Botanical barbarians - destruction to native North American plants by invasive species

Sierra, Jan-Feb, 1994 by Robert Devine

During our 200-mile journey along the California coast, we saw dozens of nonindigenous species. Sigg, who chairs the California Native Plant Society's invasive-exotics committee, began pointing out notable interlopers the moment we hit the road: French broom, Scotch broom, pampas grass, ice plant, eucalyptus.

What most worries Sigg and other conservationists is the impact of introduced plants on biodiversity. At first glance this seems contradictory: wouldn't the addition of exotics add to diversity? In some cases, yes. Sometimes an alien species will slip into a community and quietly persist there. Immigrant plants like ground ivy and butter-and-eggs have always behaved demurely in their adopted midwestern homeland. But often an exotic will displace a native species. And sometimes it turns out to be a territory-gobbling imperialist, forcing out entire complexes of native species and simplifying plant communities to the point of impoverishment.

Don Schmitz, an aquatic biologist with Florida's Department of Natural Resources and a leader in the war on exotic plants, cites the paperbark tree. This Australian native infests half a million acres of the Everglades Conservation Area (a protected area bordering Everglades National Park) and is swallowing an additional 50 acres of wet prairie a day. A pristine wet-prairie community contains 60 to 80 species of plants. After paperbark takes over, the total plummets to three or four. Due to the ripple effects from the loss of vegetation, paperbark forests are eerily quiet: virtually no insects, birds, or other animals can be heard. A paperbark forest is "for the most part biologically dead," says Schmitz. "In the last ten years we've probably lost more habitat to exotic plant species than to development."

Exotic plants affect such vital characteristics of an ecosystem as surface temperature, the pace of erosion, and the rate at which nitrogen is cycled. One introduced iceplant species on the West Coast draws salt from the soil; after it dies it leaves the topsoil poisoned for years. Tamarisk trees take in so much more water than the native plants they displace that they have desiccated southwestern and Californian desert wetlands. The frequency of fire on cheatgrass-dominated lands has increased from once every 60 to 110 years in pre-cheatgrass days to once every three to five years today.

AIDED AND ABETTED BY humans, plants slip into foreign territory in infinite ways. Exotic seeds might be mixed into a grain shipment, enter via the ballast of a ship, find their way into a sack of bird seed, or fail from the cracks of well-traveled hiking boots. The majority of exotic plants got where they are today, however, because somebody put them there.

That goes for the five central California exotics that Jake Sigg pointed out through the car window. Timber companies brought in eucalyptus to start tree plantations. Ice plant was established by the Southern Pacific Railroad to prevent erosion along train tracks. French broom, Scotch broom, and pampas grass entered the country as ornamentals. (I cringe when I remember the pampas grass my family planted in our backyard when I was a kid. Come to think of it, we planted eucalyptus and ice plant, too. I wonder how we overlooked the brooms.)