Scale-dependence in species-area relationships

Species-area relationships (SARs) are among the most studied phenomena in ecology, and are important both to our basic understanding of biodiversity and to improving our ability to conserve it. But despite many advances to date, our knowledge of how various factors contribute to SARs is limited, searches for single causal factors are often inconclusive, and true predictive power remains elusive. We believe that progress in these areas has been impeded by 1) an emphasis on single-factor approaches and thinking of factors underlying SARs as mutually exclusive hypotheses rather than potentially interacting processes, and 2) failure to place SAR-generating factors in a scale-dependent framework. We here review mathematical, ecological, and evolutionary factors contributing to species-area relationships, synthesizing major hypotheses from the literature in a scale-dependent context. We then highlight new research directions and unanswered questions raised by this scale-dependent synthesis.