Modeling occupancy and detection probabilities to update the status of threatened eastern massasauga rattlesnake populations

Knowledge of a species' distribution throughout a landscape is crucial for developing long-term conservation practices and effective management plans. Accurate distributional data are particularly important for threatened or rare species with a broad geographic range and low detectability in their preferred habitats, as surveying and successfully encountering this type of species is oftentimes difficult. One such species is the eastern massasauga (Sistrurus catenatus), a smallbodied, federally threatened rattlesnake with a distribution centered around the Great Lakes region. The current status of many massasauga populations remains uncertain, hindering effective recovery efforts. We used single-season occupancy models to reassess the status of historic massasauga sites (n = 33) distributed throughout Michigan's Lower Peninsula. We assessed siteand survey-specific covariates at both the local- and landscape-scale to inform occupancy and detection probabilities. Average canopy cover best predicted occupancy probabilities, while search effort best predicted detection probabilities. Our top model which included canopy cover (occupancy) and search effort (detection) estimated an average occupancy probability of 0.30 (CI = 0.15-0.50) and average detection probability of 0.55 (CI = 0.31-0.78). Landscape-scale covariates derived from remotely-sensed data did not improve model fit for occupancy models. Our findings suggest that persistence of eastern massasauga populations relies upon canopy cover of < 60%, so management practices aimed at reducing woody invasive species, or setting back natural succession, will benefit this species. Our results may also be used to guide future survey efforts for massasaugas and have implications for current population predictions that form the basis of recovery strategies.

Automated summary

Knowledge of species distribution is important for conservation and management. The eastern massasauga, a federally threatened rattlesnake, is difficult to survey due to its low detectability. We used occupancy models to assess the status of massasauga populations in Michigan and found that canopy cover and search effort influenced occupancy and detection probabilities. Management practices targeting woody invasive species and natural succession can benefit the species.