Facilitative interaction promotes occupancy of a desert amphibian across a climate gradient.

Biotic interactions and environmental factors influence a species' occurrence. Facilitative interactions have the potential to expand species occupancy and mitigate abiotic stress, but are often not considered. The Great Basin clade of the Columbia spotted frog (Rana luteiventris) is an aquatic amphibian found in the arid and semi-arid Great Basin, USA. Aquatic resources in this system are limited and likely to change under future climate scenarios. American beavers (Castor canadensis) are ecosystem engineers that impact the surrounding landscape by increasing water availability. Consequently, beaver-created habitat can facilitate the presence of other species by providing the aquatic resources required by many aquatic and riparian animals. Our objective was to understand patterns of co-occurrence between Columbia spotted frogs and beavers across environmental gradients in the Great Basin, USA. We used environmental DNA detections to quantify the co-occurrence of Columbia spotted frog and beaver using a two-species occupancy framework. At the lowest annual precipitation (202 mm), Columbia spotted frogs were 3× (95% CI 2.35, 3.96) more likely to occur with beavers than without; however, they were less likely to occur with beavers once precipitation was > 380 mm. Thus, beaver activity may increase aquatic resources for Columbia spotted frogs in the Great Basin that may mitigate changing precipitation patterns under climate change. Facilitative interactions likely extend to other aquatic species in arid systems and highlight an important consideration for ecologists when evaluating a species' response to climate change, and may promote the formation of refugia for species with strict abiotic tolerances and dispersal limitations.