Ehrenfeld Award 2018
The success of conservation depends on the ability to balance the needs of both people and wildlife; thus, it is imperative that we create sustainable landscapes that can be shared with biodiversity to help curb severe losses being experienced around the globe. Residential yards present a unique opportunity to simultaneously support urban wildlife and connect people to nature, but more work is needed to determine how specific features of these landscapes influence habitat quality. One major decision that homeowners can make is what kinds of plants to landscape their properties with; specifically, whether to plant native or nonnative species. For my PhD, I studied the tritrophic relationship between plants, arthropods and birds to determine whether landscaping with nonnative plants affects habitat quality for breeding birds in residential landscapes. For this research, I worked with Dr. Doug Tallamy at University of Delaware, Dr. Peter Marra at the Smithsonian Migratory Bird Center, Dr. Tomas Carlo at Pennsylvania State University, and a network of homeowners across the DC metropolitan area. The results of my dissertation demonstrate that yards landscaped with nonnative plants reduce food resources available for insectivorous birds, and food limitation results in negative consequences for both individual condition and reproductive performance. These results suggest that nonnative plants may in part explain the tendency of diet specialists to disappear in urban ecosystems and that landscaping with native plants will help restore ecological function of urban landscapes.
At the 2018 ESA meeting in New Orleans, I presented a chapter of my dissertation that used an experimental approach to investigate how nonnative plants affect trophic contributions to a wider bird and insect community. In urban ecology, much of our inference has been based on observational studies, with few experimental approaches to identify mechanisms. I addressed this research gap by using a stable isotope tracer in a large and novel manipulative experiment to track nitrogen transfer from native and nonnative trees to consumers within residential yards. I found that while nitrogen contributions to arthropods were similar between native and nonnative trees, biomass was lower, resulting in higher nitrogen contributions from native plants to both facultative and obligate insectivorous birds. This study not only experimentally demonstrates that native plants contribute more energy to vertebrate consumers, but to our knowledge is also the first study to use a stable isotope tracer to track trophic contributions to a mobile consumer in an open system.
A 15N isotope tracer experimentally demonstrates that nonnative plants limit energy flow to higher order consumers in residential yards
In urban areas, plant communities are frequently transformed from natural assemblages to biodiverse combinations of native and nonnative species due to commercial availability and homeowner preferences. Yet, the evolutionary novelty of nonnative species makes them poor hosts for herbivorous arthropods. Because of this, nonnative plants may limit food availability for consumers by reducing important arthropod prey biomass; however, no study has directly compared the contributions of native and nonnative trees to trophic interactions.
To test whether more energy is transferred to the terrestrial food web from native plants, we used a foliar-applied 15N isotope tracer to enrich leaves on either native or nonnative species at sixteen individual yards. Following the nitrogen application, we sampled arthropod communities to compare biomass of available prey and used stable isotope analysis to determine 15N of different trophic groups. Finally, we analyzed blood from locally breeding birds to determine whether more 15N was observed in higher trophic levels when only native trees were enriched.
We found that following our treatment, the proportion of 15N in leaves, caterpillars and spiders increased dramatically (up to 116% higher) and was not significantly different between native and nonnative trees (p>0.1). Spider 15N was highly variable, possibly due to the difference in nitrogen enriched prey between web-building and free hunting species. Caterpillar, spider and total arthropod biomass was consistently higher on native trees across the season thereby limiting the availability of nitrogen-enriched prey for consumers from nonnative trees. Consequently, we also found higher proportions of 15N in blood samples from birds occupying yards where native trees were treated for 5 out of 6 focal species. This study provides direct evidence that native trees supply disproportionately more nitrogen to bird diets via arthropod prey items for both obligate and facultative insectivores. These results unequivocally show that homeowners can create foundations for local food webs, and increase arthropod prey for breeding birds, by prioritizing native plants in residential landscapes.