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Interactions between introduced herbivores, plants, and their insect pollinators: density-mediated indirect effects on plant reproduction.
Vázquez, Diego1, 1
ABSTRACT- Introduced herbivores can drastically reduce plant abundance through trampling and browsing. For animal-pollinated plants, this decreased abundance can in turn affect their reproductive performance through a decreased frequency of interaction with pollinators—a density-mediated indirect effect. However, the existence of "refugia" where plants avoid herbivores could minimize this effect if these refugia housed groups of individuals dense enough to attract pollinators. I studied this problem in the understory tree Aristotelia chilensis (Eleocarpaceae). A. chilensis is dioecious, depends on its almost-exclusive pollinator Cadeguala albopilosa (Hymenoptera: Colletidae) for reproduction, and dramatically decreases in abundance in the presence of introduced ungulate herbivores. Therefore, I hypothesized that (i) the decreased abundance of A. chilensis in grazed areas should a hamper its reproductive performance at the population level, and (ii) this effect should result from the increased isolation of individuals in grazed areas. To test these hypotheses I studied pollination levels and fruit set of A. chilensis in four pairs of grazed and ungrazed sites. I found no statistically significant differences between grazed and ungrazed sites in the reproductive performance of A. chilensis. However, I found an association between the degree of isolation of individuals and their reproductive performance. Thus, my data suggest that introduced herbivores can have density-mediated indirect effects on A. chilensis, but that the magnitude of this effect at the population level is minimized by the spatial aggregation of individuals in refugia.
KEY WORDS: indirect effects, pollination, population density, introduced herbivores