Influences of beaver (Castor canadensis) activity on ecology and fish assemblages of dryland streams

Thesis (Master's)--University of Washington, 2013After near-extirpation in the early 20th century, beaver populations are increasing throughout many parts of North America. Simultaneously, there is an emerging interest in employing beaver activity for stream restoration in arid and semi-arid environments (collectively, `drylands'), where streams and adjacent riparian ecosystems are expected to face heightened challenges from climate change and human population growth. However, despite growing interest in reintroduction programs, surprisingly little is known about the ecology of beaver in dryland streams, and science to guide management decisions is often fragmented and incomplete. In my first chapter I systematically reviewed the literature addressing the ecological effects and management of beaver activity in drylands of North America, highlighting conservation implications, distinctions between temperate and dryland streams, and knowledge gaps. Well-documented effects of beaver activity in drylands include changes to channel morphology and groundwater processes, creation of perennial wetland habitat, and substantial impacts to riparian vegetation. However, many hypothesized effects lack empirical evidence, especially from dryland streams. One of the most important areas of uncertainty identified by this review is the influence of beaver activity on the proliferation and success of non-native species. Streams of the American Southwest support a highly endangered native fish fauna and abundant non-native fishes, and in my second chapter I investigated the hypothesis that beaver ponds in this region may lead to fish assemblages dominated by non-native species. I sampled fish assemblages within beaver ponds and within unimpounded stream reaches in the free-flowing upper Verde River basin, central Arizona. I found that although non-native fishes consistently outnumbered native species, this dominance was greater in pond than in stream assemblages. Multivariate analysis indicated that fish assemblages in beaver ponds were distinct from those in stream reaches, in both mainstem and tributary locations. Few native species were recorded within ponds, while some non-natives, notably green sunfish (Lepomis cyanellus) and western mosquitofish (Gambusia affinis) were abundant within ponds. Overall, this study provides evidence that, relative to unimpounded stream habitat, beaver ponds in the Verde River basin support abundant small-bodied non-native fishes, which could have negative impacts on co-occurring native fish populations