The Same Plants Are Showing Up Everywhere
North America's plant life is becoming increasingly uniform throughout the continent. It's part of a trend, that scientists have observed, towards homogenization of the planet's flora. Driving this change is the widespread introduction of plants by people into new areas.
Altogether 19,653 species of vascular plants grow wild in Canada and United States. Of these, 3,427 are not native. Concentrations of exotic plants thrive in Florida, Louisiana, the northeastern United States and the Pacific Northwest.
The 60 states and provinces each average 2,200 native species and 600 exotics. But species that have recently arrived from places like Europe and Asia are more widespread here than are native types. A native species typically survives in about eight jurisdictions, whereas an introduced species ranges through ten states and provinces on average.
Researchers at the Illinois State Museum and University of Missouri probed into the factors driving this difference. They discovered that native plants are more particular than exotic ones about where they live, and are especially sensitive to a site's climate, elevation and latitude. Native North American plants tend to be ecological specialists.
Introduced species are less fussy. Rather than associate with certain ecological conditions, they associate more with people. The greater amount of human activity in a region, the richer it is in exotic plants.
Since the number of new plant species that have arrived lately in North America is larger than the number of native species that have gone extinct, the continent's floral diversity overall has actually increased.
At some local sites, though, introduced plants have replaced native ones. New York's Staten Island, for instance, gained hundreds of exotic species in the 1930s. Meanwhile, urbanization took over native plant habitat. The result was a switch of the island's flora from a native mixture to an exotic one.
Scientists expect the Staten Island scenario to the played out, in varying degrees, across Canada and United States. As the same exotic species come to dominate many landscapes, they'll further homogenize the continent's vegetation.
Hong Qian and Robert E. Ricklefs. 2006. The role of exotic species in homogenizing the North American flora. Ecology Letters. 9(12): 1293-1298.