Geology topics

Gabriel A. Reyes

Publications and source records attributed to Gabriel A. Reyes.

5 recordsLinked to original sources

USGS2026

Effects of rice idling on occupancy dynamics and survival of giant gartersnakes ( Thamnophis gigas ) in the Sacramento Valley, California, 2016–24

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.

California↗

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

Using passive integrated transponder (PIT) tags to monitor and research bats: Applications, benefits, and limitations

Worldwide declines of bat populations underscore the value of developing and refining survey tools to assess and monitor population statuses and dynamics. Passive integrated transponder (PIT) tags can be an effective marking technique, facilitating the monitoring of individual bats at roost sites by using radio frequency (RFID) readers and antennas. We detail 5 ongoing studies, discuss the rationale for our study designs, and offer recommendations for monitoring and researching bats with PIT tags that are broadly applicable across species and locations. Using data from RFID readers, we show how population parameters can be precisely estimated across space and time, while improving our understanding of bat ecology and vulnerability to threats. We discuss PIT-tag loss as a potential limitation that varies across species, sex, and season, present minimum loss rates from 2 studies, and describe consequences and techniques to ameliorate this issue. Overall, integrating PIT tags with autonomous RFID readers and antennas provides a powerful survey method for monitoring, researching, and conserving bat populations.

Colorado↗

USGS2018

Findings from a preliminary investigation of the effects of aquatic habitat (water) availability on giant gartersnake (Thamnophis gigas) demography in the Sacramento Valley, California, 2014–17

The giant gartersnake ( Thamnophis gigas ) is a semi-aquatic species of snake precinctive to the Central Valley of California. Because the Central Valley has experienced a substantial loss of wetland habitat, giant gartersnake populations are largely found in aquatic habitats associated with rice agriculture. In dry years, less water may be available for rice agriculture, resulting in less aquatic habitat, which could have cascading effects on giant gartersnake populations. We present 2 years of data intended to examine how the demography of giant gartersnakes is affected by the availability of aquatic habitat on the landscape (2016–17), along with 2 years of (sparse) preliminary data (2014–15) collected as part of an earlier radio-telemetry study on giant gartersnake movement behavior. We sampled agricultural canals near rice fields for giant gartersnakes at 8 sites distributed throughout the Sacramento Valley. Five sites were sampled from 2014–17, and 3 sites were sampled from 2015–17. In total, we made 2,995 captures of 1,011 snakes from 2014–17. We used these capture data to fit a multi-site Jolly-Seber model to estimate the abundance of giant gartersnakes as well as the daily and annual probability of capture at each site. We used remotely sensed Landsat data to characterize the extent of flooded rice fields surrounding each site in each year. In addition, we collected 175 females from 2014–17 and delivered them to the Sacramento Zoo for health assessments and reproductive exams. The abundance of giant gartersnakes varied among sites, and abundance estimates were more precise in 2016 and 2017 when sampling effort was greatest. The probability of a giant gartersnake being captured at least once in a year was higher in 2016 and 2017 than 2014 and 2015, and recaptures of snakes marked the previous year were highest in 2016 and 2017 as well. Mean annual apparent survival was estimated to be 0.40 but varied among sites from a low of 0.14 to a high of 0.63. Five sites had diverse size distributions that included abundant sub-adult and large adult female snakes. One site had a truncated size distribution with few large adult female snakes, and 2 sites had mostly large adult-sized snakes and few small individuals. Both the probability a female was gravid and a female’s litter size were positively related to the female’s snout-vent length. Somatic growth rates varied more among years than among sites, and females grew faster (in millimeters per day) than male snakes. The proportion of the landscape around each site under active rice cultivation fluctuated over time (generally between 60–90 percent of the landscape was active rice growing, although this proportion was lower for some sites in some years), and variation in rice growing was asynchronous among sites. This study demonstrates that intensive demographic sampling enables estimation of several key demographic variables at each study site. Continued sampling would allow for investigating potential relationships between the amount of rice growing at a site and demographic parameters such as growth, survival, and reproduction.

California↗

USGS2017

Behavioral response of giant gartersnakes (Thamnophis gigas) to the relative availability of aquatic habitat on the landscape

Most extant giant gartersnake ( Thamnophis gigas ) populations persist in an agro-ecosystem dominated by rice, which serves as a surrogate to the expansive marshes lost to flood control projects and development of the Great Central Valley of California. Knowledge of how giant gartersnakes use the rice agricultural landscape, including how they respond to fallowing, idling, or crop rotations, would greatly benefit conservation of giant gartersnakes by informing more snake-friendly land and water management practices. We studied adult giant gartersnakes at 11 sites in the rice-growing regions of the Sacramento Valley during an extended drought in California to evaluate their response to differences in water availability at the site and individual levels. Although our study indicated that giant gartersnakes make little use of rice fields themselves, and avoid cultivated rice relative to its availability on the landscape, rice is a crucial component of the modern landscape for giant gartersnakes. Giant gartersnakes are strongly associated with the canals that supply water to and drain water from rice fields; these canals provide much more stable habitat than rice fields because they maintain water longer and support marsh-like conditions for most of the giant gartersnake active season. Nonetheless, our results suggest that maintaining canals without neighboring rice fields would be detrimental to giant gartersnake populations, with decreases in giant gartersnake survival rates associated with less rice production in the surrounding landscape. Increased productivity of prey populations, dispersion of potential predators across a larger landscape, and a more secure water supply are just some of the mechanisms by which rice fields might benefit giant gartersnakes in adjacent canals. Results indicate that identifying how rice benefits giant gartersnakes in canals and the extent to which the rice agro-ecosystem could provide these benefits when rice is fallowed would inform the use of water for other purposes without harm to giant gartersnakes. Our study also suggests that without such understanding, maintaining rice and associated canals in the Sacramento Valley is critical for the sustainability of giant gartersnake populations.

California↗