Friday, April 3, 2015

Invasion of the Beavers

Guest post by John Cherkas

Fifty years ago, Dr. Walter Howard presented his thoughts on invasive mammals at a symposium on colonizing (invasive) species, which was later turned into the volume "The Genetics of Colonizing Species." He speculated on the nature of predator-prey interactions, population growth limits and habitat disruptions. His ideas still resonate, but how well do they match up with a certain invasive mammal today.

May I bring your attention to some invasive beavers? Our national creature has been making quite a mess is the Southern most reaches of the Western Hemisphere. In the 1940s, Argentina was seeking economic improvements and imported beavers, mink and muskrat to Tierra del Fuego in an attempt to establish a fur trade. That fur trade didn't turn out as expected. Within a few years, beavers had colonized the entire island and were soon crossing channels to reach other Chilean islands, including Cape Horn, a UNESCO Biosphere Reserve.

Angry beaver -roar!

The ecological effects have been pretty well researched recently by Dr. Christopher B. Anderson. In seeing if beavers behave differently in their new habitat than back home, he’s been finding a few differences in the environment and beavers here. One of the most obvious changes is that the beaver colonies are at least twice as dense in Cape Horn. Is this for lack of predators or an abundance of food? So far, I couldn’t say, but I’d lean toward the latter. The Cape Horn forests are entirely southern beeches, which provide ample resources for the beavers’ engineering projects.

But how disruptive have beavers been to the environment: and environment that has no animal that makes such a massive environmental impact as the beaver. Howard suggested that an animal moving into a habitat where its niche doesn’t exist would have wider impacts than one who’s niche does exist. It’s fairly clear that the beaver’s landscaping projects is not something that other animals (except humans) partake in.

In the beaver situation this ecological disruption holds true. The floral assemblage in Cape Horn has never had to deal with beaver-like behaviour. The beavers foraging and building habits prevent forest regrowth, and provide a pathway for other plants to invade. It seems this beaver introduction might be a good example of invasional meltdown. The Chilean archipelago is home to quite a few invasive species already, and this synergistic effect is definitely concerning.

All the beaver-induced worries come with a grave concern for the natural environment. Cape Horn is referred to as pristine quite a bit by Anderson. Is this the place to have a deep political, socio-economic discussion about “pristine” environments? No, not today; you’ll have to read elsewhere for that. Cape Horn is certainly already at risk from invasive species. Beavers have a tremendous impact on the ecological structure of streams and forests. I am certainly one to wonder whether the eradication effort can be truly successful and both removing the beavers and reversing the environmental changes.

I surely hope that the environmental disruption can be reversed. Unfortunately we cannot look back to Howard to speculate on what happens when we remove an alien species. Just fifty years ago, species invasions were seen as a great research opportunity, not something to be extensively managed or eradicated.

Further Reading:

C.B. Anderson et al. (2006), The effects of invasive North American beavers on riparian plant communities in Cape Horn, Chile. Do exotic beavers engineer differently in sub-Antarctic ecosystems? Biological Conservation, 128: 467-474.

C. Choi, (2008) Tierra del Fuego: the beavers must die. Nature 453: 968. doi:10.1038/453968a

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