Dalmation toadflax (Linaria dalmatica dalmatica L.). Photo © Bob Nowierski, Montana State University
Invasive species have been part of our landscape for centuries. From early days—when smallpox-infested blankets took out entire American Indian villages—pathogens, insects and plants have seized opportunities to populate new areas where there was little or no resistance to them. Nutria imported to the West for domestic fur production multiplied, and in the absence of natural predators, chewed their way through native plants in riparian areas. Russian olive trees, sold in nurseries for southwestern gardens, escaped into the wild and began to displace native vegetation. Culinary and medicinal herbs brought from Europe, such as garlic mustard and dyer’s woad, found hospitality and little resistance in eastern forests.
But in recent years, invasive species have reached a critical mass. On public lands, their exponential growth threatens endangered plants and animals, and the outlook is grim. Soon, native species—and the biological diversity that comes with them—could all but disappear, leaving the South blanketed in kudzu, western roadsides overrun with Scotch broom and southwestrrn waterways choked with tamarisk.
Most alien infestations have their roots in human activity. As human populations grow and global trade and travel become common, so does the spread of species from one continent to another. Species from Asia and Europe arrived at our shores on boat hulls or in water in the holds, in imported wood, or even deliberately, as in the case of ornamental plants.
Once a species is established in a new region, human activity further aids the spread. Aquatic invasives, like Eurasian watermilfoil or quagga and zebra mussels, can be carried on boat propellers or in bait buckets from one lake to another. Hay bales containing unwanted seeds may be consumed by pack animals or horses, which carry seeds into wilderness areas in their digestive tracts, or they can be trucked to new areas for forestry projects such as erosion control or bank stabilization. Seeds and spores hitch rides into wilderness areas in the tread of motorized vehicles and on the boot soles of hikers, firefighters and loggers. In short, any traveler can be a carrier.
Across the landscape, the development of open spaces into subdivisions is creating new headaches. Federal weed managers, who once may have worked with one or two ranchers to control weeds, now must deal with hundreds of homeowners on small lots. Even a few horses on a ranchette can lead to overgrazing, leaving disrupted land susceptible to invasives. In addition to creating disturbed areas in former range or woodlands, roads bring another host of problems: materials to build them, such as gravel or fill soil, may be infested with undesirable seeds, and then the roads themselves create new vectors for invasives to reach into previously uninfected areas.
For decades, few admitted how severe a problem invasives were, or could become. “We didn’t have invasive species issues for many years, or we didn’t think we had them…because we weren’t being choked out by them,” says Barbara Schrader, a U.S. Forest Service ecologist in Alaska.
A Clinton-era executive order, signed in 1999, brought attention to the invasive problem through the National Invasive Species Council. In association with a non-governmental advisory committee, the council created a National Invasive Species Management Plan, which contains guidelines for restoring landscapes that were already affected by invasives. Four years ago, former Forest Service Chief Dale Bosworth included invasive species as one of the four primary threats facing the national forests.
Both of these federal actions brought welcome attention—and funding—to invasive weed programs. Across the country, Cooperative Weed Management Areas, coalitions of private landowners and state and federal government agencies, have sprung up in response to invasive weeds spreading across the landscape. In addition to assessments to evaluate the extent of the problem, and to help determine which species should receive what type of attention, the Forest Service has stepped up testing of biological controls in hopes that species that have long seed lives or deep root systems, such as Scotch broom or toadflax, can be controlled even in the absence of human intervention.
According to a 2000 Cornell University study, invasive weeds cost the public an estimated $138 billion each year, and that number is growing. In this issue, we’re exploring some of the strategies for dealing with invaders. We take a look at a Forest Service employee who sounded the alarm about the agency’s strategy for dealing with invasive species, and the concern that many citizens have over the use of chemicals to control them (see “Weapons of Choice”). We examine the effects of the hemlock woolly adelgid on the majestic trees of the Joyce Kilmer Memorial Forest in North Carolina (see “Fighting for Their Lives”). And we explore an aggressive invasive from Russia (see “Fodder for Fire”) and its effect on wildfires in the West.
But with invasive species spreading like wildfire across public lands, we’re only scratching the surface.
Steve Dewey, a professor at Utah State University, draws on the wildfire analogy in suggesting the best approach to weed management. “You can take the same four-part strategy that’s applied to fire: lookout, early detection, suppression or confinement, and restoration,” he says. Many land managers are taking this approach, but unlike wildfire, the threat of invasives is not always easily recognizable, and new species are gaining ground every day. And unlike fire, letting them run wild is not an option.