Invasion by Nonnative Brook Trout in Panther Creek, Idaho: Roles of Local Habitat Quality, Biotic Resistance, and Connectivity to Source Habitats

—Theoretical models and empirical evidence suggest that the invasion of nonnative species in freshwaters is facilitated through the interaction of three factors: habitat quality, biotic resistance, and connectivity. We measured variables that represented each factor to determine which were associated with the occurrence of nonnative brook trout Salvelinus fontinalis in Panther Creek, a tributary to the Salmon River, Idaho. Habitat variables included measures of summer and winter temperature, instream cover, and channel size. The abundance of native rainbow trout Oncorhynchus mykiss within sampled sites was used as a measure of biotic resistance. We also considered the connectivity of sample sites to unconfined valley bottoms, which were considered habitats that may serve as sources for the spread of established populations of brook trout. We analyzed the occurrence of small (,150-mm [fork length]) and large ( 150-mm) brook trout separately, assuming that the former represents an established invasion while accounting for the higher potential mobility of the latter. The occurrence of small brook trout was strongly associated with the proximity of sites to large, unconstrained valley bottoms, providing evidence that such habitats may serve as sources for the spread of brook trout invasion. Within sites, winter degree-days and maximum summer temperature were positively associated with the occurrence of small brook trout. The occurrence of large brook trout was not related to any of the variables considered, perhaps due to the difficulty of linking site-specific habitat factors to larger and more mobile individuals. The abundance of rainbow trout was not conclusively associated with the occurrence of either small or large brook trout, providing little support for the role of biotic resistance. Overall, our results suggest that source connectivity and local habitat characteristics, but not biotic resistance, influence the establishment and spread of nonnative brook trout populations. Further work is needed to confirm that the patterns observed here are relevant to other localities where brook trout have invaded and to understand the mechanisms contributing to the invasion process. Since the concern regarding invasive species was raised by Elton (1958), increasing attention has been aimed toward understanding the processes influencing the establishment and spread of biological invasions (Sakai et al. 2001). Hundreds of plant and animal species have been introduced in aquatic ecosystems in North America (Ricciardi and Rasmussen 1998). In freshwaters, Moyle and Light (1996) suggested a series of empirical rules to explain patterns of invasion in freshwaters, noting that habitat suitability and biotic resistance are important components to the success of an invading species. Habitat suitability refers to abiotic resources (e.g., temperature, flow, and chemistry) that satisfy a species’ physiological needs, whereas biotic resistance can result from interactions between an invading species and those in the receiving environment, including competitors, predators, prey, parasites, and pathogens. Moyle and Light (1996) emphasized the importance of habitat suitability over biotic resistance as a key process influencing the success of invasive species. Simply put, if the abiotic environment * Corresponding author: [email protected] 1 Present address: Department of Biological Sciences, Box 8007, Idaho State University, Pocatello, Idaho 83209-8007, USA. 2 Present address: U.S. Forest Service, Rocky Mountain Research Station, 322 East Front Street, Suite 401, Boise, Idaho 83702, USA. Received April 17, 2006; accepted January 19, 2007 Published online June 7, 2007 875 Transactions of the American Fisheries Society 136:875–888, 2007 Copyright by the American Fisheries Society 2007 DOI: 10.1577/T06-115.1 [Article]