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

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.

Keywords: body size; capture–mark–recapture; life history; population estimation; process variance; realized population growth; survival; vital rates.

Figure 1

The island region of western Lake Erie showing 14 sites included in this study. Sites, include on Kelleys Island—Long Point (A), Southeast Shore (B), South Shore (C), Minshall (D), and State Park (E); on South Bass Island—East Point (F), East Shore (G), and State Park (H); on Middle Bass Island—East Point (I), State Park (J), and West End (K); on North Bass Island—N,NE,E Shore (L) and South Shore (M); and on Gibraltar Island in its entirety (N)

Figure 2

Estimated annual adult survival, ϕ, of male (unfilled circles) and female (filled circles) Lake Erie Watersnakes and associated 95% confidence intervals. Letters identify study sites as in Figure 1, and vertical lines separate islands

Figure 3

Estimated annual adult realized population growth, λ, of male (unfilled circles) and female (filled circles) Lake Erie Watersnakes and associated 95% confidence intervals. Study sites, identified by letters as in Figure 1, are represented by separate panels. Points on the far right of each panel represent mean realized population growth across years estimated using the model ϕ(sex*site)p(site*time+sex)λ(site*sex)

Figure 4

Association between estimated annual adult survival and body size as indicated by the mean SVL of the largest 10% of animals captured from 2000 to 2015. Points represent the 14 study sites included in this analysis. Females are represented by the upper cluster of points; males are represented by the lower cluster of points. Whiskers represent 95% confidence intervals. Lines represent reduced major axis regressions for females (dashed) and males (solid)

Figure 5

Localities of studies providing estimates of annual survival among of snakes (Colubridae—circles, Viperidae—squares, Elapidae—triangles, and Pythonidae—diamonds). The location of our case study of the Lake Erie Watersnake (north central USA) is circled. Dashed lines separate tropical (<23.5°N or S), subtropical (23.5–40°N or S), and temperate (>40°N or S) climate zones

Figure 6

Box plots showing variation in adult snake survival among continents, climate zones, families, subfamilies, modes of reproduction, body size, maturation, and parity. Bars represent medians, boxes represent 50th percentiles, and whiskers represent ranges excluding outliers (points). The number of estimates and the number of species are listed above each box plot