Identifying groundwater refugia in the Pacific Northwest and key conservation issues for their associated amphibians

Sky T.C. Button

doi:10.7273/000007029

Groundwater-dependent ecosystems (GDEs) foster a high level of biodiversity, functioning as microrefugia for many habitat specialists that face looming anthropogenic threats. Impacts of these threats are poorly characterized for most species that use GDEs, in part because these species and their preferred GDEs can be challenging to detect and study. These challenges are typified by amphibians, which are biodiverse within GDEs, globally at-risk, and often challenging to detect. In this dissertation, I explore new techniques to study secretive and potentially at-risk amphibians specializing on GDEs (seeps and headwater streams) in the Pacific Northwest (PNW) region of North America, then use the best techniques available to assess their conservation status and needs. In my first chapter, I assess the utility of aquatic environmental DNA (eDNA) surveys for detecting two secretive sister species of salamanders (Plethodon idahoensis and P. vandykei) that use similar moist (yet non-aquatic) habitats along headwater streams and cliff-face seep margins. Surprisingly, I find that—despite strong genetic and ecological overlaps—these species diverge considerably in aquatic eDNA-based detection rates, with P. idahoensis being far easier to detect than P. vandykei. This finding illustrates the potential for subtle interspecific differences (e.g., in population density or microhabitat use) to substantially impact the applicability of aquatic eDNA as a research tool among watercourse-adjacent amphibians. In my second chapter, I resurvey historical P. vandykei localities and find evidence of decline over the past 15-40 years using occupancy models. These declines are linked to low contemporary cover object availability, but their primary cause is unclear from my analyses. I also find that future studies of P. vandykei may benefit from using experienced observers and conducting surveys during periods when soil temperatures are relatively cool. My findings emphasize P. vandykei as a species of high conservation need and point to multiple future monitoring and conservation possibilities. In my third chapter, I develop tools to study seep-associated species (e.g., P. idahoensis and P. vandykei) and identify potentially effective seep climatic refugia, by modeling the distribution and stability of cliff-face seep microrefugia across physioclimatic gradients. I find that these seeps are most likely to occur in steep and relatively low-lying areas, and in the presence of aquifer-generative bedrock types (e.g., basalt). Seep microrefugial properties (water and air temperature stability and perennial surface water) were most strongly associated with north-facing slopes overall. My findings provide encouraging evidence that the occurrence and stability of rare cliff-face seep microrefugia can be effectively estimated and mapped across large areas. Lastly, in my fourth chapter, I use the above seep-related outputs and other biologically-tailored variables to generate species distribution models for four PNW-endemic and GDE-specialist amphibians, including Ascaphus montanus, Dicamptodon copei, P. idahoensis, and P. vandykei. I used biologically-tailored species distribution models (SDMs) to evaluate potential climate change impacts on these four study species. These four species diverged dramatically in potential climate change-mediated trajectories despite all belonging to the same specialist ensembles, highlighting risks of generalizing climate change impacts on purported “indicator species” to broader groups. My projections indicate moderate negative potential climate change impacts on environmental suitability for A. montanus, small-to-moderate negative impacts for D. copei, large positive impacts for P. idahoensis, large negative impacts for P. vandykei in the Cascades Range, and a possibility of either minimal impact or large negative impacts on coastal P. vandykei habitat, depending on whether coastal and Cascades populations share the same fundamental niche. These findings suggest that more species-tailored approaches—and thus better conservation research funding—are needed to understand diverse potential climate change impacts on groups like GDE specialist amphibians.

Identifying groundwater refugia in the Pacific Northwest and key conservation issues for their associated amphibians

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