Invasion of exotic or non-native species is now clearly acknowledged to be a problem of global significance, and an effective means for reducing or eliminating future invasion has recently become a federally mandated priority. Amid the growing interest in the effects of exotic species on native species and the newly acquired roles of exotic species within ecosystems, there is growing concern that exotics, and all their problems, are here to stay.
While conducting a study aimed at testing a variety of theoretical ideas about the potential impact of predators on songbird reproductive ecology, my students and I collected a large data set on the use of plant species as substrates for songbird nests. In the forest where this work was conducted at The Morton Arboretum in DuPage County, exotic shrubs, in particular Asian bush honeysuckle (Lonicera maackii) and common buckthorn (Rhamnus cathartica), dominate large sections of the understory. In some sections there may be no other woody understory species present besides these two exotics. Early on in this investigation, it became clear that a majority of nests of all bird species were located in these two exotic plant species. Because exotic species like honeysuckle and buckthorn are often the targets of eradication, this led to concern about unintended effects such management may also have on the native bird species nesting in them. Does this situation pose a paradox for land managers? Or can buckthorn and honeysuckle eradication proceed in a way that minimizes negative effects on native bird species that use these shrubs for nesting sites?
To better understand the effect of substrate use for nesting location, we analyzed our data to address the question, "Does the plant species used for nest site location correlate with nest success or failure?" We restricted our analysis to two bird species, American Robin (Turdus migratorius) and Wood Thrush (Hylocichla mustelina), for which our data were most complete. From these data sets, we selected nests for which we had complete data (from date found to conclusion of the nest attempt) with unambiguous fates (success or failure). For some nests that failed due to predation, we could identify the predator. In addition, we knew from other data that the principal mammalian nest predator in our study system was the raccoon (Procyon lotor), and the principal avian predator was the Blue Jay (Cyanocitta cristata).
This analysis was revealing. American Robin nests placed in the exotic honeysuckle and buckthorn suffered significantly higher rates of predation than nests placed in comparable positions in native trees. Although currently untested, we believe the reasons for this increased nest predation in exotics may be related to a drop in average nest heights in exotics, their lack of formidable thorns that might increase foraging costs of mammalian predators, and their thick, sturdy branches, which may facilitate accessibility for large mammals like raccoons.
Raccoon caught in the act of raiding a bird nest.
For Wood Thrushes, the picture is more complicated. Over the course of the study, American Robins built progressively more and more nests in exotic substrates, a choice of nest location that increased their vertical overlap with Wood Thrushes. As this vertical overlap increased, predation on Wood Thrush nests likewise increased. Results from a further experiment indicated that alternative nesting strategies could be linked in a negative interaction chain via a common, shared predator. The result of our experiment also suggested that the same phenomenon may have been the mechanism linking increased overlap of Robin and Thrush nests with greater predation on the Thrush nests. In our study system, the data indicate that the shared predator is the raccoon.
Our results demonstrated a direct negative effect of nesting in exotic plant species on American Robins, but an indirect negative effect (via Robins and a shared predator) on Wood Thrushes. The work further begs the question, "Do these results also pertain elsewhere in the now extensive ranges of honeysuckle and buckthorn in North America?" We do know that honeysuckle is commonly used for nesting in New Hampshire and New York state, suggesting that this species may be contributing to nest failure over a wide geographic region. Our results indicate that much more work needs to be done to understand the structure-function relationships by which plant species used as nesting substrate affect the fate of those nests. More immediately, the results indicate one more reason why prevention of invasion by exotic species is a conservation priority. And finally, they suggest that replacement of exotic shrubs with the native species they apparently displaced will improve not only the biodiversity of the native plant community, but also the ecological conditions for animal species using them.
Christopher J. Whelan, Center for Biodiversity
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