The effects of nonnative plants on food webs in residential landscapes

Abstract

One of the most rapidly expanding, and least understood, ecosystems is the urban landscape. Urban-associated changes to the biological and physical environment can have cascading impacts on the ability of these landscapes to support biodiversity. One major way that these landscapes have changed is through the individual decisions of homeowners on which plant species to maintain on privately-owned, residential land. In my dissertation, I investigated whether nonnative plant species affected the tritrophic interactions between plants, foliage arthropods and insectivorous birds at three scales. In chapter 1, I speak to the importance of residential yards to the future of conservation, and call for more research in these ecosystems. In chapters 2, 3 & 4, I use the Carolina chickadee (Poecile carolinensis) as a model insectivorous bird to determine the effects of nonnative plants on chickadee behavior, diet and population growth. In chapter 5, I estimated the energy contributions of nonnative plants to predatory spiders and a broader insectivorous bird community. In chapter 2, I determined which plants produced the most caterpillar prey for birds and whether chickadees displayed preferences for some plant species over others. I also identified whether the proportion of nonnative plant biomass predicted chickadee occupancy, abundance or breeding probability. Native plants provided more caterpillar prey than nonnative species even when controlling for plant genus. In addition, chickadees preferred foraging in native plants that supported the most caterpillars and bred in yards that had the highest proportion of native plant biomass. In chapter 3, I combined insect sampling, diet analysis, and used estimates of reproduction, adult survival and fledgling survival in a population growth model to determine whether yards dominated with nonnative plants were capable of supporting sustainable populations of chickadees. My models indicated that yards with >30% nonnative plant biomass had fewer prey items, chickadee diets were composed of more predatory arthropods and these yards did not support sustainable chickadee populations. In chapter 4, I questioned whether nonnative plants influenced the nestling period for chickadees. I specifically tested the effects of nonnative plants on nestling diet, parental effort and nestling growth & condition. I found that as nonnative plants increased, the proportion of caterpillar prey declined, and provisioning visits increased. Consequently, chickadee nestlings in nonnative yards were also in poorer condition, grew slower and required more days to fledge. In chapter 5, I used a nitrogen enrichment experiment to test whether more energy is transferred to food webs from native plants compared to nonnatives. I found that nitrogen enrichment in caterpillars and spiders were similar regardless of the origin of treated plants, however, total biomass was lower on nonnative plants. At the next trophic level, both facultative and obligate insectivorous birds received more nitrogen enrichment when native plants were treated compared to nonnative, indicating these species were acquiring more prey from native plants. Overall, my dissertation indicates that for a plant-arthropod-insectivorous bird food web, nonnative plants are not ecologically equivalent to native plant species that are displaced. My results provide compelling evidence that homeowners should prioritize native plant species in residential landscapes in order to support local food webs and biodiversity.