Abstract
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.
Highlights
The availability of accurate demographic parameter estimates is central to understanding population dynamics (Caswell, 2001), evaluating life history evolution (Roff, 1992; Stearns, 1992), and modeling extinction risk (Akçakaya, 2004; Lacy, 1993, 2000)
Snakes represent a sizable fraction of vertebrate biodiversity; their 3,400 species constitute 10% of extant tetrapods and 35% of extant squamates
We identified 57 cases of Lake Erie Watersnakes that moved among our 14 study sites
Summary
The availability of accurate demographic parameter estimates is central to understanding population dynamics (Caswell, 2001), evaluating life history evolution (Roff, 1992; Stearns, 1992), and modeling extinction risk (Akçakaya, 2004; Lacy, 1993, 2000) Generating such estimates requires long-term monitoring data, often of marked individuals. Often secretive and infrequently encountered, snakes can be abundant and, as tertiary predators, exert top-down influences on ecosystem function (Jones, King, Stanford, Lawson, & Thomas, 2009; Willson & Winne, 2016 and citations therein) Despite their diversity, snakes are poorly represented among life history data compilations, comprising, for example, just four entries among the 1,927 matrix population models in the COMADRE Animal Matrix Database (Salguero-Gómez et al, 2016; http://www.compadre-db.org/ accessed 19 October 2017). Following Parker and Plummer (1987), we consider variation in annual adult survival among families, subfamilies, continents, and climate zones and its relationship to reproductive mode, body size, age at maturity, and reproductive frequency
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
