How much do simple random processes explain species-area relationships? A global synthesis and a new metric

Abstract

Abstract Context The species-area relationship (SAR) is one of the main patterns in Ecology, but its underlying causes are still under debate. The random placement hypothesis (RPH) is the simplest one to explain the SAR: larger areas are more likely to contain more individuals and, as a consequence, more species. However, it is still unclear the degree to which the RPH is supported for different taxa and regions globally. Objectives We performed the first global synthesis on the RPH to investigate which variables mediate variation in the degree of support of this hypothesis across taxa and regions. Methods We conducted a review of the global SAR literature and created a new integrative metric to estimate the degree of support of the RPH. This metric takes into account the coefficient of determination, the intercept, and the slope of the relationship between observed (empirical) and predicted (according to the RPH) species richness. We analyzed the relationship between this metric and different geographic and ecological factors. Results We found a total of 52 independent tests of the RPH. About 42% of these tests confirmed the RPH. Using the new metric, we found that the degree of support of the RPH was significantly higher for plants than animals, and increased consistently with latitude. Conclusions Simple probabilistic processes are important to determine SARs, especially for sessile organisms and at higher latitudes. Further tests of the RPH, especially using the new metric introduced here, will be vital to understand the processes underlying the SAR and to advance Landscape Ecology.