An introduced plant affects the structure and function of riparian food webs
Date
2017
Authors
Riedl, Hannah, author
Clements, William, advisor
Pejchar, Liba, advisor
Kondratieff, Boris, committee member
Fausch, Kurt, committee member
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Abstract
Aquatic and terrestrial ecosystems are closely linked by the reciprocal flux of resource subsidies, but these dynamics can be uncoupled by anthropogenic change. Introduced species are easily spread by riverine systems, but the effects of non-native riparian vegetation on reciprocal arthropod subsidies are essentially unknown. I studied the aquatic and terrestrial arthropod communities, and their flux into and out of the stream channel, in stream reaches in northwestern Colorado invaded and uninvaded by New Mexico locust (Robinia neomexicana A. Gray), a woody plant north of its native range. I also evaluated whether the aquatic- and terrestrial-derived diets of riparian songbirds were altered in stream reaches with New Mexico locust. I found that reaches with New Mexico locust had fewer terrestrial arthropods collected from vegetation, particularly in spring. Consistent with these results, some songbird species using invaded sites displayed diet shifts towards aquatic-derived resources. Aquatic resources comprised approximately 34% of the songbird assemblage's diet, which highlights the importance of aquatic subsidies to riparian consumers. In contrast, there were no impacts of New Mexico locust on the aquatic insect community associated with the invasion. Overall, seasonal and annual variation best described patterns of arthropod subsidy production and consumption. Since locust invasion did not drive strong patterns in insect communities, and because seasonal and annual factors provide little information about environmental drivers of resource subsidies, we also assessed the importance of vegetation and stream characteristics on aquatic and terrestrial arthropod communities. I found terrestrial insect biomass and richness increased with percent ground cover, forb cover, and vertical vegetation structure, and decreased with percent cover of New Mexico locust. Interestingly, vegetation characteristics best described emergence patterns of adult aquatic insects, but stream characteristics best described larval aquatic insect patterns. Our results support the use of a multi-functional approach to assessing ecosystem alteration. By evaluating the multiple pathways through which a non-native species can affect riparian systems, I found a close connection between stream and riparian ecosystems. I found the impacts of invasion were most apparent for arthropod metrics directly related to vegetation (i.e., arthropods collected from foliage), with weaker responses for indirect metrics (i.e., aquatic insect biomass, songbird diet components).
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Subject
food webs
non-native
stable isotopes
insect subsidies
ecosystem function
riparian ecology