Movement, demographics, and occupancy dynamics of a federally threatened salamander: evaluating the adequacy of critical habitat

Abstract

Critical habitat for many species is often limited to occupied localities. For rare and cryptic species, or those lacking sufficient data, occupied habitats may go unrecognized, potentially hindering species recovery. Proposed critical habitat for the aquatic Jollyville Plateau salamander (Eurycea tonkawae) and two sister species were delineated based on the assumption that surface habitat is restricted to springs and excludes intervening stream reaches. To test this assumption, we performed two studies to understand aspects of individual, population, and metapopulation ecology of E. tonkawae. First, we examined movement and population demographics using capture-recapture along a spring-influenced stream reach. We then extended our investigation of stream habitat use with a study of occupancy and habitat dynamics in multiple headwater streams. Indications of extensive stream channel use based on capture-recapture results included frequent movements of >15 m, and high juvenile abundance downstream of the spring. Initial occupancy of E. tonkawae was associated with shallow depths, maidenhair fern presence and low temperature variation (indicative of groundwater influence), although many occupied sites were far from known springs. Additionally, previously dry sites were three times more likely to be colonized than wet sites. Our results indicate extensive use of stream habitats, including intermittent ones, by E. tonkawae. These areas may be important for maintaining population connectivity or even as primary habitat patches. Restricting critical habitat to occupied sites will result in a mismatch with actual habitat use, particularly when assumptions of habitat use are untested, thus limiting the potential for recovery.

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

“…This was partly a consequence of decisions regarding the study design, such as survey interval (quarterly at most sites) and the area surveyed (limited to areas near spring outlets). However, the tendency for salamanders to migrate from the vicinity of springs (this study, Bendik et al., ), their ability to retreat to subterranean habitats (Bendik & Gluesenkamp, ), and periodic drying of surface habitats likely influenced recapture rates as well. Federal guidelines for surveys of E. tonkawae and two other federally listed congeners ( E. chisholmensis and E. naufragia ) recommend that researchers use an open‐population survey design for estimating demographic parameters (US Fish and Wildlife Service ).…”

“…Based on the study design and the use of photographs instead of physical marks, assumptions 1 through 3 are unlikely to be violated. There is some evidence of size‐biased capture and survival heterogeneity in E. tonkawae (assumptions 4 and 5) from a previous study (Bendik et al., ), although that study included very small juveniles (<16 mm SVL). Small juveniles can be particularly difficult to capture and identify reliably, and have lower survival rates compared to larger animals (Bendik et al., ); therefore, I excluded those individuals from the capture‐recapture analysis.…”

“…There is some evidence of size‐biased capture and survival heterogeneity in E. tonkawae (assumptions 4 and 5) from a previous study (Bendik et al., ), although that study included very small juveniles (<16 mm SVL). Small juveniles can be particularly difficult to capture and identify reliably, and have lower survival rates compared to larger animals (Bendik et al., ); therefore, I excluded those individuals from the capture‐recapture analysis. The last assumption is the most difficult to meet given prior evidence of movement and temporary emigration at some sites (Bendik et al., ; O'Donnell & Gluesenkamp, ).…”

“…Data are also included from other studies where individual salamanders were identified. This includes a four‐month open‐population capture‐recapture study of movement (Bendik et al., ) and a study of hormone levels (Gabor et al., ). Finally, nine sites were regularly sampled once per quarter during February/March (winter), May/June (spring), August/September (summer), and November/December (fall).…”

“…The first published account on the ecology of E. tonkawae was by Bowles, Sanders, and Hansen (2006), who examined monthly count data from a two-year period to assess relationships between relative abundance, habitat characteristics, and urbanization. Recent studies have extended this work, including estimation of demographic parameters (O'Donnell & Gluesenkamp, 2009), relative abundance in the context of land-use patterns (Bendik, Sissel, Fields, O'Donnell, & Sanders, 2014), and movement and occupancy within headwater streams (Bendik, McEntire, & Sissel, 2016).…”