Effects of Nonnative Plants on Bird Communities in Suburban Forest Fragments

Research Issue

Development of forested areas is occurring rapidly across the United States and many of the remnant forests within suburban landscapes are being fragmented into smaller patches, affecting the quality of this habitat for avian species. Forest fragmentation is linked to the invasion of nonnative plants into the ecosystem. Thousands of nonnative plants have been introduced into the United States since European settlement through landscaping and accidental release. Past studies have repeatedly shown a decrease in bird biodiversity with suburban development and the negative impacts of nonnative plants on individual bird behavior. However, few studies have explicitly examined the link between the density of native plants and avian communities and habitat use. The objective of this project is to study the effects of nonnative plants on bird populations in suburban forest fragments.

 Our Research

We surveyed the birds, plants, and insects in forests with different densities of nonnative plants. Sampling plots were located along edges of forest fragments in suburban areas of Delaware and Maryland. Point counts for birds were conducted three times per season, for a total of six times during the study. Vegetative structure and composition was analyzed within the plots by measuring understory coverage, canopy coverage, and the percentage of stems that were native. Invertebrate biomass was measured within each point by vacuum sampling to estimate the avian food supply. By comparing the forests where species of birds were found to the forests where they were absent, we can identify the factors that most strongly influence habitat choice in different bird species.

Expected Outcomes

As invasive species eradication is often an expensive and labor-intensive process, studies of the impacts of these nonnative plants on native bird species are necessary to ensure that the costs of removal are justified. Small patches of remnant forest are often overlooked by conservation planners in search of larger areas to preserve, but the results of this study may support the value of preserving even small patches.  In addition to suggesting ways that improve habitat within the forest patch, the landscape model produced by this study will provide managers with ways to improve suburban-remnant forest habitat to support biodiversity through management of the surrounding landscape.

Research Results

Field work for this project was conducted in the summer of 2009 and 2010; however only data from 2009 have been analyzed. The preliminary data suggest that although all bird species do not respond to nonnative plants in the same way, some species of birds seem to be responding to the density of nonnative plants when making habitat choices. For example, wood thrush and veery, both species of greatest conservation need, are more likely to be present in a forest with a higher proportion of native plants. Also, the data show a significant relationship between the percentage of plants within a sampling area that are native and the biomass of invertebrates collected from that site.  The relationship was positive, suggesting that as the percentage of native plants at a site increased, so did the biomass of insects collected from the site

Although our research supports that native plant density does affect birds and insects, further research is needed to fully understand how these effects occur. We hope to gain a better understanding of the interactions of birds, invertebrate populations, and plant communities as we continue to analyze our field data. After analysis, our data will be submitted to refereed journals as appropriate.

Conover, A.; Williams, C.K.; D’Amico, V. In press. The impact of nonnative plants on bird communities in suburban forest fragments. In: McManus, K.A., Gottschalk K.W., eds. Proceedings, 21st U.S. Department of Agriculture Interagency Research Forum on Invasive Species, 2010. Gen. Tech. Rep. Newtown Square, PA: U.S. Department of Agriculture, Forest Service, Northern Research Station.

Research Participants

Principal Investigator

• Vincent D’Amico, Research Entomologist, U.S. Forest Service, Northern Research Station 
• Christopher Williams, University of Delaware

Research Partner

• Amanda Conover, University of Delaware

For further details log on website :
https://www.nrs.fs.fed.us/disturbance/invasive_species/nonnative_plants_birds/