RODRIGUEZ CABAL mariano Alberto
congresos y reuniones científicas
Direct and indirect impacts of introduced species on community dynamics.
Conferencia; 97th ESA Annual Meeting; 2012
Background/Question/Methods Communities are structured by direct and indirect interactions among species. Herbivory can be intense, creating strong direct and indirect effects on plant communities. Herbivores modify plant communities by directly affecting plant survival, growth and fitness through browsing, grazing and trampling. These effects may worsen when the affected species are also keystone species. The northern portion of the temperate forest of Patagonia harbors a unique triangle of keystone mutualists comprised of a hummingbird (Sephanoides sephaniodes), a mistletoe (Tristerix corymbosus), and a marsupial (Dromiciops gliroides). This hummingbird is responsible for pollinating nearly 20% of the endemic species of woody flora in this biome. Aristotelia chilensis is the main host of the mistletoe in this forest. Thus, if herbivores reduce the number of A. chilensis, this could drastically reduce the number of mistletoes and potentially disrupt a key mutualism. Here, we examined the direct and indirect consequences of exotic ungulates on all the stages involved in this unique triangle of keystone mutualism. at thirteen invaded sites and thirteen intact sites where exotic ungulates were not present. Additionally, we experimentally excluded herbivores to assess their direct effects on an A. chilensis. Results/Conclusions We found 16× more individuals in intact than invaded sites. An exclosure experiment shows that in just a month deer herbivory damage 99% of the A. chilensis leaves outside the exclosure. By browsing on A. chilensis exotic ungulates indirectly affect the population persistence of the keystone mistletoe. The density of mistletoes was 83× greater in intact than in invaded sites. Because the mistletoe is the only resource of nectar for the hummingbird during the winter, hummingbird abundance decrease as the number of mistletoe plants decrease. Additionally, mistletoe host age distribution ranged between 6 and 42 years old in intact sites. In contrast, there were not infected host younger than 28 years at invaded sites. Finally, at all invaded site we failed to detect the presence of D. gliroides, but the marsupial was present at the 13 intact sites. Consequently, we did not find mistletoe seeds dispersed at invaded sites and fruiting plants were three times less abundant at invaded than intact sites. Our results show that exotic ungulates, by browsing on A. chilensis, indirectly affected the keystone mutualisms, and their disruption have cascading effects through the rest of the ecosystem, threatening a much larger set of interactions.