Sub‐alpine amphibian distributions related to species palatability to non‐native salmonids in the Klamath mountains of northern California

Abstract

The goal of this study was to examine how introduced trout influence the distributions and abundances of a sub‐alpine amphibian assemblage whose members display a variety of different life‐history and defence strategies. Our study was conducted in the sub‐alpine lentic habitats of three wilderness areas that form the core of the Klamath‐Siskiyou Bioregion of northern California, a biodiversity ‘hotspot’ that supports the highest diversity of sub‐alpine, lentic‐breeding amphibians in the western USA. These wilderness areas contain no native fishes, but all have been populated with non‐native trout for recreational fishing. Five of the eight amphibian species that occur in this region were sufficiently common to use in our study; these included one that breeds in both temporary and permanent waters and is palatable to fish (Pacific treefrog, Pseudacris regilla), two that breed primarily in permanent waters and are unpalatable to fish (western toad, Bufo boreas, and rough‐skinned newt, Taricha granulosa), and two that breed primarily in permanent waters and are palatable to fish (Cascades frog, Rana cascadae, and long‐toed salamander, Ambystoma macrodactylum). Based on life histories and predator defence strategies (i.e. palatable or not), we predicted that the three palatable species would likely be negatively correlated with introduced trout, but with P. regilla less impacted because of its use of both temporary and permanent waters. We predicted that B. boreas and T. granulosa would not be significantly correlated with introduced trout due to the lack of any predator/prey interactions between them. We surveyed 728 pond, lake, or wet meadow sites during the summers of 1999–2002, using timed gill‐net sets to measure trout occurrence and relative density, and visual encounter surveys to determine amphibian presence and abundance. We used semiparametric logistic regression models to quantify the effect of trout presence/absence and density on the probability of finding amphibian species in a water body while accounting for variation within and among the various lentic habitats sampled. The distributions of P. regilla, A. macrodactylum and R. cascadae were strongly negatively correlated with trout presence across all three wilderness areas. Ambystoma macrodactylum was 44 times more likely to be found in lakes without fish than in lakes with fish. Rana cascadae and P. regilla were 3.7 and 3.0 times more likely, respectively, to be found in fishless than fish‐containing waters. In contrast, the two unpalatable species were either uncorrelated (T. granulosa) or positively correlated (B. boreas) with fish presence. We found that the relative density of fish (catch per unit effort) was negatively correlated with the combined abundances of the three palatable amphibians, and also with both the length and the condition of the fish themselves. Our results are consistent with a compelling body of evidence that introduced fishes greatly alter the aquatic community structure of mountain lakes, ponds, and wet meadows.

Variation in reciprocal subsidies between lakes and land: perspectives from the mountains of California

“…Surveys in the Sierra Nevada have shown that lakes with trout are less likely to be occupied by ranid frogs (Knapp and Matthews 2000;Knapp et al 2003;Knapp 2005) and Pacific chorus frogs Knapp 2005). Similar patterns were observed in the Klamath Mountains, where Cascades frogs (R. cascadae), Pacific chorus frogs, and long-toed salamanders were found to be negatively associated with fish (Welsh et al 2006). The findings of these surveys have been reinforced by fish removal experiments, which have shown that the removal of trout leads to the recovery of ranid frog populations in both the Sierra Nevada (Vredenburg 2004;Knapp et al 2007) and the Klamath (Pope 2008) mountains.…”

Community ecology of invasions: direct and indirect effects of multiple invasive species on aquatic communities

“…The SEM did not support an alternative mechanism, whereby the increased toad growth was due to enhanced phytoplankton abundance in the presence of mosquitofish and we are unaware of a direct interaction where mosquitofish would promote toad tadpoles. There is evidence to suggest that nonnative fish also promote toads in nature; Welsh et al (2006) found western toads to be six times more abundant in the presence of nonnative salmonids in lake and pond habitats in northern California. To date, there are relatively few studies documenting this type of facilitation of native species by invaders through competitive release (e.g., Grosholz 2005).…”

Non‐native salmonids affect amphibian occupancy at multiple spatial scales

“…AICc, Akaike information criterion; SITES, number of water bodies per catchment; AREA, size of the catchment; aELEV, average elevation of water bodies in catchment; aDIST, average distance to nearest water body within catchment; FISHSTATUS, presence of non-native salmonids in at least one water body per catchment. & Marx, 2002;Welsh et al, 2006;Pearson & Goater, 2008). The habitat requirements of fish and A. macrodactylum overlap at high elevations in the northern Rocky Mountains (Howard & Wallace, 1985;Pilliod & Peterson, 2001;Pearson & Goater, 2008).…”

“…Both species respond to predatory fishes where they co-occur with them (Tyler et al, 1998a;Pilliod & Peterson, 2001;Bull & Marx, 2002;Welsh et al, 2006;Pearson & Goater, 2008), but differences in life history characteristics between A. macrodactylum and R. luteiventris could result in different sensitivities to predatory fish. Both amphibians breed in lakes and ponds ranging from sea level to nearly 3000-m elevation.…”