Friday, August 6, 2010

PS 103-113: Invasive shrubs predict the abundance of common native avian frugivores in a landscape of central Pennsylvania

Jason M. Gleditsch and Tomás A. Carlo. The Pennsylvania State University


Biological invasions pose one of the most severe threats to global biodiversity. Still, invasions can also entail positive ecological relationships and services, which can sometimes result in conundrums for conservation efforts. A case point is the invasion of alien plants that form key mutualisms with native frugivorous birds, a topic that has received insufficient attention given that many of the worse plant invaders worldwide are frugivore-dispersed plant species. Here we examined the correlation between honeysuckle (Lonicera spp.), a highly invasive alien plant in eastern North America that is bird-dispersed, and the communities of birds in a landscape of Central Pennsylvania. We conducted point counts of both birds and fleshy-fruited plant species in a landscape of 187.5 km2 that included forested, urban, and agricultural lands.


Lonicera fruits accounted for 53.6% of all the fleshy fruits available on the landscape during the fall, with the second most abundant fruit crop (Ligustrum obtusifolium) accounting for 10%. The abundance of birds showed a strong positive association with the presence of Lonicera fruits, with Turdus migratorius and Dumetella carolinensis, two migratory and generalist frugivore species, being the most abundant. The abundance of Lonicera fruits was the strongest correlation with axis-1 of a community-wide ordination (non-metric multidimensional scaling) based on the abundance of bird and plant species in the landscape. In an experiment, we also found that dense areas of Lonicera can accelerate the fruit-removal rates up to 30% for a native plant species when compared to fruit-removal rates in areas with lower densities of Lonicera. Our results suggest that Lonicera currently serves as a main axis for the organization of bird communities and the networks of frugivore-plant interactions in central Pennsylvania. This poses a puzzle for conservation efforts that try to eradicate this plant quickly, as populations of key bird frugivores may be currently depending on Lonicera fruiting resources. We argue that control measures should account for the negative effects that the loss of the most abundant fruit resources could have on populations of native consumers in highly invaded regions.