Multi-season occupancy of river otters and co-occurrence of semi-aquatic furbearers in Illinois

Background/Question/Methods

River otters (Lontra canadensis) in freshwater aquatic systems play critical roles as trophic transfer agents and apex predators, interacting with many species within the riparian community. Understanding the interactions between otters and the other semi-aquatic mammals in Illinois (mink [Neovison vison], beaver [Castor canadensis], and muskrat [Odontra zibethicus]) will lead to a more comprehensive understanding of how the reintroduction of otter in Illinois has affected the riparian community. Our objectives were to model the probability of detection, occupancy, colonization, and extinction of river otter and co-occurrence of semi-aquatic mammals to better understand the riparian community in Illinois. To estimate multi-season occupancy and co-occurrence, we surveyed for semi-aquatic mammals at 120 bridge sites in 2 seasons (Jan-Feb and Mar-Apr) each year from 2012–2014 in 11 major watersheds in the southern third of Illinois (44,526 km²). Each survey unit was a 400-m stream segment visited twice by 2 observers for a total of 4 observations per site for each of the 6 seasons.

Results/Conclusions

Probability of occupancy across all sites and seasons for river otters was 0.41. Probability of detection for river otters using a spatially and temporally uniform model was 0.53. Bank substrate availability was the best predictor of detection probability of river otter:  probability of detection exceeded 0.5 when >50% of the bank was available for track impression. The probability of otters colonizing a site increased from 0.22 to 0.35 when beavers were present. Probability of site extinction between seasons within a year decreased with each successive year from 0.38 to 0.24 to 0.00. Decreasing probability of extinction as the study progressed is evidence that otter populations are stabilizing throughout the study area. The importance of beaver occupancy for otter colonization illustrates one of the many ways semi-aquatic mammals interact within the riparian community. The ability of beavers to change the flow of a stream through the building of dams may create pools ideally suited for otter foraging, thus increasing the probability of otter colonization.