Interactions among Introduced Ungulates, Plants, and Pollinators: A Field Study in the Temperate Forest of the Southern Andes

Introduced mammalian herbivores can significantly affect ecosystems. I studied the effects of introduced ungulates on plant-pollinator interactions in the temperate forest of the southern Andes. Introduced mammalian herbivores, including ungulates, are a major conservation problem in this biome. I conducted field studies in eight forested sites where Nothofagus dombeyii was the dominant canopy tree. I described the interactions between fifteen common, insect-pollinated understory plants. I used these data to address two main questions. The first one is whether the susceptibility of plants and pollinators to disturbance by introduced ungulates is related to their degree of interaction specialization and interaction asymmetry. I found no relationship between degree of specialization and a species’ response to disturbance. I also found that plant–pollinator interactions tend to be asymmetric in this system; however, asymmetry of interactions did not explain the variability in species’ responses to disturbance. The second question I addressed is whether introduced ungulates can affect pollination and plant reproduction indirectly by modifying plant population density. This hypothesis is different from previous hypotheses of indirect effects of herbivores on plants, all of which concerned individual-level effects on vegetative and reproductive traits, whereas my hypothesis focuses on population-level effects. I found strong evidence of such an effect for only one of the species I studied, the herb Alstroemeria aurea. The general lack of evidence for indirect effects on most species may result from resistance to cattle grazing, spatial refugia, or low statistical power of my analysis. For A. aurea, additional evidence indicates that herbivores decrease the absolute and relative population density of this species through trampling., which in turn results in lower conspecific pollen deposition in stigmas and lower reproductive performance. Thus, my study suggests that introduced ungulates can in some circumstances affect plant-pollinator interactions significantly. However, in most cases plant-pollinator mutualisms appear resilient to the effects of introduced ungulates, and the direct effects of introduced ungulates on plants and pollinators are in general stronger than the indirect effect through mutualistic partners.