It’s nearly August, and as I look out on the meadow that surrounds our house in Maine, I see a profusion of goldenrods and asters across the landscape. The meadow derives from abandoned pastures that were grazed by dairy cows until the 1980s. We’ve made a special effort to survey the plants on our property, logging just shy of 200 species. What is surprising is that about 40% of them are not native to North America.
We are concerned about exotic species such as Japanese Knotweed (Fallopia japonica) that are relatively recent arrivals in the area. Indeed, we wage war on barberry and knotweed when we see them on our property. But, meanwhile, a large fraction of the flora—even some of the prettiest wildflowers—wasn’t here a couple of hundred years ago? Should we grant these species a “green card” and bless their presence in meadows that we otherwise consider natural?
Invasive plants are not only a problem in New England. The golden hills of California harbor annual grasses native to the Mediterranean region that invaded North America more than 100 years ago. Cheatgrass (Bromus tectorum) covers much of the inter-mountain west, where it has promoted the risk and extent of wildfires and the demise of pinyon woodlands. Europe and Asia are the primary homelands for exotic species that have successfully colonized North American habitat.
A few years ago, Peter Vitousek of Stanford University estimated that about 11% of the flora of the conterminous United States was composed of alien species. At Washington State University, Richard Mack estimates that about half of this alien flora was derived from deliberate introductions to North America. (The rest may stem from accidental arrival such as a contaminant in crop seeds). Most of the 10 worst weeds in lawns and farmland crops are listed as exotics by the U.S. Department of Agriculture (USDA).
Many garden centers like to supply new species that no one else has in the neighborhood. The international trade in ornamental horticultural species flourishes with little oversight over potential new invasions that might deliver weedy and problematic species for all of us. Indeed, the Trump administration proposed a complete elimination of funding for APHIS (Animal and Plant Health Inspection Service) to inspect and quarantine new plant shipments from overseas. (Perhaps they envision building a wall to keep out invasive species). Mercifully, funding for APHIS was restored to the Federal Budget by the House of Representatives. As yet, no program exists to predict the potential for new species—plant or animal–to become invasive upon arrival in the U.S. (Tree-SMART Trade is a Cary initiative that seeks to prevent the importation of new forest pests - often introduced through the nursery trade).
With so many alien species successful in establishment, how do we draw a line on the potential arrival of new exotics? Or, are we simply destined to homogenize the flora of the world?
References
Mack, R.N. and M. Erneberg. 2002. The United States naturalized flora: largely the product of deliberate introductions. Annals of the Missouri Botanical Garden 89: 176-189.
Mack, R.N. 2003. Plant naturalizations and invasions in the eastern United States. Annals of the Missouri Botanical Garden 90: 77-90.
Tallamy, D. 2007. Bringing Nature Home. Timber Press, Portland, Oregon
Vitousek, P.M. and 4 others. 1997. Introduced species: A significant component of human-caused global change. New Zealand Journal of Ecology 21: 1-16.
