In urban areas, plant communities are frequently transformed from natural assembleges to biodiverse combinations of native and nonnative species due to commercial availability and homeowner preferences. Yet, the evolutionary novelty of nonnative species make 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.
Results/Conclusions
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.