Here's a fact I suspect most people don't know: Wherever human beings have gone in the last two centuries, we have increased local and regional biodiversity. Biodiversity, in this case, is defined as species richness. For example, more than 4,000 plant species introduced into North America during the last 400 years now grow wild here; they now constitute nearly 20 percent of the continent's vascular plant biodiversity.

Yet "the popular view [is] that diversity is decreasing at local scales," as the biologists Dov Sax of Brown University and Steven Gaines of the University of California at Santa Barbara noted in a 2003 paper published by the journal Trends in Ecology and Evolution. And one alleged culprit for the purported loss of diversity is competition from invasive species—that is, plants and animals introduced into ecosystems where they are not native.

Opponents of invasive species fear that aggressive outsiders will wipe out native species. That might seem reasonable, since there are a few species, such as kudzu, purple loosestrife, and water hyacinth, that grow with alarming speed wherever they show up. But that doesn't mean other species are necessarily 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," the Macalester College biologist Mark A. Davis noted in the journal BioScience in 2003.

Yet this spurious threat of extinction is one of the chief reasons given for preventing the introduction of exotic species to new areas.

There are plenty more examples of increasing biodiversity associated with the introduction of new species. Vascular plants brought in from all over the world have doubled the species richness of the plant life on most Pacific islands. The biodiversity of some islands has increased so much that they now approach the richness of continental areas. In New Zealand, since European settlement began 160 years ago, 2,000 introduced plant species have joined 2,000 native species, and only three native plant species have gone extinct. The opening of the Suez Canal introduced 250 new fish species from the Red Sea into the Mediterranean Sea. Only a single extinction resulted.

Researchers find increases in species richness on the local level as well. Sax and Gaines cite studies finding that a corner of West Lancaster in Great Britain has seen a dramatic rise in plant species diversity during the last two centuries, gaining 700 exotics while losing 40 natives. The movement of species between ecosystems has increased reptile and amphibian diversity slightly in California, and it has increased freshwater fish diversity significantly in many drainages throughout the United States. Mammal diversity has increased on many oceanic islands, in Australia, and in North America, thanks to introduced species.

Birds are different. Many bird species, especially those endemic to isolated islands, have gone extinct, largely due to habitat loss and predation by humans or by introduced animals such as rats. Nevertheless, Sax and Gaines note, "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 more or less steady because of new species.

Some introduced species do cause harm to the environment. They become pests (that is, they set up shop where we don't want them to) or cause disease in people or in 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 nonnative species seems to be aesthetic.

The Birmingham University biologist Phillip Cassey, for example, responds to the evidence of rising local and regional biodiversity by complaining that many of the birds a visitor from the U.K. might encounter in New Zealand are the same species found back home. "The same is true for floras and faunas around the world," Cassey and three co-authors lamented in the journal Austral Ecology in 2005. "It is the biological equivalent of flying from Seattle to Paris and going to Starbucks for your coffee."

Fair enough. But that is not a scientific argument. As Sax and the New Mexico University biologist James Brown replied to Cassey and his colleagues in the same issue of Austral Ecology, whether the effects 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." Phillip 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. Science has nothing to say about which choice is better.

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 the Stony Brook University biologist Lawrence Slobodkin pointed out in a 2004 article for Restoration Ecology, 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."

The good news from biology, as Davis puts it in his BioScience article, is that the "globalization of the Earth's biota will not lead to a world composed of zebra mussels, kudzu, and starlings." In the future different regions of the world will be more similar in their floras and faunas, he concludes, but "at the same time, they will become more diverse, in some cases much more diverse."