Giant Gartersnake Research

Thamnophis gigas. By Photo: Dave Feliz, CC BY-SA 3.0,
Protecting the Giant Garter Snake
Rose et al. (2019) present work that highlights how the implications of uncertainties and unknowns can be explored by building and analyzing alternative models. We constructed  Integral projection models (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, they 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.

Rose JP, Ersan JS, Wylie GD, Casazza ML, Halstead BJ. Demographic factors affecting population growth in giant gartersnakes. The Journal of Wildlife Management. 2019.

Giant Gartersnbake Conservation
Most extant populations of Giant Gartersnakes (Thamnophis gigas) inhabit the highly modified rice agricultural regions of the Sacramento Valley, Halstead et al. (2019) evaluated whether Giant Gartersnakes are a conservation-reliant species dependent on the maintenance of rice agriculture and its infrastructure for their continued existence. Specifically, the examined the extent to which Giant Gartersnakes use rice fields, and whether survival of adult Giant Gartersnakes was influenced by the amount of rice grown near their home ranges and daily movements. They found that although Giant Gartersnakes only use rice fields minimally and then only between mid-June and early September, their survival was lower when less rice agriculture was available near the areas they inhabited. Survival was particularly low in early spring when Giant Gartersnakes emerge from brumation but the rice fields are not yet flooded. The incongruity between the phenology of rice growing and Giant Gartersnake foraging requirements suggests that although the snakes are reliant on the rice agroecosystem, rice agriculture is likely suboptimal habitat. Giant Gartersnakes’ reliance on the rice agroecosystem challenges the notion of preservation-based conservation but provides opportunities for win-win scenarios benefitting both rice farmers and Giant Gartersnakes. The study highlights that in addition to land use, the timing of land management might be crucial for conservation-reliant species.

Halstead BJ, Rose JP, Reyes GA, Wylie GD, Casazza ML. 2019. Conservation reliance of a threatened snake on rice agriculture. Global Ecology and Conservation. 2019 Jun 1:e00681.