Exotic herbivores directly facilitate the exotic grasses they graze: mechanisms for an unexpected positive feedback between invaders |
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Authors: | Rebecca J Best Peter Arcese |
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Institution: | (1) Department of Evolution and Ecology, University of California Davis, Storer Hall, Davis, CA 95616, USA;(2) Centre for Applied Conservation Research, University of British Columbia, 3004-2424 Main Mall, Vancouver, BC, V6T 1Z4, Canada |
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Abstract: | 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.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. |
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Keywords: | Annual grass Branta canadensis Poa annua Seed dispersal Selective herbivory |
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