Co-varying impacts of land use and non-native brown trout on fish communities in small streams MARK A. KIRK* ,1 , SCOTT A. WISSINGER*, BRANDON C. GOELLER* ,2 AND LESLIE O. RIECK* ,3 *Biology and Environmental Science Departments, Allegheny College, Meadville, PA, U.S.A. SUMMARY 1. Evaluating the biological integrity of stream ecosystems requires a clear understanding of biological responses to anthropogenic stressors. Co-variation of stressors with natural landscape gradients has been shown to complicate the ability of biological assessments to detect community- level responses to anthropogenic stressors. Similarly, the co-varying occurrence of non-native species would also likely confound the ability of biological assessments to accurately determine biological integrity, although these relationships have been less studied. 2. We compared fish communities in 99 wadeable tributaries (with catchments of 10–35 km 2 ) of the upper Allegheny River watershed, Pennsylvania, U.S.A., to disentangle the effects of human-induced stressors associated with agricultural development, the presence of non-native brown trout (Salmo trutta) and background variation in community composition associated with natural landscape features. 3. Multivariate analyses using both taxonomic and trait-based data revealed that environmental gradients sorted fish species into different thermal communities (coolwater versus warmwater). Layered over the shift in thermal communities was the presence of large-bodied (piscivorous) brown trout, which along with agricultural land use, was a consistent determinant of community diversity and presence–absence of nine common native species. Small-bodied taxa (minnows, darters) were the most responsive to the presence of these large non-native predators. 4. Our results highlight how the presence of a non-native species can modify the interactions between natural and anthropogenic stressor gradients and therefore confound the ability to detect signals between land use stressors and local stream community composition. The complicated interaction between brown trout and other anthropogenic stressors documented herein is likely to be a widespread phenomenon given the global introductions of many salmonid species into systems with varying anthropogenic stressor gradients. Overall, understanding how non-native species influence stream community composition should lead to biological assessments with greater capabilities for detecting non-native effects on biological integrity relative to the more frequently evaluated land use stressors. Keywords: biological assessment, brown trout, non-native species, stream fishes, thermal regimes Introduction Stream ecosystems are among the most imperilled sys- tems globally with community-level threats from biodi- versity loss (Xenopoulos & Lodge, 2006; Hawkins et al., 2015), biotic homogenisation (Gido, Schaefer & Falke, 2009; Rahel, 2010) and non-native species (Scott & Helf- man, 1999; Mooney & Cleland, 2001; Gozlan et al., 2010). Perhaps the single greatest threat to stream ecosystems is the intensification of landscape-level development, the cumulative effects of which degrade in-stream conditions at multiple spatial scales (Fausch et al., 2002; Allan, 2004; Correspondence: Mark A. Kirk, Biology and Environmental Science Departments, Allegheny College, Meadville, PA 16335 U.S.A. E-mail: kirkma18@gmail.com 1 Present address: Department of Zoology and Physiology, University of Wyoming, Laramie, WY, U.S.A. 2 Present address: Centre for Integrative Ecology, School of Biological Sciences, University of Canterbury, Christchurch, New Zealand. 3 Present address: School of Environment and Natural Resources, Ohio State University, Columbus, OH, U.S.A. © 2017 John Wiley & Sons Ltd 1 Freshwater Biology (2017) doi:10.1111/fwb.12889