Using transect sampling and road cruising surveys to estimate abundance and distribution of sidewinders (Crotalus cerastes) in Saguaro National Park, Tucson Mountain District: Final report

Gardner, Coral; Goode, Matt; Philippi, Tom; Swann, Don

doi:10.36967/2302204

Saguaro National Park, Tucson Mountain District (TMD) includes a portion of the Tucson Mountains and adjacent lower bajada landscapes along the western boundary in Avra Valley. Threats to TMD include urban encroachment, busy commuter roads running through the interior and along the boundaries, agricultural development, and the Arizona Canal and water recharge basin. Plans to route Interstate 11 within 400 m of the western boundary have led to concerns over projected negative impacts on wildlife. Among the species of concern is the Sidewinder (Crotalus cerastes), primarily because the population residing in TMD represents the easternmost distribution of this iconic species. Historical records are limited to a handful of localities; however, given their cryptic lifestyle, and the lack of intensive survey effort, there was a need to determine if the species is truly rare or just apparently rare but difficult to detect. To better inform management, we used distance sampling along transects and washes and road cruising surveys to examine density and distribution of Sidewinders in TMD. From May-September, 2022, encompassing both the dry pre-monsoon season and the wet monsoon season, we conducted 519 distance sampling surveys totaling 814.61 km and 69 road cruising surveys totaling 1,997.47 km in upland and wash environments. Our efforts produced 421 total snake observations, which included only nine Sidewinder observations. The small number of Sidewinders observations proved to be insufficient for model convergence. Although we were unable to obtain density estimates of Sidewinders, data from both distance sampling and road cruising surveys provided valuable information on distribution of the species. As expected, we only observed Sidewinders in the westernmost portion of TMD in sandy environments dominated by yellow palo verde and creosote bush. Because we observed a relatively large number (n = 229) of Western Diamond-backed Rattlesnakes (Crotalus atrox), the most commonly observed species, we were able to obtain robust density estimates, which has rarely been done for snakes of any kind. Results revealed that C. atrox density varied by season, with more than double the number of detections during the rainy monsoon season, compared to the dry pre-monsoon season. We recommend that TMD staff conduct repeated road cruising surveys over time to make inferences about population trends that can be compared before and after construction of I-11.

Using transect sampling and road cruising surveys to estimate abundance and distribution of sidewinders (Crotalus cerastes) in Saguaro National Park, Tucson Mountain District: Final report

Series