Here's a fact that I suspect most people don't know: Wherever we humans have gone in the past two centuries, we have increased local and regional biodiversity. Biodiversity, in this case, is defined as increasing species richness. Yet, "the popular view [is] that diversity is decreasing at local scales," Brown University biologist Dov Sax and University of California, Santa Barbara biologist Steven Gaines report [PDF].
Ample scientific evidence shows that this popular view is wrong, however. For example, more than 4,000 plant species introduced into North America during the past 400 years grow naturally here and now constitute nearly 20 percent of the continent's vascular plant biodiversity.
The fear among opponents of "invasive species" is the aggressive outsiders will cause a holocaust among the native plants. That might initially seem reasonable because there are a few species, like kudzu, purple loosestrife, and water hyacinth, that grow with alarming speed wherever they show up. But that doesn't mean other species are in danger. "There is no evidence that even a single long term resident species has been driven to extinction, or even extirpated within a single U.S. state, because of competition from an introduced plant species," Macalester College biologist Mark Davis notes [PDF]. Yet this spurious threat of extinction persists as one of the chief reasons given for trying to prevent the introduction of exotic species.
Meanwhile, there are plenty more examples in which local and regional species richness has been increasing. Introduced vascular plants have doubled the species richness of the plant life on most Pacific Islands. In fact, the species richness of some islands has increased so much that they now approach the richness of continental areas. In New Zealand 2,000 introduced plant species have taken up residence with the islands' 2,000 native species and only three native plant species have gone extinct. The opening of the Suez Canal introduced 250 new fish species into the Mediterranean Sea from the Red Sea which resulted in only a single extinction.
Researchers find increases in species richness on the local level as well. Sax and Gaines cite studies [PDF] which find that a corner of West Lancester in Britain has seen a dramatic rise in plant species diversity over the past two centuries, gaining 700 exotics while losing 40 natives. They note that reptile and amphibian diversity has increased slightly in California. Mammal diversity has increased on many oceanic islands, and in Australia and North America. Freshwater fish diversity has increased significantly in many drainages throughout the U.S.
Birds are different. Many species, especially those endemic to isolated islands, have gone extinct, largely due to habitat loss and predation from humans or introduced predators such as rats. Nevertheless, Sax and Gaines note that "net bird diversity (in spite of large changes in species composition) has remained largely unchanged on oceanic islands." In other words, despite extinctions of endemic species, the number of avian species on any given island remains about steady because new species are introduced to them.
So why then are so many ecologists and environmentalists on a jihad against introduced species? Of course, some introduced species do cause harm to the environment. They become pests (which means they set up shop where we don't want them to) or cause disease in people or creatures we care about. But the vast majority of introduced species blend in more or less unobtrusively with the natives. The main objection to spreading non-native species seems to be aesthetic.
For example, Birmingham University biologist Phillip Cassey and colleagues respond to the evidence of rising local and regional biodiversity by complaining that many of the birds that a visitor from the U.K. would encounter in New Zealand are the same species found back home. "The same is true for floras and faunas around the world," lament [PDF] Cassey and colleagues. "It is the biological equivalent of flying from Seattle to Paris and going to Starbucks for your coffee."
Fair enough. But this is not a scientific argument. Sax and New Mexico University biologist James Brown correctly observe that whether the impacts of introduced species "are considered to be positive or negative, good or bad is a subjective value judgment rather than an objective scientific finding. Scientists are no more uniquely qualified to make such ethical decisions than lay people." Cassey may wish to quaff his café au lait at Les Deux Magots while others enjoy their Venti Café Misto in the familiar purlieus of a Parisian Starbucks.
Nevertheless, aesthetic reasons are still reasons and science can be validly deployed in their service. Some people may prefer landscapes restored to a condition prior to the introduction of outside species. As Davis and his colleague Stony Brook University biologist Lawrence Slobodkin point out [PDF], architecture uses mathematics, physics, and engineering to achieve aesthetic and social goals. "Perhaps 'ecological architecture' might be a more apt characterization of the work of ecological restoration," they suggest. "Because the term acknowledges the central role played by both values and science."
Ultimately, Davis argues that the good news from biology is that the "globalization of the Earth's biota will not lead to a world composed of zebra mussels, kudzu, and starlings." Instead, while in the future different regions of the world will be more similar in their floras and faunas, Davis concludes, "At the same time, they will become more diverse, in some cases much more diverse."
Ronald Bailey is Reason's science correspondent. His book Liberation Biology: The Scientific and Moral Case for the Biotech Revolution is now available from Prometheus Books.