An endangered snake thrives in a highly urbanized environment

Abstract

The endangered San Francisco garter snake Thamnophis sirtalis tetrataenia occurs in only 2 counties on the densely populated San Francisco Peninsula, California, USA. Due to its limited geographic range and destruction of populations by the rapid pace of urban development in the 1950s and 1960s, T. s. tetrataenia was the only snake included in the inaugural US Endangered Species Act. Several large areas preserved as public parks, utility lands, or private trusts support remnant populations that are the focus of recovery and conservation efforts. To assist in assessment of status and trend for this subspecies, we fit Huggins' closed capture models in RMark to mark−recapture data collected in 2007 and 2013 to estimate population size of an isolated population on a small (72.8 ha) property surrounded by residential development and transportation infrastructure. Our best supported models indicate a population of 1763 individuals (95% CI: 1302−2446) in 2007 and 1761 (95% CI: 1211−2639) in 2013. In addition, total captures of an important prey species increased 2 orders of magnitude from 829 in 2007 to 17 180 in 2013. These findings highlight the potential for well-managed but small habitat units ('the good'), to compliment vast natural areas ('the ideal') in recovery and conservation strategies for threatened and endangered species that can be supported by a small home range.

Combining genetic and demographic monitoring better informs conservation of an endangered urban snake

“…We used population abundance estimates (N a ) of adult T. s. tetrataenia to approximate census size (N) for each site during its respective survey period. We used a size threshold of 300 mm SVL to classify individuals as mature adults, following [35]. At all sites except San Bruno, we used a Bayesian multinomial N-mixture model [36] with random effects of date, site, and individual on capture probability (p) to estimate N a for the five sites monitored in 2018.…”

“…San Bruno was the only site that exceeded the minimum short-term threshold with an N e of 254, which was coupled with highest abundance estimate (N a = 1317). Habitat enhancement activities over the past two decades at San Bruno may have facilitated the higher abundance estimates at this site despite being embedded in an urban matrix since the 1960s [35,77], suggesting that habitat restoration and management may alleviate local loss of genetic diversity in this species.…”

Integrating growth and survival models for flexible estimation of size‐dependent survival in a cryptic, endangered snake

“…Increasing recapture probabilities compared with the current study could require increased spatial coverage of trap arrays within a site (to reduce the effect of temporary emigration) and a longer duration of sampling within each year. For example, Reeder et al ( 2015 ) sampled a population of San Francisco gartersnakes with a much greater density of trap arrays and for a longer duration each year (>70 days) than in the current study, and were able to estimate abundance of snakes >300 mm SVL with good precision. Still, increased sampling effort (i.e., increased trap density) alone cannot overcome low recapture rates for individuals of certain life stages if the sampling method exhibits bias in the sizes captured (Rose, Wylie et al, 2018 ; Willson et al, 2008 ), or if the sampling period is not aligned seasonally with the availability of certain life stages (e.g., if sampling takes place before parturition and therefore neonates are unavailable).…”

“…Fish & Wildlife Service, 2020 ; Wood et al, 2020 ). The minimum reported size at sexual maturity for female San Francisco gartersnakes at one population (not studied here) was 368 mm snout–vent length (SVL) with a modal SVL of 420 mm for gravid females (Reeder et al, 2015 ). Females are expected to mature at 2–3 years of age (U.S.…”

“…Here, we fit an integrated growth–survival model to long‐term CMR data from an endangered snake subspecies, the San Francisco gartersnake ( Thamnophis sirtalis tetrataenia ). Capture probabilities for this species are low (Halstead et al, 2011 ; Reeder et al, 2015 ); therefore, fitting a growth model was necessary to assess how individual size and sex affected survival, recapture probability, and availability for capture. We also evaluated whether and how individual growth and survival differ among populations.…”