Abstract
Amphibian declines have been reported in mountainous areas around the western USA. Few data quantify the extent of population losses in the Pacific Northwest, a region in which amphibian declines have received much attention. From 2001–2004, we resurveyed historical breeding sites of two species of conservation concern, the Western Toad (Bufo [=Anaxyrus] boreas) and Cascades Frog (Rana cascadae). We detected B. boreas breeding at 75.9% and R. cascadae breeding at 66.6% of historical sites. When we analyzed the data using occupancy models that accounted for detection probability, we estimated the current use of historically occupied sites in our study area was 84.9% (SE = 4.9) for B. boreas and 72.4% (SE = 6.6) for R. cascadae. Our ability to detect B. boreas at sites where they were present was lower in the first year of surveys (a low snowpack year) and higher at sites with introduced fish. Our ability to detect R. cascadae was lower at sites with fish. The probability that B. boreas still uses a historical site for breeding was related to the easting of the site (+) and the age of record (-). None of the variables we analyzed was strongly related to R. cascadae occupancy. Both species had increased odds of occupancy with higher latitude, but model support for this variable was modest. Our analysis suggests that while local losses are possible, these two amphibians have not experienced recent, broad population losses in the Oregon Cascades. Historical site revisitation studies such as ours cannot distinguish between population losses and site switching, and do not account for colonization of new habitats, so our analysis may overestimate declines in occupancy within our study area.
Highlights
Amphibian declines have been reported from many parts of the world, with concentrations in Australia, Central America, and western North America [1]
Our objective was to estimate the proportion of historical breeding sites that were currently used for breeding by B. boreas and R. cascadae
We found evidence for effects of YEAR1 and FISH on detectability of B. boreas (Table 2)
Summary
Amphibian declines have been reported from many parts of the world, with concentrations in Australia, Central America, and western North America [1]. Declines in species from mountainous areas such as the western USA have been difficult to quantify and their causes can be complex [2,3,4]. The Cascade Range spans the latitudinal length of the northwestern USA, and is an example of a montane region that has not experienced the direct anthropogenic disturbance present in most western lowlands, but where amphibian declines have been reported [2,3,5,6]. Most of the research on putative amphibian declines in the Cascade Range has focused on experimental investigations of hypothesized stressors such as UV radiation (e.g., [3]). One approach to quantifying amphibian declines is to resurvey sites of known historical occurrence to assess whether the species remains present [7,8]. Studies of historical sites have helped clarify the scope of declines in some other western amphibians [2,4,9,10,11]
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