scale environmental heterogeneity, which is a pattern that has not been described previously, to the best of our knowledge. The abundance of accumulator species and the similarity among the ISAR curves suggest that nurse‐like species were not present, thereby indicating that a different ecological mechanism led to the emergence of this pattern.