Urban stormwater runoff limits distribution of platypus

The platypus (Ornithorhynchus anatinus), like many other stream‐dependent species, is reportedly sensitive to catchment urbanization. However, the primary mechanism limiting its distribution in urban environments has not been identified. We created species distribution models for three platypus demographic classes: adult females (which are exclusively responsible for raising young), adult males (which are more mobile than females), and first‐year juveniles. Using live‐trapping data collected in Melbourne, Australia, we tested whether distributions of the three demographic classes were better predicted by catchment urban density (total imperviousness), by urban stormwater runoff (catchment attenuated imperviousness), or by stream size (catchment area). Two variants of each predictor variable were developed, one that accounted for platypus mobility, and one that did not. Female distribution was most plausibly predicted by stormwater runoff (accounting for mobility), with a steep decline in reporting rate from 0 to 10% attenuated imperviousness. Male distribution was equally plausibly predicted by stormwater runoff and urban density (both accounting for mobility), with a less steep and more uncertain decline with imperviousness than females. Juvenile distribution was most plausibly predicted by stream size (accounting for mobility), but both stormwater runoff and urban density (accounting for mobility) were nearly equally plausible predictors. The superior performance of models that accounted for mobility underscores the importance of accounting for this in species distribution models of highly mobile species. Platypus populations in urban areas are likely to be affected adversely by urban stormwater runoff conveyed by conventional drainage systems, with adult females more limited by runoff‐related impacts than adult males or juveniles. Urban platypus conservation efforts have generally focused on restoring riparian and in‐stream habitats on a local scale. This is unlikely to protect platypus from adverse impacts of urban stormwater runoff, which is most effectively managed at the catchment scale.