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Exotic herbivores facilitate the exotic grasses they graze : mechanisms for an unexpected "invasional meltdown" Best, Rebecca Jane

Abstract

Native communities increasingly host exotic species at multiple trophic levels, but most current hypotheses about community invasibility consider only a single invader. In addition to the simultaneous effects of native herbivores and native plant competitors on community invasibility, we must also consider the possibility of positive or negative feedback cycles between exotic species at multiple trophic levels. Though examples of mutually-beneficial interactions between exotic species have been described, and the possibility of subsequent "invasional meltdown" scenarios raised, few studies have been quantitative or mechanistic. I studied the co-invasion of an endangered island ecosystem by exotic Canada geese (Branta canadensis) and eight exotic annual grasses to estimate the synergistic effects of novel inter- and intra-trophic interactions on the invasion success of the grasses. Geese fed selectively on the exotic grasses, causing them to produce a higher number of short stems. This appeared to be an efficient reproductive and competitive strategy, allowing the grasses to form dense lawns with reduced occurrence of native forbs not adapted to compete under grazing or with grass. The demographic success of the exotic grasses was thus a product of both novel inter-trophic interactions with geese and novel intratrophic interactions with the native plant community. In combination, these interactions produced an unexpected outcome. Current theory suggests the grasses should be limited by a selective enemy, but my work shows that co-evolved grazer and grass strategies benefited exotic species at both trophic levels. Selective herbivory by geese also facilitated their dispersal of exotic grass seed between heavily invaded feeding areas and small islands used for nesting. In sum, selective herbivory by geese resulted in the spread and proliferation of their preferred food source. This unexpected case of positive feedback between invaders suggests two avenues for strengthening current theoretical frameworks. Robust hypotheses for predicting invasion success must account for multiple novel interactions, and for the degree of shared evolutionary context between multiple invaders.

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