Applying Bergmann's Rule to Species Conservation Planning

ABSTRACT Body size is often strongly linked to species abundance and range size. Although Bergman's rule, which relates body size to climate, has been extensively studied, the implications of this rule for conservation remain unclear. Climate warming may reduce the habitat area of some large‐bodied species at higher latitudes or altitudes, where they need large ranges to survive, and may also cause some larger‐bodied species to shrink, as there is no critical need for them to remain large in order to preserve heat in very cold conditions. Under such circumstances, Bergmann's rule may offer some insights regarding species extinction, as body size is related to other critical traits, such as generation time, especially for species that follow the “rule.” Here, we explore the potential applications of Bergmann's rule in species conservation, using the IUCN's (2022) Red List and species' body size as a reference. Although a greater number of larger birds and mammals (endothermic species) are threatened, a higher proportion of smaller reptiles and amphibians (ectothermic species) face endangerment. Threatened species show stronger body size–range size relationships and are more restricted in monotypic genera. We discuss how Bergmann's rule might be considered in future long‐term species conservation planning within the broad context of climate change, human impacts, and species invasions.