Birds rely on riparian areas of Southeast Utah and the Colorado Plateau. As the ecosystems change along the banks of these rivers, there is real impact on bird populations and species. Dr. Sean Mahoney spoke with Science Moab about his work with birds that inhabit and migrate through the rivers of the Colorado Plateau. We talk about how they are being affected as climate, water levels, and invasive species make their mark on the banks of these rivers.
Science Moab: Where are birds in our landscape? Are birds that live along rivers also traveling away from the rivers and the riparian zone into the desert at all?
Mahoney: There’s an abundance of food in the riparian zone (along rivers), that’s why birds are in that area to begin with. What’s interesting is that some of these upland species away from the river actually come into the riparian area to exploit the food resources that are available. So the riparian habitats are not just important for the birds that are living in them, but also important for the upland community because those birds are moving in, at least for foraging purposes.
Science Moab: I know your work has specifically looked at invasive Russian olive trees in riparian areas. Can you comment on how non-native Russian olive and tamarisk has affected bird species?
Mahoney: Both tamarisk and Russian olive are super common in the western United States, and they’re really common along rivers, particularly in the Southwest. Most of the research focus has been placed on understanding the biology and ecology of tamarisk and trying to understand how tamarisk affects birds. One group of birds that really drops out of tamarisk habitat are woodpeckers. The thought is that tamarisk structurally can’t support the cavity nests that woodpeckers need. But there’s a lot of research that shows tamarisk is suitable habitat for insectivorous (insect-eating) birds. So the response, at least from birds, to tamarisk habitat seems to be species-specific. The impact of Russian olive is less well known and that’s why we sought out to provide some data and focus on how birds respond to Russian olive. The San Juan River was the obvious place to do it because Russian olive is extensive along the river.
Science Moab: What has your study found about how Russian olive is impacting bird communities along the San Juan?
Mahoney: What we found was that sites that were mostly dominated by Russian olive had a lower number of different species and a lower number of functional groups. In our study, we define functional groups as basically birds with different diets. Russian olive-dominated habitats had lower numbers of functional groups relative to these native/non-native mixed sites. We also just looked at the overall species composition of mixed habitats versus Russian olive habitats and found that the species compositions were different between those two.
So what does that mean? We know that Russian olive is a drought-tolerant plant. We also know that the Southwest is becoming hotter and drier and is projected to continue to do that. Given those two pieces of information, we expect Russian olive to continue to naturalize, not just along the San Juan, but along other river systems. Russian olive is also shade tolerant, which means that it can grow in the understory of native cottonwoods and native willows and we know that cottonwoods and willows are drought intolerant. So if we lose those native species, due to increasing drought conditions, Russian olive is going to be in the understory, ready to take advantage of that void. In terms of bird habitat, this suggests that with increasing temperatures in the Southwest, we expect more Russian olive habitats, and because of that, altered bird communities. We know that along river systems are important breeding areas and stopover sites for birds. If we start losing those habitats this could be a real conservation concern for native birds.
Science Moab: So what kind of conservation efforts could happen for these birds if they’re losing this habitat in the face of these large scale changes?
Mahoney: What I repeatedly hear from geomorphologists and hydrologists is that dams have completely altered what native vegetation has evolved for in the Southwest and promote the establishment of invasives, like Russian olive and tamarisk. We can plant as many willows and cottonwoods and remove as much Russian olive and tamarisk as we want, but if those plants don’t receive the water necessary to become established, it’s just going to be a lost cause. The willows and cottonwoods need a lot of water at just the right time of year, in order for seeds to germinate. Promoting policies where we do experimental flows, where we can release a bunch of water that will allow native cottonwoods and willows to germinate is one way where we could keep dams but also promote a really important conservation effort. So I see that as a major challenge for riparian ecology.
This interview has been edited for length and clarity. To learn more and listen to the rest of Sean Mahoney’s interview, visit www.sciencemoab.org/riparian-birds/.
Science Moab speaks with Dr. Sean Mahoney about the birds of the Colorado Plateau