Driving around in the Patagonian Andes in December, my wife and I were enchanted by the masses of luminous blue lupines and brilliant yellow scotch broom lining many of the roads. We stopped frequently to take photos of the floral abundance. How insensitive of us! Both, it turns out, are evil foreigners. Lupine is from North America and scotch broom hails from Europe.
Since 1992, the nations of the world have been waging a war against such foreign invaders under the Convention on Biological Diversity. In the United States the public regularly reads anguished stories about the "damage" being caused by alien invaders such as zebra mussels and purple loosestrife. Environmentalist groups including the Natural Resources Defense Council, the Union of Concerned Scientists, and the National Wildlife Federation fiercely denounce these foreign intruders, urging Americans to band together to force these invaders from our shores.
In response, Congress passed the National Invasive Species Act and the executive branch has adopted a National Invasive Species Management Plan aimed at closing our borders to alien species. NASA warned recently, "Non-indigenous invasive species may pose the single most formidable threat of natural disaster of the 21st century." But is all this jingoistic furor justified? Some biologists and other analysts are beginning to doubt it.
For example, University of California-Santa Barbara biologist, Daniel Botkin, points out in his article "The Naturalness of Biological Invasions," that "[b]iological invasion is a natural process everywhere, requisite for the persistence of essentially all species on Earth over the long term. Being able to seek new habitats and survive in them is essential in an environment that changes at all scales of space and time."
In the May 2005 cover article for Discover, senior editor Alan Burdick asks the startling question, "Are Invasive Species Really So Bad?" (not yet available online). The article concludes, "Fifty years of invasion biology has failed to identify a clear ecological difference between an ecosystem rich in native species and one chock-full of aliens. Invasions don't weaken ecosystems—they simply transform them into different ecosystems, filled with different organisms of greater and lesser value to us." (To be immodest, this is exactly the point I made in my "Bioinvaders" article nearly 5 years ago.) Introducing new species generally boosts the total number of species dwelling in any given ecosystem.
What about the claim that invasive species pose "the single most formidable threat of natural disaster"? It is certainly the case that some introduced species have detrimental effects. Think West Nile virus and Norway rats. We should take steps to prevent the introduction of disease organisms and parasites that show a high likelihood of harming species that we value.
But even the NRDC admits that over the past two centuries, only one in seven of the thousands of introduced species have caused environmental, health, or economic harm. In fact, most, such as wheat and cows, have provided people with far more benefits than harms. And while some species are threatened with extinction by the introduction of outside species—most infamously the case of the brown tree snakes that killed off several bird species on the isolated island of Guam —in fact, fewer than 6 percent of species considered endangered are menaced by non-native species, according to Burdick.
Ecologists had assumed that introducing alien species would be detrimental because these species would disrupt ecosystems in which species had co-evolved for millions of years. Species from different ecosystems would harm tightly functioning "natural" plant and animal communities. This assumption has recently been called into question by the creation of an "accidental rainforest" on Ascension Island in the middle of the Atlantic Ocean. As New Scientist (subscription required) points out, Ascension's bare central peak once called White Mountain is now covered with an extensive cloud forest consisting in guava, banana and wild ginger, bamboo, the white-flowered Clerodendrum and Madagascan periwinkle, Norfolk Island pine and, eucalyptus from Australia and is renamed Green Mountain. This new rainforest, less than 150 years old, is an affront to conventional ecological wisdom that species must co-evolve in order to function together. Instead the Ascension rainforest supports the dissident notion that species engage in "ecological fitting." That is, species make the best of what they have.
Ascension's rainforest is evidence that nature is super resilient and that moving species around the globe is unlikely to cause wide-scale ecosystem collapses. Ecological puritans loathe the new Ascension Island rainforest as a pastiche and lupine and scotch broom in Patagonia as sinful aberrations. However, less conservative temperaments welcome foreign species as fascinating scientific and aesthetic experiments that can enrich landscapes such as Patagonian roadsides. Ultimately, the battle against exotic species is a cultural and aesthetic war, not one compelled by scientific evidence.