Influences of Wildfire, Habitat Size, and Connectivity on Trout in Headwater Streams Revealed by Patterns of Genetic Diversity

—Wildfire is an important natural process in many stream ecosystems, but the ability of fish to respond to wildfire-related disturbances is increasingly constrained by human activities that fragment and degrade stream habitats. In this study, we used molecular genetic markers (nuclear microsatellites) to examine the effects of wildfire and related disturbances along with habitat fragmentation on native rainbow trout in the Boise and Payette River basins, Idaho. We surveyed the genetic diversity of fish in 55 tributary streams to compare the level of diversity in samples without the recent influence of wildfire with that of those influenced by stand-replacing wildfire and those influenced by both wildfire and a severe channel-reorganizing disturbance. Stream habitats also varied substantially in size (catchment basin area) and isolation caused by road culverts. Based on prior work in our study streams, we expected that both wildfire and channel reorganization would reduce local population sizes significantly. Accordingly, we expected that wildfirerelated disturbances would reduce genetic diversity via founder effects or population bottlenecks. Our results, however, showed little evidence of these influences. In contrast, the level of genetic diversity was lower in fish collected upstream of culvert barriers, probably because of restricted gene flow. We also observed the expected positive correlation between habitat size and genetic diversity, which suggested the importance of larger local population sizes and habitat diversity in maintaining genetic diversity. An unexpected finding was that 15 of the 55 samples showed genetic evidence of hybridization between rainbow trout Oncorhynchus mykiss and nonnative cutthroat trout O. clarkii. The results of this study suggest that human influences such as barriers to dispersal and introductions of nonnative fish may pose greater threats to populations of native trout than wildfire itself. In western North America, wildfire can be a major driver of disturbance in headwater streams (Reeves et al. 1995; Rieman and Clayton 1997; Gresswell 1999). Though these disturbances can be dramatic and in some cases devastating to stream habitats and species in the short term, the prevailing view is that wildfirerelated disturbances are an important contributor to the natural functioning of stream ecosystems over longer time frames (Bisson et al. 2003; Minshall 2003). The loss of vegetation and decreased infiltration capacity of the soil that accompany wildfire can render small, steep headwater streams particularly vulnerable to extreme flooding and debris flows, leading to channel-reorganizing disturbances (Benda et al. 2003; Miller et al. 2003; Wondzell and King 2003). In some instances, these events have led to the local extirpation of fish (Rieman and Clayton 1997; Brown et al. 2001; Minshall 2003; Burton 2005; Sestrich 2005). Over time, populations may recover as habitat conditions * Corresponding author: [email protected] Received August 5, 2008; accepted May 26, 2009 Published online October 1, 2009 1314 Transactions of the American Fisheries Society 138:1314–1327, 2009 Copyright by the American Fisheries Society 2009 DOI: 10.1577/T08-162.1 [Article]