Density-dependent and density-independent drivers of population change in Barton Springs salamanders

Abstract

Understanding population change is essential for conservation of imperiled species, such as amphibians. Worldwide amphibian declines have provided an impetus for investigating their population dynamics, which can involve both extrinsic (density-independent) and intrinsic (density-dependent) drivers acting differentially across multiple life stages or age classes. In this study, we examined the population dynamics of the endangered Barton Springs Salamander (Eurycea sosorum) using data from a long-term monitoring program. We were interested in understanding both the potential environmental drivers (density-independent factors) and demographic factors (interactions among size classes, negative density dependence) to better inform conservation and management activities. We used data from three different monitoring regimes and multivariate autoregressive state-space models to quantify environmental effects (seasonality, discharge, algae, and sediment cover), intraspecific interactions among three size classes, and intra-class density dependence. Results from our primary data set revealed similar patterns among sites and size classes and were corroborated by our out-of-sample data. Cross-correlation analysis showed juvenile abundance was most strongly correlated with a 9-month lag in aquifer discharge, which we suspect is related to inputs of organic carbon into the aquifer. However, sedimentation limited juvenile abundance at the surface, emphasizing the importance of continued sediment management. Recruitment from juveniles to the sub-adult size class was evident, but negative density-dependent feedback ultimately regulated each size class. Negative density dependence may be an encouraging sign for the conservation of E. sosorum because populations that can reach carrying capacity are less likely to go extinct compared to unregulated populations far below their carrying capacity. However, periodic population declines coupled with apparent migration into the aquifer complicate assessments of species status. Although both density-dependent and density-independent drivers of population change are not always apparent in time series of animal populations, both have important implications for conservation and management of E. sosorum.

Keywords: Eurycea sosorum; MARSS; density‐dependence; density‐independence; endangered species; karst aquifer; long‐term monitoring; time series analysis.

Figure 1

An adult Barton Springs salamander at Eliza Spring (in situ)

Figure 2

Time series of counts of E. sosorum by size class from 2004 to 2014 from Parthenia and Eliza springs. Data from each site are plotted to different y‐axes

Figure 3

Cross‐correlation between counts of E. sosorum and mean monthly discharge of Barton Springs at lags k = 0–15 months. (a) Juvenile (≤ 25 mm TL) abundance at Eliza; (b) Juvenile abundance at Parthenia; (c) Adult (≥ 50 mm TL) abundance at Eliza; (d) Adult abundance at Parthenia. Dashed lines represent 95% confidence limits (calculated at lag k = 1)

Figure 4

Model‐averaged size class interactions and environmental drivers of E. sosorum abundance at Eliza (green lines) and Parthenia (purple lines) from the top two AIC b models (total weight = 0.99). Line weights represent the size of model‐averaged coefficients; dashed and solid lines indicate negative and positive relationships, respectively. Lighter colored lines indicate coefficients that included zero within their 95% confidence intervals