Invasive Species

Let me preface this by saying: I am not a biologist, and therefore not qualified to truly debate the issue of invasive species. Nor should the thoughts which follow be taken as an indictment on the field of biology or any of its scholars. I'm certain that the central question I wish to raise has been thought of and discussed in those circles. (In fact, I know some research has been done.) What I am uncertain of, however, is if a satisfactory conclusion has been reached. My question: are invasive species always bad?

This question is pretty simple in terms of syntax, and yet my research into this has suggested that it is complex and even difficult to asses as a valid question. The first problem is that the term "invasive species" has a fluid meaning. It is sometimes defined as a "non-native species with an adverse effect on the habitat it is invading." Adherents of that definition of the term would say "yes," and the discussion would end. At the other end of the spectrum, the term is synonymous with "introduced species," broadening its horizons to include all non-native species. Under this definition, I think it would be safe to say "no" to my original question and move on... after all, the apple is an introduced species, and I don't think anyone is blaming it directly for any particular problems (if I'm wrong about this, I'm certain there is another example - perhaps the tomato).

The above, infuriatingly, gets us nowhere: we have an absolute question with a simultaneous yes and no answer (yuck!). So how do we move past the semantics here? The first definition above is the more widely used, so I will use that as "the" definition for the purposes of this discussion. To avoid the dead-end in the debate, I must rephrase the question as follows: do biologists and/or naturalists too readily classify a species as invasive? The crux of this question is whether or not we have enough data and understanding of our ecosystem to accurately evaluate harm.

I don't have an answer to that question; I do have suspicions that naturalists are generally predisposed to see any change as negative. That predisposition, I suspect, leads us to view any effect of an introduced species as a negative effect. I'm going to attempt to share my thought process as it occurred to me: through a series of examples encountered in my own life over the past year or so.

Mile-a-minute and its distinctive triangular leaves and red
vine. The plant produces small, blue-to-purple berries.

Last summer, I put in some volunteer hours removing invasive species with the Friends of Sligo Creek. There were several such species marked for destruction, but the hardy mile-a-minute stands out in my mind. This stuff earns its name. Its vines snaked through an entire field, choking off other flora to a staggering degree. It climbed bushes and trees, out-competing not only the other ground cover but the taller flora as well. This seems like a clear-cut case of an invasive species which is truly bad for its environment. In this environment, mile-a-minute thrives so well that it threatens every other plant, endangering food sources and habitats for the area's fauna. There's no one species it is in competition with - it risks a severe drop in the biodiversity in the local area.

This is mile-a-minute doing what it does.

I've been hearing a lot about deer exclosures lately. These are tall fences put in place to exclude deer from a small area. The vegetation inside and outside the exclosure is later studied comparatively. On the surface, this would seem tangential to the subject of invasive species - the white-tailed deer is an example of a native species which has become a problem. However, per NPR reports on these studies in Silver Spring (Sligo Creek again) one of the more striking differences between the vegetation growth on either side of the fence involves the success of native vs. introduced species. Where deer are free to roam, they decimate populations of native species, while leaving the invasive species alone. Looked at in one light, this means that the native plants are under attack on two fronts. However, without the non-native plants, would the deer simply consume ALL of the area's vegetation? In this interaction, I wonder if perhaps the introduced plants are in fact acting to preserve the biodiversity of suburban wooded areas in the region. I don't know, but it seems a question worth asking.

This red mulberry grows near my apartment. As I will explain below, it may actually be a hybrid with the introduced white mulberry. One cannot really tell without a DNA study of the specimen these days.

It was some new information about mulberries that finally inspired me to write on this topic. I learned that the native red mulberry (Morus rubra) is often hybridizing with the invasive white mulberry (Morus alba). When I first read of this, it was presented as a sinister attack on the genetic purity of M. rubra. Yet the more I read the less I understood why this is a problem. Both species occupy the same niche, and the hybrids produce viable offspring. I'm sure the rationale is that if one species eliminates the other it would constitute a decrease in biodiversity. But would it? If two closely-related species interbreed to become one with a greater range of genetic variability, isn't that positive? Doesn't a larger gene pool mean a more robust population? The line between species is arbitrary in the first place - it is sometimes even loosely defined by the ability to produce viable offspring, which calls into question whether these are two species or one with a broad genetic base. At its best, the idea that M. rubra is somehow threatened in this interaction seems analogous to dog breeders' aversion to mutts, or to the concept of preserving a royal line through inbreeding.

There is a range of ambiguity in the examples above. The first seems a clear-cut case of a correctly identified invasive species. The second seems nothing but gray area to me. Clearly the overpopulated deer are damaging the ecosystem, but are the introduced species truly invasive in this case, or will they have a net positive effect given the destruction already wrought? The third seems a very likely candidate for just the type of mistake I suspect happens. I certainly could be missing a key point there, so I don't mean to imply that I have answered my question... I cite it instead as evidence that my question is valid. Is M. alba truly "invasive" under the strictest definition of the term?

It is certainly a tricky - and even uncomfortable - question. Where is the line between healthy competition and destructive contaminant? How do conservationists act without a complete answer to this question? I don't wish to sound pessimistic, or to frighten anyone into inaction. I do hope, though, that readers will think on these matters when they act to preserve the natural environment.