Shrinking salamanders, reproducing refugees, and pliable populations: Drought response of a neotenic amphibian, the Jollyville Plateau salamander (Eurycea tonkawae)

Background/Question/Methods

Amphibians respond to drought in many different ways. For example, drought can induce reproductive failure, migration events, and even result in local extinction. Most studies documenting these effects have focused on biphasic amphibians that require temporary ponds for reproduction. However, droughts may also have profound effects on aquatic species. In this study, we examine the effects of a recent drought on populations of an obligate aquatic salamander, Eurycea tonkawae (the Jollyville Plateau salamander).  Eurycea tonkawae belongs to a clade of neotenic salamanders that inhabit the aquifer waters of the Edwards Plateau in central Texas. Eurycea tonkawae, like other central Texas Eurycea, inhabits both surface (e.g. springs) and subterranean waters (e.g. cave streams). Little is known about how surface populations respond when spring water ceases to flow during periodic droughts, and individuals are either forced to retreat underground with the receding water table, or perish. We used data from a mark-recapture study at two sites (Lanier and Wheless Springs) to examine growth rates (measurements from digital photographs) and changes in population size (estimated using program MARK) during and after an extended dry period, in order to better understand the response of E. tonkawae to natural environmental fluctuations.

Results/Conclusions

Between March 2008 and March 2009, salamanders shrank in size an average of 0.5mm (±0.96mm). This corresponds to an average 2% decrease in body length, and to our knowledge, is a phenomenon previously undocumented in amphibians. Measurement errors for each sampling period were distributed ~N(0,σ²) suggesting that negative growth was not an artifact of data collection. The following year (March 2009- March 2010), spring flow resumed and salamanders grew 3.1mm (±2.38mm) on average. Additionally, estimates of population size were higher (N_Lanier=284±19; N_Wheless=353±87) after an extended dry period, compared to pre-drought population size estimates (N_Lanier=197±14, N_Wheless=185±7). Higher post-drought population sizes may be due to individuals clustering around the springs (our sample area) just after re-emergence from subterranean refugia. We also documented reproduction during this period, as indicated by observations of small juveniles just after flow resumed at Lanier. Reproduction, and a growing (or stable) population size, despite decreasing body length, suggest that E. tonkawae surface populations are resilient to dry surface conditions lasting several months to over a year. Given the frequency of droughts in central Texas and the evolutionary history of central Texas Eurycea, it is unsurprising to learn that E. tonkawae are well adapted to such conditions.