How important is passive sampling to explain species-area relationships? A global synthesis

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 passively sample more individuals and, consequently, more species. However, it is still unclear the degree to which this null hypothesis is supported for different taxa and locations 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 literature and estimated the degree of support of the RPH. The degree of support (effect size) was inferred through the coefficient of determination of the relationship between observed (empirical) and predicted (according to the RPH) species richness. We analyzed the relationship between this effect size metric and different geographic and ecological factors. Results About 31% of the studies explicitly considered the RPH. From these, only 14% tested the RPH in a total of 52 independent case studies. About 42% of these case studies confirmed the RPH. The degree of support was significantly higher for plants than animals, and increased consistently with latitude for animals. Conclusions Passive sampling is important to determine SARs, especially for animals at higher latitudes and plants. Further tests of the RPH, which is still scarcely explored in the literature, are vital to understanding the stochastic and ecological processes underlying the SAR and to advancing Landscape Ecology.