Estimates of Survival Probability from Two Populations of Giant Gartersnakes in California’s Great Central Valley

Hansen, Eric C.; Scherer, Rick D.; White, Gary C.; Dickson, Brett G.; Fleishman, Erica

doi:10.1643/ce-15-233

Cited by 13 publications

(24 citation statements)

References 42 publications

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“…The quadratic size-survival relationship in this study differs from strictly positive linear size-survival relationship found in the only comparable study for giant gartersnakes (Hansen et al 2015). We found that survival is positively related to individual size in giant gartersnakes up to a peak, after which survival may decrease or remain constant for the largest individuals.…”

Section: Discussioncontrasting

confidence: 91%

“…Although the precipitation covariate had a lower probability of being included in the model compared to the emergent and floating vegetation covariates, the positive relationship between precipitation and survival was clear. A previous study reported a negative relationship between the survival of Thamnophis gigas and the amount of precipitation that fell between 15 April and 15 May; high precipitation during this period can delay rice planting, which affects the condition of canal habitat (Hansen et al 2015). In the Sacramento Valley, rainfall is minimal from May through September, which comprises the majority of the active season for giant gartersnakes.…”

Section: Discussionmentioning

confidence: 98%

“…One solution is deterministic imputation, where values are predicted for the missing covariate data, and these predicted values are treated as observed data in a CMR model (Hansen et al 2015). This problem is exacerbated for species with low capture probability, because the vast majority of individuals may be captured only once or twice, so most individual covariate data are missing.…”

Section: Introductionmentioning

confidence: 99%

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

et al. 2018

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Citation: Rose, J. P., G. D. Wylie, M. L. Casazza, and B. J. Halstead. 2018. Integrating growth and capturemark-recapture models reveals size-dependent survival in an elusive species. Ecosphere 9(8):Abstract. 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 sizesurvival 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.

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Section: Discussioncontrasting

confidence: 91%

Section: Discussionmentioning

confidence: 98%

Section: Introductionmentioning

confidence: 99%

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

et al. 2018

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“…Only a single study included size as a continuous covariate. Using a growth function to estimate the size of snakes when not encountered, Hansen, Scherer, White, Dickson, and Fleishman (2015) demonstrated that, for Thamnophis gigas (Giant Gartersnake), survival increased as function of SVL from 0.37 among neonates to 0.73 among large adults. Similarly, survival consistently increased with increasing size, stage, or age in studies using these grouping variables.…”

Section: Resultsmentioning

confidence: 99%

Sunning themselves in heaps, knots, and snarls: The extraordinary abundance and demography of island watersnakes

King

Stanford²,

Jones

2018

Ecology and Evolution

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Snakes represent a sizable fraction of vertebrate biodiversity, but until recently, data on their demography have been sparse. Consequently, generalizations regarding patterns of variation are weak and the potential for population projections is limited. We address this information gap through an analysis of spatial and temporal variation in demography (population size, annual survival, and realized population growth) of the Lake Erie Watersnake, Nerodia sipedon insularum, and a review of snake survival more generally. Our study spans a period during which the Lake Erie Watersnake was listed as threatened under the U.S. Endangered Species Act, recovered, and was delisted. We collected capture–mark–recapture data at 14 study sites over 20 years, accruing 20,000 captures of 13,800 individually marked adults. Lake Erie Watersnakes achieve extraordinary abundance, averaging 520 adults per km of shoreline (ca. 260 adult per ha) at our study sites (range = 160–1,600 adults per km; ca. 80–800 adults per ha) and surpassing population recovery and postdelisting monitoring criteria. Annual survival averages 0.68 among adult females and 0.76 among adult males, varies among sites, and is positively correlated with body size among study sites. Temporal process variance in annual survival is low, averaging 0.0011 or less than 4% of total variance; thus, stochasticity in annual survival may be of minor significance to snake extinction risk. Estimates of realized population growth indicate that population size has been stable or increasing over the course of our study. More generally, snake annual survival overlaps broadly across continents, climate zones, families, subfamilies, reproductive modes, body size categories, maturation categories, and parity categories. Differences in survival in relation to size, parity, and maturation are in the directions predicted by life history theory but are of small magnitude with much variation around median values. Overall, annual survival appears to be quite plastic, varying with food availability, habitat quality, and other ecological variables.

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“…We compiled data on growth, reproduction, and survival of giant gartersnakes from United States Geological Survey studies in the Sacramento Valley, California from 1995 to 2017 (Rose et al , b, c ) and from published studies of giant gartersnake populations in the Sacramento and San Joaquin valleys, California from 1978 to 1990 and 2001 to 2012 (see study area descriptions in Hansen and Hansen , Hansen et al ). The Sacramento Valley has a Mediterranean climate with cool, wet winters and hot, dry summers, and giant gartersnakes primarily occur in wetlands associated with rice agriculture (Halstead et al ).…”

Section: Study Areamentioning

confidence: 99%

Demographic factors affecting population growth in giant gartersnakes

et al. 2019

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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.

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Estimates of Survival Probability from Two Populations of Giant Gartersnakes in California’s Great Central Valley

Cited by 13 publications

References 42 publications

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

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

Sunning themselves in heaps, knots, and snarls: The extraordinary abundance and demography of island watersnakes

Demographic factors affecting population growth in giant gartersnakes

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