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Invasive Species – Good or Bad?

July 13, 2011

A recent letter to Nature has spawned a lot of conversation among ecologists. It makes the argument that we should judge species on te ecological impact rather than on whether or not they are “native” to the ecosystem they are now inhabiting. They bolster their argument by citing that species responsible for the greatest number of tree deaths in the USA is a native species of beetle and that some alien species are either neutral (tamarisk) or beneficial (shrub honeysuckle) in ecosystems.They even invoke the argument (often forwarded by Ariel Lugo, one of the co-authors) that alien species may even increase species diversity.

The unfortunate thing about this letter is that, while shrub honeysuckle may provide food for bird species, it has devastating and long-term impacts on the plant community. And this is one of the problems with all the examples presented in the Nature letter: they concentrate on one positive  (or not negative) aspect of a particular invasive species while ignoring all the other literature having to do with the impacts of that species on native communities. The primary point that they make, that a species is not necessarily “bad” simply because it is non-native, is good but when conservation biologists refer to a species as invasive, they are generally making a series of statements about that species. First, it is not native to the geographic space being referred to. Second, it is growing in such a way that it is coming to dominate the ecosystem of interest either numerically, by biomass or in terms of net ecological effect. And thirdly, this dominance is occurring at the expense of other native species or with correlated effects on ecosystem function. The taxonomy of non-native species is complicated (native, exotic, naturalized, invasive, etc.) and we should be careful about how we use terms but we should neither paint all exotic species with the same brush of being invasive NOR should we paint all exotic species with the same brush of being benign. I agree with the authors that we should label species according to their effects and allocate resources to control on the same basis, but we should look at all the available information on a particular species  when making these designations and resource allocations – not just cherry-picking a single aspect or two to bolster a particular claim.

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