Brian J. Halstead

The giant gartersnake (Thamnophis gigas) is a semi-aquatic species of snake endemic to marshes and sloughs in the Central Valley of California. Because the Central Valley has lost a substantial amount of wetland habitat, giant gartersnake populations are typically found in aquatic habitats associated with rice (Oryza sativa) agriculture. Therefore, changes to the amount and location of rice agriculture on the landscape could affect giant gartersnake populations. We present analyses of 9 years of data examining how the occurrence of giant gartersnakes is affected by rice idling. We sampled agricultural canals associated with active rice fields and fallowed fields at 119 sites from 2016 through 2024. We captured giant gartersnakes at 95 of 119 sites sampled during our study, totaling 5,269 captures of 2,687 individuals. We used these capture data, as well as data on the trapping effort, habitat, vegetation, prey availability, and amount of active rice growing at each site, to estimate occupancy and detection probabilities of giant gartersnakes. Mean daily detection probability at a site was 0.38 (95-percent credible interval 0.22–0.56). Given the mean daily detection probability, the probability of detecting giant gartersnakes at an occupied site at least once during a 21-day sampling period within a given year was greater than (>) 0.99 (>0.99 to much greater than [>>] 0.99). The spatial extent of active rice growing within a 2-kilometer buffer of a site was positively correlated with initial site occupancy and the probability of site colonization. Giant gartersnake population density (hereinafter, density) estimated using spatially explicit capture-recapture models, varied among traplines and years, with densities consistently highest in the Butte basin. The annual population growth rate also varied among site-clusters and years, with no temporal trend of growth or decline among sites. Annual apparent survival of giant gartersnakes varied slightly between years and among site clusters, with overall mean annual apparent survival of 0.51 (95-percent credible interval 0.34–0.71) for female snakes and 0.45 (0.30–0.63) for male snakes. The amount of rice in the surrounding landscape was positively correlated to giant gartersnake survival. Rice contributes positively to giant gartersnake occurrence, colonization, and survival, and maximizing the amount of rice agriculture within 4 kilometers of canals with giant gartersnake populations would likely minimize the effects of rice idling on these populations.

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.

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.

We describe the results of long‐term population monitoring of the island night lizard Xantusia riversiana on San Nicolas Island, California, following the species' removal from the U.S. Endangered Species list in 2014. Monitoring activities were carried out from October 2014 through November 2023, but we also incorporate data from earlier work dating back to 1993. Because of habitat loss on the western part of the island, the island night lizard is almost entirely confined to the island's eastern half. The species' distribution remains largely the same since studies in the 1990s, though small increases in distribution were noted at the island's west end. Numbers at most long‐term monitoring sites appeared to show a slight decrease associated with a multi‐year drought, with numbers returning to previous levels by the end of the monitoring period. Models fit to counts of newly recruited lizards suggested a positive association between winter precipitation and abundance; however, we found a simpler model including constant abundance throughout the study to be more parsimonious. Two monitoring sites showed marked decreases in numbers, with one site declining to zero following infestation of the area by non‐native Argentine ants Linepithema humile . The island's vegetation and natural communities continue to gradually recover from a century of overgrazing by sheep and the widespread introduction of aggressive non‐native plants. Habitat restoration efforts by U.S. Navy natural resources staff on the island may lead to increasing population numbers and genetic connectivity, but persistent threats remain for the island night lizard on San Nicolas Island.

Disease, alone or combined with other stressors such as habitat loss and contaminants, affects wildlife populations worldwide. However, interactions among stressors and how they affect demography and populations remain poorly understood. The amphibian chytrid fungus (Batrachochytrium dendrobatidis; Bd) is a sometimes-lethal pathogen linked with population declines and extirpations of amphibians globally. Laboratory evidence shows ubiquitous contaminants like methylmercury (MeHg) can reduce vigor and survival of amphibians, but population-level effects remain unclear. We used non-lethal sampling to assess how Bd and MeHg affected survival of juvenile and adult amphibians in 20 populations across the USA. Survival of several species declined with increasing Bd loads, including some species previously considered resistant to Bd (e.g., eastern newt [Notophthalmus viridescens]). Although our sampling for MeHg was less intensive than for Bd, we found MeHg can both directly reduce survival and synergistically magnify the effects of Bd infection. For a population of foothill yellow-legged frogs (Rana boylii), the estimated reduction in survival from MeHg exceeded that from Bd. Although effects varied widely among populations and species, our results help clarify the potential for synergistic effects of disease and contaminants and emphasize the complexity of identifying and quantifying the population-level effects of interactions among stressors.

Batrachochytrium dendrobatidis (Bd) is a globally distributed fungal pathogen of amphibians that has contributed to one of the largest disease‐related biodiversity losses in wildlife. Bd is regularly viewed through the lens of a global wildlife epizootic because the spread of highly virulent genetic lineages has resulted in well‐documented declines and extinctions of multiple amphibian species. However, the current state of Bd occurrence, host range, host impacts, and ecological drivers remains poorly understood outside of the most negatively affected amphibian species and regions. Our objective was to describe the macroecology of Bd occurrence and infection intensity on caudates (salamanders) across the United States and to compare these patterns with better‐studied anurans (frogs and toads). We collected swabs from 11,183 amphibians at 609 sites from 54 species across the United States from 2015 to 2017. We analyzed the prevalence and intensity of Bd infection jointly using a Bayesian hurdle model with covariates of site‐level temperature and precipitation, as well as individual characteristics and species identification. Bd was distributed widely across sites and species sampled across the spatial extent of the conterminous United States. We found that Bd prevalence and intensity were most strongly influenced by temperature in the month preceding sampling and by differences among taxon groups. We estimated that temperature had a strong and nonlinear influence on both Bd prevalence and intensity with peak infection at intermediate temperatures and lower infection at low and high temperatures. We found Caudate hosts tended to have higher prevalence than Anuran hosts and Anuran hosts tended to have higher intensity at optimal temperatures for Bd infection. Our findings suggest that Bd has an amphibian‐wide host range, temperature gradients exert a strong influence on Bd, and enzootic transmission likely encompasses a much larger spatial and species distribution than previously recognized across North America.

Gaps in dispersal data limit habitat protection efforts. We sought to characterize such gaps and compare the utility of dispersal data from demographic and genetic methods in making habitat protection decisions. Here, we used a search string to retrieve dispersal studies for amphibians, reptiles, birds, and mammals. We included studies based on a set of selection criteria. We used this sample of selected studies to assess for persistence of taxonomic and geographic biases. We extracted non-effective (i.e., demographic) and effective (i.e., genetic) dispersal rates. We weighted these dispersal rates by associated sample size and standard deviation to indicate the ability to capture population-level dispersal. We then tested for variation in weighted dispersal by study type using Bayesian mixed-effects models. Amphibians were the most under-represented taxonomic group in our sample. Dispersal studies were mostly retrieved from developed nations indicating the distribution of dispersal research reflected GDP rather than the number of threatened species. The magnitude of dispersal from genetic methods exceeded demographic methods in all vertebrate groups considered in our study. Further, genetic studies consistently sampled a larger number of individuals. Thus, genetic methods may be better suited to characterize population-level dispersal. However, demographic and genetic approaches enable examination of the dispersal process at varying spatial and temporal scales and a combination of these approaches can be used to address persistent gaps in dispersal and enable land-management decisions.

Rio Grande Cooters (Pseudemys gorzugi) occupy the Rio Grande watershed and have among the smallest ranges of all North American freshwater turtles. Anthropogenic dewatering is considered to have caused range contractions and population declines. We sought to facilitate management recommendations by determining the extent of movement and potential associations with extrinsic cues. We conducted a GPS-enabled telemetry study from August 2015 to May 2017 on the Devils River in Texas, USA. We included Capture–Mark–Recapture data from 2011, 2014, and 2015–2018 to determine population status in conjunction with movement ecology. Turtles showed increased movement as streamflow and water depth increased. Larger movements were also made mid-year, coincident with the peak nesting season. We speculate that seasonality and increases in streamflow facilitate switches from slower, localized movement to transiting modes. We observed individual heterogeneity in transitory movements. Such movements led us to maintain our population estimate of 726 to 1219 individuals is representative of the entire Devils River. The extent of movement in P. gorzugi has been previously underestimated and long-range movements could explain observed genetic structure. Future efforts to re-establish natural flow regimes in the Rio Grande basin could potentially be the most effective management approach for this range-restricted chelonian.