, Volume 159, Issue 1, pp 139–150 | Cite as

Exotic herbivores directly facilitate the exotic grasses they graze: mechanisms for an unexpected positive feedback between invaders

  • Rebecca J. Best
  • Peter Arcese
Community Ecology - Original Paper


The ability of an exotic species to establish in a system may depend not only on the invasibility of the native community, but also on its interactions with other exotic species. Though examples of mutually beneficial interactions between exotic species are known, few studies have quantified these effects or identified specific mechanisms. We used the co-invasion of an endangered island ecosystem by exotic Canada geese (Branta canadensis) and nine exotic annual grasses to study the effects of an invading herbivore on the success of invading grasses. On our study islands in southwestern Canada, we found that geese fed selectively on the exotic grasses and avoided native forbs. Counter to current theory suggesting that the grasses should be limited by a selective enemy, however, the grasses increased in proportional abundance under grazing whereas forbs showed declining abundance. Testing potential mechanisms for the effects of grazing on grasses, we found that the grasses produced more stems per area when grazing reduced vegetation height and prevented litter accumulation. Forming dense mats of short stems appeared to be an efficient reproductive and competitive strategy that the Eurasian grasses have evolved in the presence of grazers, conferring a competitive advantage in a system where the native species pool has very few annual grasses and no grazers. Germination trials further demonstrated that selective herbivory by geese enables their dispersal of exotic grass seed between heavily invaded feeding areas and the small islands used for nesting. In summary, the exotic geese facilitated both the local increase and the spatial spread of exotic grasses, which in turn provided the majority of their diet. This unexpected case of positive feedback between exotic species suggests that invasion success may depend on the overall differences between the evolutionary histories of the invaders and the evolutionary history of the native community they enter.


Annual grass Branta canadensis Poa annua Seed dispersal Selective herbivory 



We thank D.S. Srivastava, M. Vellend, R. Guy, L.B. Marczak, the Arcese lab, the UBC Florum discussion group, J. Hille Ris Lambers, and one anonymous reviewer for helpful comments, and M. Flint for field assistance. We thank the Gulf Islands National Park Reserve and, especially, A.J. Brumbaum and W. and H. Hesse for their very generous contributions to our research. R.J. Best was funded by a Julie-Payette NSERC Research Award and by the UBC Faculty of Forestry. All research reported herein complies with current Canadian law.

Supplementary material

442_2008_1172_MOESM1_ESM.doc (60 kb)
Online supplement S1 (DOC 60 kb)


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Copyright information

© Springer-Verlag 2008

Authors and Affiliations

  1. 1.Department of Evolution and EcologyUniversity of California DavisDavisUSA
  2. 2.Centre for Applied Conservation ResearchUniversity of British ColumbiaVancouverCanada

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