Although exotic plant invasions are known to threaten native plants, surprisingly little research has evaluated the specific impacts of these invasions on wildlife. In order to fully understand the impacts of exotic weeds and protect native species in the face of current weed threats, land managers need to understand which species are affected by weed invasions, to what extent these species are being affected, and the mechanisms by which weeds impact these populations. The present research on songbirds provides an example of how we are approaching this problem and what new information is arising from our work.
When weeds invade, wildlife may be affected in simple direct ways through loss of critical habitat components such as nesting sites or in more complex indirect ways resulting from the loss of critical food resources dependent on native plants displaced by weeds. Understanding the process by which weeds impact native species is critical for managers to predict which wildlife species will be most affected by various weed invasions without having direct information on every wildlife-weed interaction. Such understandings also provide the necessary information to mitigate weed impacts on wildlife by delineating which critical life history stages of a species are threatened. To explore this problem for native songbirds, we have focused our research on chipping sparrows (Spizella passerina). This migrant species is common in open forest habitats during the breeding season, where it nests in trees and forages for seeds and insects on the ground. It is a model species for understanding the more complex indirect effects of weeds on songbirds because its nest-site requirements are met by trees and removed from the direct influence of weeds. Therefore, impacts of weeds on this species would likely be due to indirect effects of weeds on their food sources rather than direct effects of weeds on nesting sites. We are evaluating potential impacts of weeds on songbirds by studying open forest habitats of western Montana that are heavily invaded by spotted knapweed (Centaurea maculosa), one of the most aggressive and widespread weeds in western North America.
Our research shows that chipping sparrows exhibit delayed breeding, diminished productivity, and reduced site fidelity in knapweed-invaded habitats compared to uninvaded but otherwise similar habitats (Ortega et al. 2006). Because this species forages primarily on insects associated with grassland vegetation but nests in trees, these results suggest that knapweed invasion indirectly affects chipping sparrows by impacting their food supply rather than by directly degrading their nesting sites. Data gathered on invertebrate populations in this study show that grasshoppers and other important insects significantly decline with knapweed invasion, supporting the contention that waning food resources are driving impacts of knapweed on chipping sparrows. This research not only establishes that noxious weeds can impact songbird populations, but also helps to build understandings of the mechanisms by which plant invaders can affect native fauna. Since these insects are important foods for many wildlife species, it is likely that other species are declining through similar means. The next phase of this research is examining the efficacy of weed control methods (herbicides) aimed at habitat restoration to examine the degree to which negative impacts of weed invasion can be mitigated by such management efforts.
- University of Montana
- Montana Department of Natural Resources and Conservation
- Lolo National Forest
- Plum Creek Timber Company