Bauder, J.M., A. Pawlicki, and M. Goode. 2024. Northern Mexican gartersnake demographics and movement ecology. Final report for Task Order 23-3 to Arizona Game and Fish Department.
Abstract
The northern Mexican gartersnake (Thamnophis eques; hereafter NMGS) is a federally threatened species occurring in riparian areas in central and southeast Arizona and west-central New Mexico. While previous studies have examined the ecology of this species in central Arizona, less is known about NMGS ecology in grassland landscapes of southeast Arizona. This project continues a long-term mark-recapture study of NMGS in the upper Santa Cruz River in the San Rafael Valley of southeast Arizona. We analyzed mark-recapture data collected by Arizona Game and Fish Department from 2008–2019 and included data we collected during 2022 and 2023. We also studied NMGS movement ecology using a combination of externally attached GPS transmitters and surgically implanted VHF transmitters. We had 517 NMGS captures over 26 survey sessions across the entire study, 186 of which were captures of previously marked snakes. Most of our captures were females and evidence of a statistically significant female-biased sex ratio was present on three survey sessions. Trapping success was generally highest June-September and catch-per-unit-effort (CPUE) was strongly correlated with the number of individuals captured. We used mark-recapture data from 288 individuals to estimate annual apparent survival and session-specific recapture probabilities. Annual apparent survival was higher for females (0.72, 95% highest posterior density intervals [HPDI] = 0.63–0.81) than for males (0.58, 95% HPDI = 0.44–0.72). Session-specific recapture probabilities were similar for males and females and were highly correlated with CPUE. During 2023, we monitored the movements of 13 adult females using GPS transmitters, two adult males using VHF transmitters, and one adult female using a GPS transmitter followed by a VHF transmitter. GPS transmitters were deployed from 7-18 days at a time as limited by battery life. After filtering out low-quality GPS locations, we obtained a total of 1,688 hourly GPS locations. Mean and maximum distances moved per hour ranged from 5.54–10.92 m and 15.24–135.71 m, respectively, across GPS transmitter attachment periods. Space use estimates from 100% minimum convex polygons ranged from 0.04–1.16 ha across GPS transmitter attachment periods. All documented NMGS movements during 2023 were relatively close (< 100 m) to the Santa Cruz River. These results continue to build upon our knowledge of NMGS ecology and suggest the continued presence of a relatively robust population within the upper Santa Cruz River.