Geology topics

Brian J. Halstead

Publications and source records attributed to Brian J. Halstead.

At least 73 records · Page 4Linked to original sources

USGS2020

Occurrence of a suite of stream-obligate amphibians in timberlands of Mendocino County, California, examined using environmental DNA

Stream-obligate amphibians are important indicators of ecosystem health in the Pacific Northwest, but distributional information to improve forest management is lacking in many regions. We analyzed archived DNA extracted from water samples in 60 pools in streams on private timberlands in Mendocino County, California, for 3 California Species of Special Concern—Coastal Tailed Frogs ( Ascaphus truei ), Foothill Yellow-legged Frogs ( Rana boylii ), and Southern Torrent Salamanders ( Rhyacotriton variegatus )—to better understand their distributions in the region. Detection probabilities for eDNA of Foothill Yellow-legged Frogs and Coastal Tailed Frogs were positively influenced by water temperature. eDNA occurrence for both frogs was affected by whether silt or organic matter was a dominant substrate in the sampled pool, and Foothill Yellow-legged Frog eDNA occurrence was also affected by water temperature. Foothill Yellow-legged Frog eDNA occurrence had a strong, positive association with water temperature, with occurrence unlikely below 14°C and very likely above 16°C, and a positive association with silt or organic substrates in pools, which was likely an indicator of higher-order stream reaches. In contrast, Coastal Tailed Frogs had a negative association with silt or organic substrates. Historical visual detections were generally congruent with findings using eDNA, but differences highlight important areas for further study. We did not detect Southern Torrent Salamanders using eDNA at any sites. Our study reinforces that ecological relationships of these species are varied, and shows the importance of maintaining the integrity of streams with diverse characteristics for conserving stream amphibians.

California↗

USGS2020

Hatchling emergence ecology of Ouachita map turtles (Graptemys ouachitensis) on the lower Wisconsin River, Wisconsin, USA

Despite its biological importance in shaping both individual fitness and population structure, much remains to be learned about the hatchling emergence ecology of most freshwater turtles. Here, we provide some of the first details on these early life stages for the Ouachita map turtle ( Graptemys ouachitensis ) obtained during 2015–2017 along the lower Wisconsin River, Iowa County, Wisconsin, and integrate our results into related research within the genus Graptemys . Dedicated trail cameras over in situ turtle nests provided otherwise difficult to obtain observational data relevant to natural hatchling emergence without disturbing nests or hatchlings. In contrast to some earlier reports for Graptemys , hatchling emergence was mostly diurnal and synchronous, primarily in the morning soon after soil temperatures began to rise from overnight low values. Data suggest a temperature change model of cueing hatchling emergence, which may represent a local or regional adaptation to reduce nocturnal predation risks, mostly from raccoons ( Procyon lotor ), or may simply reflect default diurnal hatchling activity patterns when not affected by thermal constraints. Aside from predation, hatchlings on this small study site are affected by vegetative shading, leading to relatively long times to first emergence periods (mean, 82.3 d), low mean nest temperatures (25.9°C), and a likely male-biased sex ratio. These findings highlight the value of hatchling emergence studies in revealing important influences on population viability and in guiding appropriate habitat management in conservation efforts.

Wisconsin↗

USGS2020

Dispersal of hatchling Ouachita map turtles (Graptemys ouachitensis) from natural nests on the lower Wisconsin River, Wisconsin, USA

Despite its importance to individual fitness and population dynamics, the dispersal behaviors of most neonate freshwater turtles after nest emergence are poorly known. We studied the initial dispersal tendencies of neonate Ouachita map turtles ( Graptemys ouachitensis ) exiting natural nests during 2015–2017 along the Wisconsin River, Wisconsin. Overall, dispersal was nonrandom, and hatchlings largely oriented toward the nearest substantial vegetative cover, a woodland north of the nesting area. However, variation sometimes occurred in routes taken among hatchlings within a clutch. Directional changes within an individual's dispersal track, including route reversals, were also observed. As our work appears to be the first to use standalone trail cameras as a primary data-gathering tool for a hatchling dispersal study, it highlights the potential benefits and limitations of this technique for similar research.

Wisconsin↗

USGS2019

Demographic factors affecting population growth in giant gartersnakes

Demographic models provide insight into which vital rates and life stages contribute most to population growth. Integral projection models (IPMs) offer flexibility in matching model structure to a species’ demography. For many rare species, data are lacking for key vital rates, and uncertainty might dissuade researchers from attempting to build a demographic model. We present work that highlights how the implications of uncertainties and unknowns can be explored by building and analyzing alternative models. We constructed IPMs for the threatened giant gartersnake ( Thamnophis gigas ) based on published studies to determine where management efforts could be targeted to have the greatest effect on population persistence and what unknowns remain for future research. Given uncertainty in the survival of snakes during their first year, and in the form of the size‐survival relationship, we modeled a range of scenarios and evaluated where models agree about factors influencing population growth and where discrepancies exist. For most scenarios, the survival of large adult females had the greatest influence on population growth, but the relative importance of juvenile versus adult somatic growth for population growth was dependent on the recruitment probability and the shape of the size‐survival function. More data on temporal variation and covariance among vital rates would improve stochastic models for the giant gartersnake. This paper demonstrates the effectiveness of IPMs for studying the demography of reptiles and the value of the model‐building process for formalizing what is known and unknown about the demography of rare species. Published 2019. This article is a U.S. Government work and is in the public domain in the USA.

California↗

USGS2019

Monitoring protocol development and assessment for narrowly endemic toads in Nevada, 2018

Several species and subspecies of toads are endemic to small spring systems in the Great Basin, and their restricted ranges and habitat extent make them vulnerable to environmental perturbations. Very little is known about several of these toad populations, so a group of stakeholders including the U.S. Geological Survey, U.S. Fish and Wildlife Service, Bureau of Land Management, Nevada Department of Wildlife, the U.S. Navy, U.S. Forest Service, and Oregon State University met to discuss information needs on these populations and to develop a monitoring protocol that would detect population changes over time. In cooperation with the U.S. Fish and Wildlife Service, the U.S. Geological Survey implemented the proposed survey protocol, a multi-state occupancy design, for three sites: Dixie Valley, Railroad Valley, and Hot Creek, to evaluate its ease of implementation and effectiveness. We found that the multi-state occupancy protocol worked well in the Dixie Valley and, with some refinement, would likely work well in the Railroad Valley. We suggest that capture-mark-recapture of adults might be a more effective approach at Hot Creek. For most life stages of most populations, detection probabilities were positively related to survey duration up to 20 minutes, and the best time of day to conduct surveys varied by life stage and population. We make population-specific suggestions for the number of surveys and their timing and duration. Annual surveys using the suggested survey protocols will likely allow estimation of trends in the proportion of area of each population existing in different population states (occupied, occupied with evidence of reproduction, and unoccupied) and in most cases can be readily implemented with minimal training or handling of toads.

California, Idaho, Nevada, Oregon, Utah↗

USGS2019

Effect of amphibian chytrid fungus (Batrachochytrium dendrobatidis) on apparent survival of frogs and toads in the western USA

Despite increasing interest in determining the population-level effects of emerging infectious diseases on wildlife, estimating effects of disease on survival rates remains difficult. Even for a well-studied disease such as amphibian chytridiomycosis (caused by the fungus Batrachochytrium dendrobatidis [Bd]), there are few estimates of how survival of wild hosts is affected. We applied hierarchical models to long-term capture-mark-recapture data (mean = 10.6 yrs, range = 6–15 yrs) from >5500 uniquely-marked individuals to estimate the effect of Bd on apparent survival of four threatened or endangered ranid frog species ( Rana draytonii , R. muscosa , R. pretiosa , R. sierrae ) at 14 study sites in California and Oregon (USA) and one bufonid toad ( Anaxyrus boreas ) at two study sites in Wyoming and Montana. Our models indicated that the presence of Bd on an individual reduced apparent survival of ranid frogs by ~6–15% depending on species and sex. The estimated difference between toads with and without Bd was 19% for the Montana population and 55% for the Wyoming population; however, the 95% Credible Interval of these estimates included zero. These results provide evidence for negative effects of Bd on survival in wild populations even in the absence of obvious die-offs. Determining what factors influence the magnitude of the effects of Bd on wildlife populations is an important next step toward identifying management actions. These estimates of Bd effects are important for understanding the extent and severity of disease, whether disease effects have changed over time, and for informing management actions.

California, Montana, Oregon, Wyoming↗

USGS2018

Linking variability in climate to wetland habitat suitability: Is it possible to forecast regional responses from simple climate measures?

Temporary wetlands have value to both ecological and social systems. Interactions between local climate and the surrounding landscape result in patterns of hydrology that are unique to temporary wetlands. These seasonal and annual fluctuations in wetland inundation contribute to community composition and richness. Thus, predicting wetland community responses to environmental change is tied to the ability to predict wetland hydroregime. Detailed monitoring of wetland hydroregime is resource-intensive, limiting the scope and scale of forecasting. As an alternative, we determine which freely available measures of water availability best predict one component of wetland hydroregime, habitat suitability (i.e., the predictability of water in a wetland) within and among geographic regions. We used data from three North American regions to determine the climate index that best explained year-to-year variation in habitat suitability during a key phenological period—amphibian breeding. We demonstrate that simple, short-term climate indices based solely on precipitation data best predict habitat suitability in vernal pools in the northeast, montane wetlands in the west and coastal plain wetlands in the southeast. These relationships can help understand how changes in short-term precipitation patterns as a result of climate change may influence the overall hydroregime, and resulting biodiversity, of temporary wetlands across disparate biomes.

Wetlands Ecology and Management↗

USGS2018

Effects of prescribed fire on San Francisco gartersnake survival and movement

The application of fire is prescribed for management of habitats for many plant and animal communities, but its effects on herpetofauna are diverse and remain poorly understood. To date no studies have examined the effects of prescribed fire on endangered San Francisco gartersnake ( Thamnophis sirtalis tetrataenia ) populations, despite a call for prescribed fire in the recovery plan for the species. We used multi‐state capture‐mark‐recapture models to assess whether effects of prescribed fire were ephemeral, occurring only during the year of the fire, or persisted for 3 years following the fire, and to estimate the effects of prescribed fire on demographic parameters of San Francisco gartersnakes inhabiting California coastal prairie. Ephemeral fire effects were better supported for transitions (movement) between burned and unburned areas, but persistent fire effects were better supported for apparent survival and recapture probabilities. Movement between burned and unburned areas decreased during the year of the fire, but transition rates from burned to unburned areas decreased less than transition rates from unburned to burned areas in the year of the fire. Apparent survival probabilities increased in the unburned areas following the fire but were largely unchanged, though more uncertain, in burned areas following the fire. Recapture probabilities decreased site‐wide following the fire, though the decrease was greater in burned areas than in unburned areas. Although imprecise, our estimates of the effects of prescribed fire suggest that under the conditions of this fire (low‐intensity fall burn applied to a small area within a robust population and followed by wet weather), prescribed fire is a viable management tool for maintaining open habitats where San Francisco gartersnakes occur.

California↗

USGS2018

Integrating growth and capture–mark–recapture models reveals size‐dependent survival in an elusive species

Survival is a key vital rate for projecting the viability of wild populations. Estimating survival is difficult for many rare or elusive species because recapture rates of marked individuals are low, and the ultimate fate of individuals is unknown. Low recapture rates for many species have made it difficult to accurately estimate survival, and to evaluate the importance of individual and environmental covariates for survival. Individual covariates such as size are particularly difficult to include in capture–mark–recapture models for elusive species because the state of the individual is unknown during periods when it is not captured. Here, we integrate a von Bertalanffy growth model with a multi‐state robust‐design Cormack‐Jolly‐Seber model to test for a relationship between body size and survival in the elusive, threatened giant gartersnake, Thamnophis gigas . We take a Bayesian approach to model the size of an individual during periods when it was not captured and measured, which fully propagates uncertainty in this unobserved covariate. We found strong support for a positive relationship between snake size and annual survival, with survival increasing with size up to a peak for adult snakes, after which survival either declines slightly or plateaus for the largest individuals. Few captures of very small and very large individuals led to high uncertainty in the survival rates of these sizes. Survival of giant gartersnakes was also positively related to the amount of precipitation and the cover of emergent and floating vegetation at a site. To our knowledge, our study is the first to estimate a size–survival relationship in a snake while fully accounting for uncertainty in the size of unobserved individuals. Our results have implications for the management of this threatened species and illustrate the utility of integrating hierarchical Bayesian models to the study of survival in elusive species.

California↗

USGS2018

Distribution and demography of San Francisco gartersnakes (Thamnophis sirtalis tetrataenia) at Mindego Ranch, Russian Ridge Open Space Preserve, San Mateo County, California

San Francisco gartersnakes ( Thamnophis sirtalis tetrataenia ) are a subspecies of common gartersnakes endemic to the San Francisco Peninsula of northern California. Because of habitat loss and collection for the pet trade, San Francisco gartersnakes were listed as endangered under the precursor to the Federal Endangered Species Act. A population of San Francisco gartersnakes resides at Mindego Ranch, San Mateo County, which is part of the Russian Ridge Open Space Preserve owned and managed by the Midpeninsula Regional Open Space District (MROSD). Because the site contained non-native fishes and American bullfrogs ( Lithobates catesbeianus ), MROSD implemented management to eliminate or reduce the abundance of these non-native species in 2014. We monitored the population using capture-mark-recapture techniques to document changes in the population during and following management actions. Although drought confounded some aspects of inference about the effects of management, prey and San Francisco gartersnake populations generally increased following draining of Aquatic Feature 3. Continued management of the site to keep invasive aquatic predators from recolonizing or increasing in abundance, as well as vegetation management that promotes heterogeneous grassland/shrubland near wetlands, likely would benefit this population of San Francisco gartersnakes.

California↗

USGS2018

Construction and analysis of a giant gartersnake (Thamnophis gigas) population projection model

The giant gartersnake ( Thamnophis gigas ) is a state and federally threatened species precinctive to California. The range of the giant gartersnake has contracted in the last century because its wetland habitat has been drained for agriculture and development. As a result of this habitat alteration, giant gartersnakes now largely persist in and near rice agriculture in the Sacramento Valley, because the system of canals that conveys water for rice growing approximates historical wetland habitat. Many aspects of the demography of giant gartersnakes are unknown, including how individuals grow throughout their life, how size influences reproduction, and how survival varies over time and among populations. We studied giant gartersnakes throughout the Sacramento Valley of California from 1995 to 2016 using capture-mark-recapture to study the growth, reproduction, and survival of this threatened species. We then use these data to construct an Integral Projection Model, and analyze this demographic model to understand which vital rates contribute most to the growth rate of giant gartersnake populations. We find that giant gartersnakes exhibit indeterminate growth; growth slows as individuals’ age. Fecundity, probability of reproduction, and survival all increase with size, although survival may decline for the largest female giant gartersnakes. The population growth rate of giant gartersnakes is most influenced by the survival and growth of large adult females, and the size at which 1 year old recruits enter the population. Our results indicate that management actions benefitting these influential demographic parameters will have the greatest positive effect on giant gartersnake population growth rates, and therefore population persistence. This study informs the conservation and management of giant gartersnakes and their habitat, and illustrates the effectiveness of hierarchical Bayesian models for the study of rare and elusive species.

Open-File Report↗

USGS2018

Multiple drivers, scales, and interactions influence southern Appalachian stream salamander occupancy

Understanding how factors that vary in spatial scale relate to population abundance is vital to forecasting species responses to environmental change. Stream and river ecosystems are inherently hierarchical, potentially resulting in organismal responses to fine‐scale changes in patch characteristics that are conditional on the watershed context. Here, we address how populations of two salamander species are affected by interactions among hierarchical processes operating at different scales within a rapidly changing landscape of the southern Appalachian Mountains. We modeled reach‐level occupancy of larval and adult black‐bellied salamanders ( Desmognathus quadramaculatus ) and larval Blue Ridge two‐lined salamanders ( Eurycea wilderae ) as a function of 17 different terrestrial and aquatic predictor variables that varied in spatial extent. We found that salamander occurrence varied widely among streams within fully forested catchments, but also exhibited species‐specific responses to changes in local conditions. While D. quadramaculatus declined predictably in relation to losses in forest cover, larval occupancy exhibited the strongest negative response to forest loss as well as decreases in elevation. Conversely, occupancy of E. wilderae was unassociated with watershed conditions, only responding negatively to higher proportions of fast‐flowing stream habitat types. Evaluation of hierarchical relationships demonstrated that most fine‐scale variables were closely correlated with broad watershed‐scale variables, suggesting that local reach‐scale factors have relatively smaller effects within the context of the larger landscape. Our results imply that effective management of southern Appalachian stream salamanders must first focus on the larger scale condition of watersheds before management of local‐scale conditions should proceed. Our findings confirm the results of some studies while refuting the results of others, which may indicate that prescriptive recommendations for range‐wide management of species or the application of a single management focus across large geographic areas is inappropriate.

Georgia, North Carolina↗

USGS2018

Identifying management-relevant research priorities for responding to disease-associated amphibian declines

A research priority can be defined as a knowledge gap that, if resolved, identifies the optimal course of conservation action. We (a group of geographically distributed and multidisciplinary research scientists) used tools from nominal group theory and decision analysis to collaboratively identify and prioritize information needs within the context of disease-associated amphibian decline , in order to develop a strategy that would support US management agency needs. We developed iterated influence diagrams to create and assess a unified research strategy. We illustrated a transparent process for identifying specific knowledge gaps in amphibian disease ecology relevant to environmental management, and then constructed a research plan to address these uncertainties. The resulting priorities include a need to: (1) understand the drivers of the community-disease relationship, (2) determine the mechanisms by which exposure to contaminants influence disease outcomes, (3) identify elements of terrestrial and aquatic habitats that stabilize host-pathogen dynamics, (4) discuss how metapopulations may be managed to reduce the speed and intensity of disease outbreaks, and (5) define the relationship between habitat management and the environmental and host microbiomes. Along with identifying research priorities for disease management, we present the details of the process used to develop a consensus plan for addressing disease-related declines in amphibians on federally managed lands of the United States.

Global Ecology and Conservation↗

USGS2018

The relative importance of intrinsic and extrinsic drivers to population growth vary among local populations of Greater Sage-Grouse: An integrated population modeling approach

Consideration of ecological scale is fundamental to understanding and managing avian population growth and decline. Empirically driven models for population dynamics and demographic processes across multiple spatial scales can be powerful tools to help guide conservation actions. Integrated population models (IPMs) provide a framework for better parameter estimation by unifying multiple sources of data (e.g., count and demographic data). Hierarchical structure within such models that include random effects allow for varying degrees of data sharing across different spatiotemporal scales. We developed an IPM to investigate Greater Sage-Grouse ( Centrocercus urophasianus ) on the border of California and Nevada, known as the Bi-State Distinct Population Segment. Our analysis integrated 13 years of lek count data ( n > 2,000) and intensive telemetry (VHF and GPS; n > 350 individuals) data across 6 subpopulations. Specifically, we identified the most parsimonious models among varying random effects and density-dependent terms for each population vital rate (e.g., nest survival). Using a joint likelihood process, we integrated the lek count data with the demographic models to estimate apparent abundance and refine vital rate parameter estimates. To investigate effects of climatic conditions, we extended the model to fit a precipitation covariate for instantaneous rate of change ( r ). At a metapopulation extent (i.e. Bi-State), annual population rate of change λ ( e r ) did not favor an overall increasing or decreasing trend through the time series. However, annual changes in λ were driven by changes in precipitation (one-year lag effect). At subpopulation extents, we identified substantial variation in λ and demographic rates. One subpopulation clearly decoupled from the trend at the metapopulation extent and exhibited relatively high risk of extinction as a result of low egg fertility. These findings can inform localized, targeted management actions for specific areas, and status of the species for the larger Bi-State.

The Auk↗

USGS2018

Spatial and temporal variability in growth of giant gartersnakes: Plasticity, precipitation, and prey

The growth rate of reptiles is plastic and often varies among individuals, populations, and years in response to environmental conditions. For an imperiled species, the growth rate of individual animals is an important component of demographic models, and changes in individual growth rates might precede changes in abundance. We analyzed a long-term dataset on the growth of Giant Gartersnakes ( Thamnophis gigas ) to characterize spatial and temporal variability and evaluate potential environmental predictors of growth. We collected data on the growth in snout–vent length (SVL) of Giant Gartersnakes over 22 yr (1995–2016) from eight sites distributed throughout the Sacramento Valley of California, USA. The von Bertalanffy growth curves indicated male Giant Gartersnakes grew faster toward shorter, asymptotic SVL than did females. Nearly equal variability in growth was attributable to differences among years and among sites. From 2003–2016 we collected data on precipitation, temperature, and the abundance of fish and anuran prey at each site and used these variables as predictors in growth models of Giant Gartersnakes. Snake growth was positively related to the amount of precipitation that fell during the prior water year and the abundance of anurans at a site. Fish and frog abundance interacted to affect snake growth: at low abundances of one prey type, the other positively affected growth, but the slope of this relationship decreased as alternative prey abundance increased. Our results highlight the plasticity of growth in this threatened snake species, point to potential environmental drivers of growth, and provide valuable data for demographic modeling efforts.

California↗

USGS2018

Time-to-detection occupancy modeling: An efficient method for analyzing the occurrence of amphibians and reptiles

Occupancy models provide a reliable method of estimating species distributions while accounting for imperfect detectability. The cost of accounting for false absences is that detection and nondetection surveys typically require repeated visits to a site or multiple-observer techniques. More efficient methods of collecting data to estimate detection probabilities would allow additional sites to be surveyed for the same amount of effort, which would support more precise estimation of covariate effects to improve inference about underlying ecological processes. Time-to-detection surveys allow the estimation of detection probability based on a single site visit by one observer, and therefore might be an efficient technique for herpetological occupancy studies. We evaluated the use of time-to-detection surveys to estimate the occupancy of pond-breeding amphibians at Point Reyes National Seashore, California, USA, including variables that affected detection rates and the probability of occurrence. We found that detection times were short enough, and occupancy was high enough, to estimate reliably the probability of occurrence of three pond-breeding amphibians at Point Reyes National Seashore, and that survey and site conditions had species-specific effects on detection rates. In particular, pond characteristics affected detection times of all commonly detected species. Probability of occurrence of Sierran Treefrogs ( Hyliola sierra ) and Rough-Skinned Newts ( Taricha granulosa ) was negatively related to the detection of fish and pond area. Time-to-detection surveys can provide an efficient method for estimating detection probabilities and accounting for false absences in occupancy studies of reptiles and amphibians.

California↗

USGS2018

Distribution, habitat, and population size of Island Night Lizards on San Nicolas Island, California

The Island Night Lizard ( Xantusia riversiana ) was removed from the federal list of threatened species in May 2014. This strongly differentiated species is endemic to 3 of the southern California Channel Islands—San Clemente, San Nicolas, and Santa Barbara. Suitable habitat for Island Night Lizards is extensive on San Clemente Island, and the species is abundant there. Habitat is limited and fragmented, however, on San Nicolas Island and small Santa Barbara Island. Bringing together extensive field surveys and mark-recapture sampling, we synthesize available data for Island Night Lizards on San Nicolas Island and calculate population and density estimates for the species in major habitats on the island. Island Night Lizards are widely distributed across most of the eastern half of San Nicolas Island. In contrast, they are nearly absent over the western third of the island except for isolated populations in boulder beach habitats. We combined mark-recapture population estimates with comprehensive measurements of the extent of cactus, boxthorn, and other habitat types on the island to arrive at a more accurate assessment of the status of Island Night Lizards on San Nicolas Island. High densities of Island Night Lizards on the island are found in small areas of cholla cactus ( Cylindropuntia prolifera; mean of 4100 lizards/ha), boulder beach habitat (mean of 3400 lizards/ha), and prickly pear cactus ( Opuntia spp.; mean of 1700 lizards/ha); low numbers are found in more extensive mixed-shrub habitat (mean of 250 lizards/ha). The U.S. Fish and Wildlife Service requires a post-delisting program for “monitoring the overall health of the Island Night Lizard” to assure the continued long-term viability of the species in its restricted distribution. The information on population size and habitat presented here will inform and guide conservation and management efforts by the U.S. Navy on San Nicolas Island over the coming years.

California↗

USGS2018

Reproductive frequency and size-dependence of fecundity in the Giant Gartersnake (Thamnophis gigas)

How reproductive output changes with age or size is a key life-history trait that can affect which demographic rates most influence population growth. Although many studies have investigated the reproductive ecology of gartersnakes, we know little about reproduction in the threatened Giant Gartersnake, Thamnophis gigas . We used X-radiography to determine reproductive status and estimated fecundity for 73 female T. gigas collected from several regions within the range of this species in the Sacramento Valley of California, USA, and synthesize these data with data from litters born in captivity to improve our understanding of reproduction in this species. Average total litter size determined from X-rays (15.9) and captive-born litters (15.5) are within the ranges reported from other gartersnakes, but captive-born litters had high rates of stillbirth. Only 154 of 202 neonates from captive snakes were born alive, and seven of 13 litters contained at least one stillborn neonate. We found that fecundity was positively related to maternal snout-vent length, and some evidence that larger litters contained smaller neonates. The proportion of X-rayed females that were gravid was 0.50 in 2014, 0.47 in and 2015, and 0.64 in 2016. Central California experienced an exceptional drought from 2012–2015, which may have affected the reproductive output and frequency of T. gigas . Our estimates of reproductive frequency and size-dependent fecundity in T. gigas provide valuable information that can be used in demographic models of this threatened species. Our results demonstrate that X-radiography is a useful, minimally invasive means to study fecundity in wild populations of snakes.

California↗