Conservation Of Snakes

Contributed Paper
ROOM: CC, Room 20

8:10AM Modeling Growth Variability in the Western Terrestrial Garter Snake: Influences of Prey Availability on Long-Term Growth and Body Size
Carli Dinsmore
In environments where conditions vary significantly from year to year, the rate of growth constitutes a link between resource acquisition and partitioning. Without commensurate changes in life history (i.e. age at maturity, fecundity), environmentally driven change in growth rate and maximum body size has important implications for the evolution of growth trajectories, which precede changes in population dynamics. The purpose of this study is to evaluate how growth rate and maximum size vary with climate (i.e. proxy for prey availability), and how climate impacts growth across a life history continuum. I have analyzed 30 years of mark-recapture data to understand the responses across two ecotypes of the Western Terrestrial Garter Snake (Thamnophis elegans) to climate in the Lassen National Forest, California. Ecotypes of T. elegans differ in key life-history traits, wherein the fast-living lakeshore ecotype reproduce younger, have larger litters and experience lower survival than their slow-living mountain meadow counterparts. Using Bayesian methods, I fit individual specific von Bertalanffy growth models to estimate growth rate and maximum body size. Results show that lakeshore snakes grow at faster rates than meadow snakes, reaching larger average asymptotic body sizes. Sexual size dimorphism, a common characteristic in ectotherms, is significantly more pronounced in fast-living lake shore snakes, indicating a stronger ecotypic differentiation in females than in males. Individual growth rates serve as a critical component of population-wide demographic processes. This study highlights how we can use spatial and temporal variability in the environment to predict population level responses in growth parameters of reptiles.
8:30AM Conservation Genetics of the Concho Water Snake, Nerodia Harteri Paucimaculata, a De-Listed Endemic in an Impounded River System.
Mary Janecka; Jan E. Janecka; Charles D. Criscione
The construction of dams is one of the most pervasive and dramatic contemporary habitat modifications of the 20th and 21st centuries. Impoundments alter river and stream habitats directly by changing flow regime and increasing sedimentation. Dams are among the leading factors contributing to population reductions, isolation for freshwater organisms, and loss of biodiversity. The Concho watersnake (Nerodia harteri paucimaculata) is a natricine endemic to central Texas. It was federally listed as a threatened species due to its small range, specific habitat requirements, and potential threats of habitat modification, specifically the construction of a major reservoir in 1990. The Concho water snake was delisted in 2013 after surveys suggested it was able to persist in the reservoir. We conducted population genetic surveys of this species as part of the federally required post-delisting monitoring plan throughout the historic range on the Colorado and Concho Rivers. We developed eighteen species-specific microsatellites to assess effective population size and connectivity of 110 individuals captured from the upper Colorado River (UCR), within the reservoir (O.H. Ivie), and lower Colorado River (LCR)below the reservoir along the Colorado River. Despite extensive sampling, no Concho watersnakes were encountered or captured in the Concho River. Overall, the Concho water snake exhibited low genetic diversity (An: LCR = 4.615, O.H. Ivie = 3.385, and UCR = 3.23), (Hs : LCR = 0.516, O.H. Ivie = 0.479, and UCR = 0.504). Multiple structure analyses and Fst estimates support three genetic clusters corresponding to populations located above, below and within the dam. The O.H. Ivie Reservoir population exhibited the highest divergence, likely due to a founder effect. Our study suggests that low connectivity, coupled with prolonged drought conditions, may put the Concho water snake at risk from bottlenecks and inbreeding, further promoting genetic drift. These effects could be ameliorated with a translocation program.
8:50AM Inferring Landscape-Scale Connectivity between Local Populations of the Eastern Massasauga Rattlesnake Using Genomic Markers
Scott A. Martin; Greg Lipps; H. L. Gibbs
Effective management of rare species relies on knowing the spatial structuring and connectivity between populations. For example, the ability of individuals to move between populations increases the likelihood of long-term persistence of a species by promoting gene flow and buffering populations against stochastic demographic events, whereas a lack of movement leads to population isolation and an increase in genetic drift. Genetic markers, such as single nucleotide polymorphisms (SNPs), can be used to determine if individuals successfully disperse between populations with a high degree of resolution. We used genome scale genetic markers to study the population connectivity of the federally threatened Eastern Massasauga Rattlesnake (Sistrurus catenatus) which exists across the US portion of its range in isolated populations with small effective sizes. Specifically, we generated SNP data for six putative populations in NE Ohio that have previously shown some level of genetic structuring. We sequenced 114 individuals using ddRADseq generating over 15K polymorphic loci per individual. Then, we used resistance modeling based on circuit theory with land cover data to map how different landscape types influence the ability of individuals to disperse between populations. Our results show that Massasaugas move between adjacent fields and may have the potential to colonize nearby habitat; however, not all populations are highly connected. Our study provides insights into how metapopulations of this species persist in a highly fragmented landscape. This information can be used to better plan management efforts to increase the amount of available habitat and support the formation of new populations.
9:10AM Genetic Signatures of Small Effective Population Sizes and Recent and Historical Declines in an Endangered Rattlesnake
H. Lisle Gibbs; Michael G. Sovic; Anthony Fries; Scott A. Martin
Endangered species that exist in small isolated populations are at elevated risk of losing adaptive variation due to genetic drift. Analyses that estimate short-term effective population sizes, characterize historical demographic processes, and project the trajectory of genetic variation into the future are useful for predicting how levels of genetic diversity may change. Here, we use data from two independent types of genetic markers (SNPs and microsatellites) to evaluate genetic diversity in 17 populations spanning the geographic range of the endangered eastern massasauga rattlesnake (Sistrurus catenatus). First, we estimate contemporary genetically effective population sizes and show that most populations have contemporary Ne estimates less than 50, suggesting that genetic drift will be strong in these populations in the future. Second, model-based analyses of the demographic histories of individual populations indicates that all have experienced declines, with the onset of about half of these declines occurring within the timescale over which humans have impacted the landscape of North America. Third, forward simulations of the expected loss of variation in relatively large (Ne = 50) and small (Ne = 10) populations show they will lose a substantial amount of their current standing neutral variation (63% and 99%, respectively) over the next 100 years. Our results argue that genetic drift is poised to have a significant and increasing impact on levels of genetic variation in isolated populations of this snake, and efforts to assess and mitigate the impacts of drift on adaptive variation should be components of the management of this endangered reptile.
9:30AM Adaptive Variation in Venom Genes in Small Isolated Populations of Eastern Massasauga Rattlesnakes
Alexander Ochoa; Michael Broe; H. Lisle Gibbs
Small isolated populations of endangered species can experience genetic costs through the loss of adaptive variation and/or the accumulation of deleterious mutations through genetic drifts. The endangered Eastern Massasauga Rattlesnake (Sistrurus catenatus) occurs in isolated populations with small effective sizes throughout its range in the U.S. and Canada, but little is known about the levels of adaptive genetic variation in existing populations. Here, we used DNA capture probes and Next Generation Sequencing to assess the genetic diversity of a wide range of venom genes in 93 Eastern Massasauga Rattlesnakes from 12 populations in Ohio, Pennsylvania, New York, Illinois, and Ontario. Specifically, we characterized the genetic diversity of genes encoding PLA2, BPP, CRISP, SVSP, and SVMP venom proteins, as well as an additional set of ~1400 non-toxin and neutral loci. Within populations, we find that variation—defined as the presence of nonsynonymous single nucleotide polymorphisms in venom genes—is common and not related to effective population sizes, as determined from neutral genetic markers. This suggests that small populations of this species still retain high levels of adaptive genetic variation due to selection despite the impact of strong genetic drift. In contrast, levels of population divergence in toxin and non-toxin loci are similar, thus making the roles of selection versus genetic drift in maintaining population differences in venom gene alleles uncertain. Broadly, we discuss the implications of our results for management activities for this endangered snake from a conservation genetics perspective.


Contributed Paper
Location: Cleveland CC Date: October 8, 2018 Time: 8:10 am - 9:50 am