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Effects of an introduced tree, New Mexico locust (Robinia neomexicana), on riparian birds




Stinson, Lani Treadway, author
Pejchar, Liba, advisor
Clements, William H., committee member
Hufbauer, Ruth A., committee member

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Biological invasions are global drivers of environmental change and riparian ecosystems are particularly susceptible to the effects of non-native species. While much research has focused on understanding the impacts of non-native introductions from other biogeographic regions, effects of plant species that spread close to or within their native ranges are relatively understudied. My research investigated the effects of the near-range introduction of a non-native woody plant, New Mexico locust (Robinia neomexicana), on a variety of ecological responses within the Clear Creek drainage of northwestern Colorado, USA. I used riparian songbirds as model species to investigate invasion-mediated effects on 1) changes to avian habitat use, including species richness, occupancy, and abundance; 2) effects on songbird reproductive success, including nest survival and productivity; and 3) changes to aquatic insect subsidies and avian diet compositions. My first chapter provides a general synthesis of fitness consequences of plant invasion through a global systematic review of introduced plant effects on songbird reproductive success. Only 16% of the 137 songbird responses examined resulted in significant effects, and these were predominately negative and highly context-dependent. However, non-significant trends were much more prevalent and mixed in direction. The literature review highlighted the lack of study on fitness-level impacts of invasion, uncovered a strong geographic bias for North American studies, and identified knowledge gaps, such as the lack of studies on juvenile survival during the postfledging period. In chapter two, I examine the effects of New Mexico locust invasion on the habitat use and reproductive success of breeding songbirds in the Clear Creek drainage of northwestern Colorado. Despite evidence of biotic homogenization of the vegetation community with increasing invasion intensity, I found few negative impacts on the songbird community. Rather, the introduced locust provided quality breeding habitat to support diverse assemblages of riparian birds. Not only did I find increased bird species richness with increasing invasion intensity, but nest productivity was significantly higher in invaded habitats compared to those where locust was absent. Overall, I found that environmental characteristics other than invasion (i.e., elevation and shrub density) were more important predictors of avian habitat use. There was also no evidence of reduced nest survival in nests built in locust or in nests in native plants situated in locust-invaded habitats. My third chapter, a collaboration with Colorado State University M.S. student Hannah Riedl, uses stable isotope analysis of avian fecal samples and insect prey to investigate invasion-mediated changes in the aquatic resource subsidies provided to avian insectivores. We quantified the amount of terrestrial- and aquatic-derived insect prey consumed by a riparian songbird assemblage and assessed whether relative prey contributions differed between non-invaded reference sites and sites invaded by New Mexico locust. We found that songbird diets were comprised of approximately one third aquatic resources and two-thirds terrestrial resources, highlighting the importance of aquatic resource subsidies for riparian consumers. Although differences in diet composition between reference and locust habitats were highly species-specific and inconsistent across years, we found significant diet shifts towards more aquatic-derived prey at locust sites for two species of avian insectivores. Collectively, my findings provide little support for negative impacts on riparian bird communities associated with the near-range introduction and establishment of New Mexico locust within Clear Creek drainage. Similar or stronger effects might be expected for invasive alien species introduced from more geographically isolated native ranges. Combining multiple ecological response variables to obtain a more complete and mechanistic understanding of invasion impacts is critical for advancing invasion biology and facilitating effective conservation of native communities.


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