Linking an invasive ecosystem engineer with its community and ecosystem effects
Anderson, Christopher Brian
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Exotic species are a major component of current, global-scale ecological change. Even otherwise remote and pristine areas, such as the Cape Horn Biosphere Reserve (CHBR), Chile, are replete with introduced species. The North American beaver (Castor canadensis), which was introduced to Tierra del Fuego island in 1946, in particular has become highly invasive in the Chilean subantarctic archipelago. Nonetheless, little previous research has addressed its role as an invasive ecosystem engineer. I sought to determine the role of introduced beavers by addressing their impacts on several levels. Nearly all of the CHBR has been colonized by beavers with the only remaining non-invaded areas found in Cape Horn National Park and the far southwestern portion of the archipelago. Furthermore, where it has become established it is found in nearly all watersheds at very high densities. The foraging and dam building activities of beaver have consequently provoked wide-scale deforestation of riparian forests with little regeneration of the two dominant species (Nothofagus betuloides and N. pumilio). In addition, I found that the beaver meadows that are created by drained pondsprovided an avenue for the invasion of exotic plant species into the interior of watersheds. The plant communities created by beaver activities in these meadows were generally more speciose than unimpacted forested reaches. However, the forest understory assemblage was a subset of those found in meadows, rather than each community constituting a unique vegetation community assemblage. In streams, beavers significantly impacted benthic community and ecosystem properties as well. Their effect on benthic macroinvertebrates was to reduce diversity, but increase production in pond sites, while downstream sites were similar to unimpacted reaches. The overall influence of beavers on streams was to make them more retentive of organic matter, which increased the magnitude of terrestrially derived organic matter flows throughout the benthic food web. Overall, I found that beavers have profound effects on both terrestrial and aquatic habitats and the linkages between them. These results highlight the strong need for management of invasive beavers. These findings also provide baseline information for future research in subantarctic streams and investigating the impacts of exotic species in novel ecosystems.