Clarifying the relationship between body size and extinction risk in amphibians by complete mapping of model space

In vertebrates, large body size is often a key diagnostic feature of species threatened with extinction. However, in amphibians the link between body size and extinction risk is highly uncertain, with previous studies suggesting positive, negative, u-shaped, or no relationship. Part of the reason for this uncertainty is 9researcher degrees of freedom9: the subjectivity and selectivity in choices associated with model specification. Here I clarify the size - threat association in amphibians using Specification Curve Analysis, an analytical approach from the social sciences that attempts to minimize this problem by complete mapping of model space. I find strong support for prevailing negative associations between body size and threat status, the opposite of patterns typical in other vertebrates. This pattern is largely explained by smaller species having smaller geographic ranges, but smaller amphibian species also appear to lack some of the life history advantages (e.g. higher reproductive output) that are often assumed to 9protect9 small species in other taxa. These results highlight the need for a renewed conservation focus on the smallest species of the world9s most threatened class of vertebrates, as aquatic habitats become increasingly degraded by human activity.