Biocontrol agents, such as insects, are often released outside of their native ranges to control invasive plants. …As early as 1971, U.S. scientists began releasing gallflies in an effort to reduce populations of invasive [spotted knapweed]. Like all biocontrol agents, the gallflies were selected because of their specificity to their host plant, leaving little risk of direct harm to other plants…
Scientists and managers expected that this seed deficiency would lead to limited knapweed population growth. An unanticipated side effect, however, involves the flies’ furry neighbors. At the foot of the Sapphire Mountains in western Montana, omnivorous deer mice, whose diet usually consists of native seeds and insects, have also begun to prey on the introduced gallflies.
The abundance of knapweed leads to lots of gallfly larvae, which serve as a food subsidy for the mice. Pearson and his coauthor, Ragan Callaway of the University of Montana, found that this extra nourishment bolsters mouse population size, increasing the numbers of hungry mice feeding on their original source of food: the seeds of native plants. As mouse consumption of native plant seeds increases, fewer native plants survive past the seed stage.”
Full reference to the paper:
Pearson DE, Callaway RM (2008) Weed-Biocontrol Insects Reduce Native-Plant Recruitement through Second-Order Apparent Competition. Ecological Applications: Vol. 18, No. 6 pp. 1489–1500.
A very nifty study, if a bit depressing. It’s really impossible to do just one thing when dealing with ecological systems. The complex community interactions among mice, native and invasive plants, insects, and seeds in this study is a nice demonstration of that principle.
More info on Spotted Knapweed: