Demographics, reproduction, growth, and abundance of Jollyville Plateau salamanders (Eurycea tonkawae)

Abstract

Insights into the ecology and natural history of the neotenic salamander, Eurycea tonkawae, are provided from eight years of capture-recapture data from 10,041 captures of 7,315 individuals at 16 sites. Eurycea tonkawae exhibits seasonal reproduction, with peak gravidity occurring in the fall and winter. Size frequency data indicated recruitment occurred in the spring and summer. Open-population capture-recapture models revealed a similar seasonal pattern at two of three sites, while recruitment was dependent on flow at the third site. Females can reach sexual maturity within one year, and oviposition likely takes place below ground. The asymptotic body length of 1,290 individuals was estimated as 31.73 mm (at ca. two years of age), although there was substantial heterogeneity among growth trajectories. Longevity was approximately eight years, and the median age for a recaptured adult was 2.3 years. Abundance estimated from closed-population and robust-design capture-recapture models varied widely within and among sites (range 41-834), although, surprisingly, dramatic changes in abundance were not observed following prolonged dry periods. Seasonal migration patterns of second-year and older adults may help explain lower ratios of large individuals and higher temporary emigration during the latter half of the year, but further study is required. Low numbers of captures and recaptures precluded the use of open-population models to estimate demographic parameters at several sites; therefore, closed-population (or robust-design) methods are generally recommended. Based on observations of their life history and population demographics, E. tonkawae seems well adapted to conditions where spring flow is variable and surface habitat periodically goes dry.

Keywords: Plethodontidae; natural history; reproductive phenology; von Bertalanffy.

Figure 1

Adult Eurycea tonkawae in Bull Creek, Travis County, Texas

Figure 2

Eurycea tonkawae localities and study sites

Figure 3

Histogram of snout–vent length (mm) for all E. tonkawae captures at 16 sites from 2008 to 2015 during surveys in February and March (a), May and June (b), August and September (c), and November and December (d). Values are shown for the highest density modes

Figure 4

Gravidity in E. tonkawae at 16 sites, 2008–2015 (n = 708). (a) Proportion of gravid E. tonkawae captures by month. (b) Histogram of snout–vent length. (c) Proportion of gravid E. tonkawae by size class; total number of gravid individuals observed is indicated above each bar

Figure 5

Abundance ($\widehat{N}$) of E. tonkawae at Lanier, Lower Ribelin, and Wheless springs, 2008–2012. Error bars represent 95% confidence intervals. *indicates estimates following a protracted dry period at the respective site

Figure 6

(a) Per capita recruitment ($\widehat{f}$) (b) quarterly apparent survival ($\widehat{\mathit{\phi }}$) and (c) capture probability (${\widehat{p}}^0$) for E. tonkawae at three sites, 2012–2015. Error bars represent 95% confidence intervals. Estimates for the last period encompassing multiple seasons are not shown

Figure 7

Estimated von Bertalanffy growth curve from hatching (initial SVL = 8 mm) to three years of age for 1,290 E. tonkawae recaptured at ten sites, 2008–2015. The dark solid line indicates age‐at‐length estimated from the mean parameter values for a and k while dashed lines represent age‐at‐length estimated from the upper and lower 95% credence intervals for a and k, respectively. Light gray lines are from 30 randomly generated growth curves from the mean parameter estimates to demonstrate individual variation