Disentangling effects of invasive species and habitat while accounting for observer error in a long‐term amphibian study

Abstract

The invasive American bullfrog (Lithobates catesbeianus) and a variety of non‐native sport fish commonly co‐occur in lowland lentic habitats of the western United States. Both invasive taxa are implicated in declines of native amphibians in this region, but few long‐term studies of communities exist. Further, field studies of invasive–native interactions are complicated by confounding habitat modifications and observation errors. We surveyed amphibians and measured habitat characteristics for 12 yr across 38 wetland sites within the Willamette Valley, Oregon, USA. We assessed the influence of invasive species, habitat, and their interactions on the distributions of five native amphibian species using a multispecies dynamic occupancy model that accounted for false‐negative and false‐positive detections. In general, habitat characteristics—such as within‐pond vegetation cover, surrounding forest, and drought severity—were important for local persistence of native species when bullfrogs co‐occurred. We also found evidence of a cumulative negative effect of bullfrogs and non‐native fish (families Centrarchidae and Ictaluridae) on northern red‐legged frog (Rana aurora) local persistence that was mediated by the dominance of invasive reed canarygrass (Phalaris arundinacea). Non‐native fish and bullfrogs had variable effects on native amphibian species, but neither invasive taxon appears to be causing declines in occupied sites within our study area. Moreover, species relationships with habitat differed when invaders were present, indicating that certain habitats may increase persistence of native amphibians in the invaded landscape.

Floodplains provide important amphibian habitat despite multiple ecological threats

“…While bullfrogs have been implicated with amphibian declines, much of this work was correlative and potentially explained by confounding variables, such as non-native fish or disease (Hayes and Jennings 1986), or was experimental (Lawler et al 1999, Adams 2000, which may amplify effects not seen in the wild. In field studies within the Pacific Northwest, most studies have found no effect of bullfrog presence on native amphibian species occurrence (Richter and Azous 1995, Adams et al 1998, 2011, Adams 1999, Pearl et al 2005, but see Rowe et al 2019). It is possible that our native amphibian assemblage was not as susceptible to bullfrogs due to their use of terrestrial habitats post-metamorphosis.…”

“…, but see Rowe et al. ). It is possible that our native amphibian assemblage was not as susceptible to bullfrogs due to their use of terrestrial habitats post‐metamorphosis.…”

“…, Rowe et al. ). Emergent vegetation can provide refuge by reducing the foraging efficiency of fish predators (Diehl , Sass et al.…”

“…, Holzer and Lawler ) and could actually benefit native amphibians by providing refuge habitat from non‐native predators (Rowe et al. ). Yet other effects of reed canary grass on wetland ecosystems remain unknown; for instance, selected plant traits (e.g., lignin, phenolics, and nutrient ratios) may affect amphibian performance (Cohen et al.…”

“…Based on areal cover, intermediate levels of emergent vegetation cover increased occupancy for all native amphibian species. A number of other studies have highlighted emergent vegetation as important to the same suite of native amphibians (Monello and Wright 1999, Pearl et al 2005, Adams et al 2011, Rowe et al 2019. Emergent vegetation can provide refuge by reducing the foraging efficiency of fish predators (Diehl 1988, Sass et al 2006 or maintaining cooler water temperatures (Miller and Fujii 2010) and can support abundant highquality food resources (Cattaneo andKalff 1980, James et al 2000).…”