Across the Plains, leafy spurge is overtaking the grasslands that feed cattle, forcing ranchers to find creative ways to battle the invasive weed, which can grow to a yard or more in height. Government agencies, farmers and businesses pass on the cost in the form of higher taxes and prices. On the ecological side, the cost is measured in native species that are being pushed to the brink of extinction. Invasive species make preservation harder because they usually overrun the biological niches that native flora and fauna have carved out for themselves over the course of millennia. Researchers estimate that the prolific zebra mussel, the mollusk now found as far south as New Orleans, has eliminated 98 percent of the native mussels in western Lake Erie in little more than a decade.
Humans have a long history of carrying animals and plants around the world.
In fact, Americans rely on non-native species, such as wheat and cattle, for about 98 percent of their food. Cornell's Pimentel estimates that 50,000 non-indigenous species have been introduced in the United States, most of which are either beneficial or pose no threat. Most biologists, while acknowledging that not all foreign species become threats, still maintain that the world's increasingly mobile plants and animals threaten to turn the separate, highly evolved ecosystems of the planet into just a few big, boring habitats. The World Conservation Union, one of the leading scientific organizations studying threats to the world's 1.4 million-plus species, says that the impact of invasive species is second only to habitat destruction, such as the felling of rain forests, among the causes of extinction. The threat of alien species is most acute on islands, where native birds or plants have evolved in isolation and are either defenseless against new predators or are small in numbers to begin with.