The Case for Invasive Species

Starlings, introduced to the United States in the late 19th century by a wealthy New Yorker and avid Shakespeare fan who wanted to bring all of the birds mentioned in the bard's work to the United States, are an invasive species. (Wiki Commons/Ingrid Taylar)

Can we win the war against invasive species? Should we even fight it?

Before answering these questions, let’s look at what constitutes an invasive species. Executive Order 13112, aimed at addressing the problem and signed by President Clinton in 1999, defines it like so:

“an alien species whose introduction does or is likely to cause economic or environmental harm or harm to human health.”

The biology-focused wiki provides a more concise definition for invasive species:

“non-native species disrupting and replacing native species.”

Invasive species should not be entirely conflated with alien or exotic species which, according to, are not always disruptive:

plant or animal species introduced into an area where they do not occur naturally, non-native species.”

The Anti-Invasive Species Club

While exotic species by definition can be more innocuous than invasive species, both are anathema to many conservationists. (See here, here, here, here and here.) Martin Schlaepfer of the State University of New York at Syracuse and co-workers observe [pdf] in the journal Conservation Biology that

“several authors have demonstrated that a bias persists against non-native species among scientists. These biases are reflected in the assumptions commonly made about the intrinsic and instrumental values of non-native species, the language used when describing them. … For example, in a landmark study … [the investigators] considered non-native species only as potential threats. … Furthermore, in studies in which an index of biotic integrity was used, the presence of non-native species decreases the index value even if the non-native species have no or little detectable biological effect. … Finally, the language used to describe non-native species in the scientific literature is frequently scattered with militarized and xenophobic expressions (e.g., “war on aliens” and “American ecosystems under siege by alien invaders”).”

And this antagonism toward invasive species is reflected in the large number of projects dedicated to, in the words of just such a program in the state of New York, "kill[ing] and/or permanently remov[ing] plants or animals that meet the definition of terrestrial invasive species." (See also here and here.) Just like skinning a cat there is more than one way to do in an invasive. For example, enthusiasts of the burgeoning "invasivore" movement promote invasive species as food for the discerning gourmet. Asian carp fritters or feral pig, anyone?

gypsy moth
The gypsy moth is an invasive species that has killed millions of trees in the Northeast. (U.S. FWS)

It's also reflected in the ways you can earn points to prove you're a good green citizen. For example, obtaining LEED certification [pdf] for homes from the U.S. Green Building Council mandates "no invasive species" in the landscape.

Why We Hate, Kill and (Try to) Eradicate Invasive Species

This avid antagonism aimed at invasive species is not without reason. Time and again we have seen a species from some distant shore sneak into our home ecosystems and bring about a serious ecological disruption. Consider just three of thousands of examples.

Kudzu, introduced from Japan in 1876, now buries landscapes beneath the weight of its vines. (But there are those who see great potential for the green vine; check out this short video.)

Sea lampreys, native to the Atlantic Ocean, found their way into the Great Lakes and decimated that fishery.

Cryphonectria parasitica, a devastating fungus commonly known as the chestnut blight and believed to have been introduced to the United States on fungus-bearing chestnut trees imported from Japan, all but wiped out the magnificent American chestnut tree in the 20th century, which in turn collapsed a number of moth species dependent on it. (More on the American chestnut tree here and here.)

Kudzu, which was introduced to the United States in 1876, is a
. What can be done about it? You'll find some ideas
. (Gsmith/Wiki Commons)

Invasive species are also seen as an almost existential threat to biodiversity. CSIRO, Australia's national science agency, calls invasive species “one of the greatest threats to biodiversity, and to the ecological and economic well-being of society and the planet.”

The University of Illinois Extension Service reports that

“introduced species are a greater threat to native biodiversity than pollution, harvest (agriculture, forestry, fisheries), and disease combined. Compared to other threats, invasive introduced species rank second only to habitat destruction, such as forest clearing. Of all 1,880 imperiled species in the United States, 49% are endangered because of introduced species alone or because of their impact combined with other forces. Finally, damage to the U.S. economy inflicted by introduced species is estimated at $137 billion per year.”

If that isn't an indictment of invasive species, I can't imagine what would be. But are these statements accurate portrayals of the impact of invasive species? Some, like Chris Thomas of the University of York writing last week in the journal Nature, argue maybe not.

Are Invasives All That Insidious?

While the accepted wisdom is that invasive species suppress biodiversity, the data supporting that contention is not all that strong. In fact, Jessica Gurevitch and Dianna Padilla of Stony Brook University argue in the journal Trends in Ecology and Evolution that

“[t]he link between species invasions and the extinction of natives is widely accepted by scientists as well as conservationists, but available data supporting invasion as a cause of extinctions are, in many cases, anecdotal, speculative and based upon limited observation.”

In fact, there is a good deal of data that suggest something quite different.

  • In a “meta-analysis on the effects of invasive plants on the species richness of invaded communities” published in 2011 in the American Journal of Botany, Kristin Powell of Washington University and co-authors found that the effect on biodiversity depended upon the spatial scale considered. At larger regional scales studies have concluded that “no native species have gone extinct from the introduction of plant competitors ... even in locations that are infamous for being devastated by species invasions, such as remote oceanic island.” (They obviously did not consider humans in this analysis but more on that later.)
  • James Brown and Dov Sax, writing in the University of Washington's Conservation magazine, report that "there is locally fewer than one extinction of a native species for every successful colonization of an alien species. This [which implies a net increase in local diversity] will come as a surprise to many who believe that biodiversity is decreasing everywhere on earth."
  • Schlaepfer (of SUNY) and co-workers argue in a 2012 article [pdf] in Conservation Biology that "the net effect of regional species introductions [because of hybridization] is generally an increase in diversity. Such an increase has occurred with plants, mammals, birds, fishes, and many other groups on both islands and continents worldwide."
  • Thomas (of the University of York) reports that "Britain, for instance, has gained 1,875 established non-native species without yet losing anything to the invaders."
  • And Gurevitch and Padilla report that of the 18,318 species listed by the International Union for Conservation of Nature as “extinct, endangered, or threatened globally,” only 6 percent “list direct and indirect effects of all kinds from naturalized alien species as contributing to their decline.” The greatest threat, by the way: loss of habitat caused by you know who.

Is it just possible that invasive species have been given a bad rap?

What Is Invasive When Everything is Changing?

To me, the whole notion of invasive species gets kind of muddied when we recognize that we live in a warming world. Think about it: our notion of how species will survive with global warming is that they will be able to migrate to higher latitudes and altitudes. (See here and here.) In fact in an article from 2011 in Conservation Biology Schlaepfer and co-authors

"speculate that non-native species might contribute to achieving conservation goals in the future because they may be more likely than native species to persist and provide ecosystem services in areas where climate and land use are changing rapidly and because they may evolve into new and endemic taxa.”

Interestingly, while conservationists plot campaigns to eradicate invasive and even exotic species, they are at the same time working to create wildlife migration corridors to facilitate species movement in anticipation of further warming. (See also here.) In other words to help species migrate to -- invade -- a new habitat. Do these species get a hostile reception as invasives once they arrive, or a warm welcome as climate-change survivors? I vote the latter.

And that wouldn't be without precedent. After all, we've welcomed many a plant species for a very long time. Think apples and tomatoes for starters. (The vast majority of the fruits and veggies we eat are non-native.) And where would we be without the European honey bee?

We Are All Invaders

The way I see it: much of the arc of life on planet Earth is the story of invasive species. There is a never-ending search for new niches and new habitats. And there is sex, genetic mixing that results between two individuals (e.g., a native and invasive individual) to create new and unique genetic codes. I suspect that efforts to keep ecosystems genetically static and pure are akin to trying to outlaw the second law of thermodynamics and are ultimately doomed to failure. They may be wrong-headed as well. We earthlings are blessed with a genetic diversity that is in many ways a product of species invasion. Why would we want to stop now?

Ultimately, invasives do not represent a zero sum game. And more conservation scientists are recognizing this. As our ability to hold on to pristine, functioning environments free of human influences dwindles ($ub req’ed), scientists on either side of the debate are looking at new ways to proceed.

And finally, when we talk about invasive species, let's be honest -- by far the most invasive, the most destructive species on the planet is Homo sapiens. Despite all our faults, I vote we leave Homo sapiens off the list for eradication. And if the worst of the worst is left off the list ...

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