Concerns regarding the use of amphibians as metrics of critical biological thresholds: a comment on Welsh & Hodgson (2008)

Abstract

Welsh & Hodgson (2008) argued that stream-associated amphibians (Ascaphus truei, Dicamptodon tenebrosus and Rhyacotriton variegatus) are reliable indicators of ecosystem health in the Pacific Northwest (PNW), U.S.A. We demonstrate that their assertions rely on circular reasoning as well as logical and empirical assumptions that have not received rigorous evaluation. 2. Welsh & Hodgson (2008) based their arguments on data collected in northwestern California, U.S.A. However, all three taxa occur across a wide range of environmental gradients in the PNW, and other research results suggest that the taxa respond differentially to environmental perturbations across their geographical distribution. Hence, differences in abundance are not always reliable indicators of the state of an ecosystem, as population numbers can be influenced by factors that may not be associated with habitat quality (e.g. predation and disease). (2008) present indices (rather than estimates) of abundance, which contain an unquantifiable amount of uncertainty not reliably attributable to either environmental or management factors. Evaluating parameters such as reproductive success and survival are more likely to provide strong inference about factors influencing population size than correlative studies that associate some measure of relative abundance with individual habitat features. 4. We argue that direct measurement of physical features (water temperature, sediment and wood), for which Welsh & Hodgson (2008) suggest using amphibians as surrogates, is more accurate, efficient and cost effective compared with monitoring cryptic species with methods that are unable to quantify spatial and temporal uncertainty in biological responses. 5. Headwater streams are critical components of catchments in the PNW, and all three amphibian taxa show strong associations with low-order streams. However, we think research and monitoring of headwater streams should be guided by hypotheses that evaluate causal mechanisms, rather than relying on descriptive patterns that are limited in geographical scope and which support only weak inference about ecosystem processes. Other studies provide direct links between physical features and responses of target populations (i.e. fish). Adding a second correlative link (i.e. amphibians) provides no Correspondence: A. J. Kroll, Welsh & Hodgson

Stream amphibians as metrics of critical biological thresholds in the Pacific Northwest, U.S.A.: a response to Kroll et al.

“…health) of these networks in terms of their ability to sustain native fauna that evolved in these clear, cold water systems. Kroll et al. (in press) agreed with our contention that the approach of using animal metrics is conceptually well established, but argued that it is poorly developed.…”

“…We recently proposed using three amphibians as metrics of critical biological thresholds in headwater streams of the Pacific Northwest (PNW) of North America (Welsh & Hodgson, 2008), an idea that has prompted concerns and criticism from timber industry biologists and a state Fish and Wildlife biologist (Kroll, Hayes & MacCracken, in press; see also Welsh, Pope & Wheeler, 2009). We welcome the opportunity to address their concerns here and to bring greater focus to the related issues of habitat alteration and fragmentation in the stream networks of the PNW.…”

“…Kroll et al. (in press) noted three objectives for their comments; the first was to address four assumptions they deemed we failed to evaluate empirically in our paper (Welsh & Hodgson, 2008).…”

“… Summary 1. Kroll, Hayes & MacCracken (in press) Concerns regarding the use of amphibians as metrics of critical biological thresholds: a comment on Welsh and Hodgson 2008. Freshwater Biology , criticised our paper [Welsh & Hodgson (2008) Amphibians as metrics of critical biological thresholds in forested headwater streams of the Pacific Northwest.…”

Response of stream salamanders to experimental drought in the southern Appalachian Mountains, USA

“…Plethodontid salamanders can be indicators of habitat integrity in their respective ecosystems (Welsh & Droege 2001;Southerland et al 2004;Welsh & Hodgson 2008; but see Kroll et al 2009). The southern Appalachian Mountains are a global hotspot for plethodontid salamander diversity, but this diversity may become increasingly threatened by altered precipitation regimes as a result of global climate change (Bates et al 2008;Milanovich et al 2010).…”