Beavers at work: good for native fish?

Walk along a river down in Arizona or New Mexico and you might see fallen trees, distinctive pencil-shaped gnawed stumps, or even a small dam across the stream – all signs of a beaver at work. Beavers, of course, are famous for their ability to build dams and change their environment; but, although beavers in North America range throughout most of the desert Southwest and into northern Mexico, very little is known about their ecological role in desert streams.  Recently, scientists and resource managers have become interested in how beaver activity might contribute to restoration of stream ecosystems. Reintroduction of beaver populations could be particularly valuable in the desert Southwest, where it has been suggested that, by retaining surface water year-round, beaver dams might be able to counteract some of the effects of drought, such as the extreme drought that is currently occurring throughout much of the area.

In the Olden lab, we’re especially interested in what consequences this beaver engineering might have for native fishes. Although beaver ponds usually make great habitat for native trout and salmon here in the Pacific Northwest, we suspect that this is not necessarily true for native fish down in the desert Southwest. Fish native to the lower Colorado River basin are notable for their adaptations to harsh desert conditions. Unfortunately, they’re also a highly endangered group, due in part to widespread introductions of non-native fish. Many of these non-native species are either large predators or superior competitors, and where they spread, populations of native fishes tend to decline. For my Master’s research I’m interested in how beaver ponds might influence this process. The deep pools, slow water, and abundant backwater habitat typically found in a beaver pond could provide ideal habitat for some species of non-native fish. Beaver dams might even be able to help non-native fish survive tough conditions like droughts and floods, conditions which they’re not well adapted to cope with. And anything that’s good for non-native fish could translate into bad news for the native fish.

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Sampling fish with a backpack electroshocker

Last spring I led a research team down to the Verde River in central Arizona to study beaver ponds and desert fish in action. Our goal was to sample the fish communities within beaver ponds, and compare that with the group of fish found in nearby, un-dammed stream reaches. We snorkeled, set traps and nets, and used electrofishing through 38 beaver ponds and stream reaches, working to get as complete as possible a picture of the fish communities. So far we are seeing some differences between the groups of fish that live in beaver ponds and those found in flowing streams. The next challenge will be to figure out whether the different fish communities that we observed within beaver ponds will have consequences for fish in the larger stream system. Are beaver ponds associated with different fish densities elsewhere in the stream? What does this mean for native fish populations? Answering these questions will help answer the question of whether it is always a good idea to reintroduce beaver in desert streams.

-Polly Gibson