Terrestrial and aquatic drivers of occupancy in four semiaquatic mammals

Abstract

Semiaquatic mammals serve as ecosystem engineers and indicator species in their environment. While they play important roles in both terrestrial and aquatic systems, the relative importance of each ecosystem in shaping semiaquatic mammal distributions remains unclear. Additionally, occupancy studies generally focus on a single type of waterbody (e.g., lentic or lotic systems), limiting a holistic understanding of factors impacting these species distribution. To address the relative importance of terrestrial and aquatic environments to semiaquatic mammal distributions, we surveyed 67 sites across four counties in southern Illinois, USA, from March to May 2023 for American beaver (Castor canadensis), muskrat (Ondatra zibethicus), river otter (Lontra canadensis), and American mink (Neogale vison). Sites were distributed evenly between waterbody type and size. Given the elusive nature of these species, we combined two detection methods, sign surveys and camera traps, to increase detection. We applied a Bayesian multi-method occupancy model that incorporates both detection methods to estimate a single occupancy probability for each target species. To evaluate the relative importance of aquatic and terrestrial factors on occupancy, we built candidate models of aquatic and terrestrial covariates separately to identify the most important covariates of each category. The individual top model varied by species, but a combined model of the top aquatic and terrestrial models provided the best overall predictions for each species. Beaver, otter, and mink occupancy showed positive associations with large waterbodies, while muskrat occupancy was positively linked to lotic systems. Additionally, muskrat and mink occupancy were positively related to increasing distance from roads. Our results suggest that while aquatic and terrestrial factors have varying influences in predicting semiaquatic mammal occupancy, considering both yields the most accurate results. All four semiaquatic mammal species we studied were impacted differently by lentic and lotic waterbodies, highlighting the importance of considering both types to better understand their distributions. Our framework is applicable to numerous environments and has the potential to enhance efforts to sustain semiaquatic mammal populations and their habitats.