Geology topics

Allison M. Nguyen

Publications and source records attributed to Allison M. Nguyen.

4 recordsLinked to original sources

USGS2026

Natomas basin giant gartersnake annual monitoring report 2024

The giant gartersnake ( Thamnophis gigas ) is a semi aquatic snake endemic to the Central Valley of California. After losing 95 percent of its historic wetland habitat (Frayer and others, 1989), giant gartersnakes became state and federally listed as a threatened species (California Fish and Game Commission, 1971; U.S. Fish and Wildlife Service 1993, 1999). Continued monitoring of current populations and implementation of suggested management actions is necessary to recover the species. The Natomas basin in Sacramento, California, supports a population of giant gartersnakes persisting in restored marshes and rice agriculture. This annual report summarizes the giant gartersnake monitoring project for 2024, focusing on the apparent survival, abundance, density, and distribution of the giant gartersnakes and the connectivity of habitat throughout the Natomas basin. In 2024, 131 giant gartersnakes were captured 216 times at 44 sites by hand or trap. The catch-per-unit effort decreased from 2023 to 2024 but was similar to other years of the study. Estimates of occupancy increased between 2023 and 2024, although the trend of occupancy from 2011 through 2024 is still decreasing overall at a mean annual rate of 3 percent per year. Apparent survival was much higher at Betts-Kismat-Silva from 2018 to 2019 and from 2021 to 2022 than in other years, but this may be partly attributed to different sampling efforts over the years. Trapping effort was more consistent in the Sills tract, and apparent survival was slightly higher in later years (2022–23 and 2023–24). Giant gartersnake populations appeared to remain stable in 2024, but abundance, density, survival, and distribution is highly variable across different sites and years of the study. Continued monitoring of the populations would allow for better trend estimates over time and assessment of the effects of management activities. Giant gartersnake populations throughout the basin and on reserve lands would likely benefit from the following: (1) creating more managed marsh; (2) increasing the amount of emergent tule vegetation in existing marshes (for example, Cummings, Natomas Farms, and Lucich South); (3) continuing to flood existing marshes in early spring; (4) maintaining rice agriculture; and (5) continuing research into conservation actions that target the giant gartersnake, such as habitat and water management and translocation.

California↗

USGS2025

Rice cultivation supports growth and survival of a threatened semi-aquatic reptile

Integration of agroecosystems and other working landscapes with protected lands and waters is critical to the conservation of Earth's biodiversity. Rice agroecosystems support many species by providing aquatic habitat where natural wetlands have been altered or drained. In regions with long dry seasons, rice fields and associated irrigation canals provide essential habitat for wetland-dependent species. We quantified the spatial scale and magnitude of the effect of rice growing on the growth and survival of the giant gartersnake ( Thamnophis gigas ), a threatened species that persists primarily in areas of rice agriculture in the Central Valley of California, USA. We used structural causal models to identify drought condition as a key confounder to adjust for when estimating the total effect of rice growing on demographic rates. We analyzed capture-mark-recapture data from 19 populations of giant gartersnakes with an integrated growth–survival model and used distance-weighted covariates to account for the decline in influence of rice with increasing distance from our study sites. We found strong support for a positive effect of rice grown within 1.9 km of a canal on giant gartersnake growth. There was also support for a positive effect of rice on giant gartersnake survival, although the spatial scale extended out to 5 km or more. Our results demonstrate how active rice growing benefits giant gartersnakes inhabiting irrigation canals and demonstrate an approach for studying landscape effects on wildlife in agroecosystems.

California↗

USGS2025

Assessing the Feasibility of Reintroducing San Francisco Gartersnakes ( Thamnophis sirtalis tetrataenia ) to La Honda Creek Open Space Preserve, San Mateo County, California

Reintroductions are used worldwide to increase the viability of species and restore native ecological communities. The success of reintroductions is usually judged by the establishment of self-sustaining populations, restoration of naturally occurring ecological communities, and the species resuming its ecological function. Recovery for the endangered San Francisco gartersnake (SFGS, Thamnophis sirtalis tetrataenia ), a subspecies with a small range in San Mateo and Santa Cruz counties in California, will likely require reintroduction and establishment of new populations within its historical range. La Honda Creek Open Space Preserve (LHC), managed by the Midpeninsula Regional Open Space District (MROSD), is one potential site for the reintroduction of SFGS. The La Honda Creek Open Space Preserve is a preserve managed for wildlife, recreation, grazing, and agriculture located near extant populations of SFGS inhabiting other open space preserves managed by MROSD (Cloverdale Ranch Open Space Preserve [CR]; Russian Ridge Open Space Preserve [RR]). We compared the habitat and prey communities at LHC to nearby open space preserves that support extant SFGS populations. Based on pond surveys done annually since 2008, the occurrence of California red-legged frogs ( Rana draytonii ), Sierran chorus frogs ( Pseudacris sierra ), and Pacific newts ( Taricha spp.) at LHC indicates a similar prey community at this preserve to those at CR and RR. Likewise, the landscape at LHC is a similar mosaic of wetlands, open grassland, shrub-dominated scrub, and coast redwood ( Sequoia sempervirens ) and Douglas fir ( Pseudotsuga menziesii ) forest that meets the habitat requirements for the life history of SFGS at CR and RR. One difference between LHC and preserves with SFGS populations is the lack of vegetative cover immediately adjacent to some wetlands at LHC, which could affect the ability of SFGS to disperse from wetlands and find terrestrial refuges. To evaluate alternative reintroduction strategies, we simulated population viability for a fixed number of SFGS released at LHC into one to six subpopulations (where each wetland represents a subpopulation) over a period from 5 to 20 years. Population simulations indicated that the highest average viability (in other words, the lowest probability of quasi-extinction) occurred when all SFGS were released into a single subpopulation and releases continued annually for 15 to 20 years. Our results indicate that LHC is a good candidate for reintroducing SFGS with suitable habitat, climate, and prey for this snake subspecies. Supporting SFGS populations at LHC could require habitat management to provide sufficient vegetative cover in the terrestrial environment near wetlands. Maintaining genetic diversity in the reintroduced population will also be paramount to ensure negative effects of inbreeding and homozygosity do not affect population viability.

California↗

USGS2023

Survival and establishment of captive-reared and translocated giant gartersnakes after release

Many imperiled species face increasing extinction risk that requires interventional management like translocation or captive rearing. The use of translocations to successfully restore or create populations requires that animals survive at recipient sites, information that is often lacking for imperiled species and that can be risky to acquire if not obtained before a species has dwindled in number. The giant gartersnake ( Thamnophis gigas ), a semiaquatic snake endemic to the Central Valley in California, USA, has declined after losing >90% of its historical habitat and may benefit from successful translocations to protected habitat. We released adult and captive-reared juvenile snakes from 2 donor sites into a recently restored wetland in 2019 and 2020 and compared their survival, movement, and activity using radio-telemetry through 2021. We monitored juvenile survival for 2 years in captivity after birth and then estimated post-release survival at the recipient site using radio-telemetry. Just 8% of translocated adult snakes survived >801 days (95% CI = 1–64%) compared with 39% of resident snakes at the donor sites surviving >1,154 days (95% CI = 23–68%). This equated to annualized survival rates of translocated adults ( x̄ = 0.32, 95% CI = 0.12–0.82) that were roughly half that of resident snakes ( x̄ = 0.74, 95% CI = 0.63–0.89). Translocation was negatively correlated with survival, but movement and activity received little support in models. Seventy-six percent of juvenile snakes survived captivity and, once released, juvenile survival was 60% (95% CI = 38–94%) for the 4-month monitoring period before winter. It is unclear why survival of translocated adult snakes was lower than that of resident snakes, but there was little evidence for differences in behavior (e.g., increased surface activity, average distance moved, emigration from the translocation site) that are commonly cited causes of mortality in other translocation studies. Our results suggest that, in the absence of a clear understanding as to what contributes to adult survival after release to a new site, future work using captive-rearing and juvenile translocation may be more promising for establishing and recovering populations than just translocating adults given the high survival of juveniles.

California↗