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. 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.
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 Cassey and colleagues. “It is the biological equivalent of flying from Seattle to Paris and going to Starbucks for your coffee.”
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.”
Wednesday, August 11, 2010
Invasive species & biodiversity
Remember the panic about zebra mussels and purple loosestrife? Relax, says Ronald Bailey at Reason: invaders rarely result in the extinction of native species.