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Exotic Plants And Biocontrol

Faith Thompson Campbell, Ph.D.
Executive Secretary
National Association of Exotic Pest Plant Councils

The region included in the Southern Appalachian Man and Biosphere Reserve (SAMAB) is an important center of plant diversity. It is also a highly scenic area with resulting importance of tourism. Other segments of the regional economy, including timber production and the reservoirs, are also tied to the health of the forests.

Unfortunately, the forests of the SAMAB region have been hard hit by a variety of invasive alien species, or biological pollution. These range across the taxonomic spectrum from European wild boars to the chest nut blight. Other participants in the conference will provide updates on the pathogens and insects attacking trees native to the area. Dr. Randy Westbrooks and I will focus on the alien plant species that have also invaded forests.

Review of a list of "worst" invasive plants for continental North America compiled from nearly 30 sources has resulted in a list of 57 plant species that probably are important invaders in the SAMAB region. These include such familiar species as crown vetch (Coronilla varia), purple loosestrife (Lythrum salicaria), English ivy (Hedera helix), privet (Ligustrum spp.), autumn olive (Elaeagnus umbellata), and empress tree (Paulownia tomentosa).

Another hundred species may pose a threat to biological integrity of the region.

Invading alien plant species are not widely recognized as serious threats in the SAMAB region. I suggest this lack of attention -- dangerous, in my view -- stems from the following factors:

  1. these plant invaders cause less conspicuous ecosystem changes unlike situation in Everglades National Park, where the Melaleuca tree is replacing sawgrass, in the SAMAB region we have invading plants that are of similar structure to the natives they replace. In other words, shrubs are replacing shrubs -- or sometimes creating a shrub layer where none previously existed. Trees replace trees, herbaceous understory plants replace native wildflowers. The exotic vines are the most conspicuous, but even they somehow escape notice.

  2. Fewer drastic ecosystem changes are caused by invading alien plant species in the SAMAB region than in some other ecosystems. For example, in Hawai`i, many of the invaders are changing soil chemistry. Also in Hawai`i, as well as the Intermountain West, alien grasses are fueling much larger fires -- which impinge on human activities and command considerable attention. (Unfortunately, the role of exotics in fueling the fires is often not acknowledged at the time of crisis.)

  3. Our inability to show harm resulting from the invading plant to some vertebrate species. In fact, many of these plants have been promoted on the basis that their fruits are wildlife food.

  4. The region's economic interests are not harmed as directly as are those of other regions. For example, livestock ranchers are harmed by invasions of unpalatable leafy spurge, knapweeds, and yellow starthistle of on the Great Plains and farther West.

This lack of economic links is not absolute. Farmers and utility operators hate several of the thistles and kudzu. However, in most cases, economic interests favor these invasive species: between 80% and 100% of the plant species listed here are in the nursery trade. What is being done to counter the most damaging of the invasive alien plant species?

For the past three years, Great Smoky Mountains National Park has been carrying out a control or eradication program against 33 exotic plant species. This work has been funded by a grant under the Natural Resources Protection Program (NRPP). The assumption was that this expanded eradication effort would sufficiently reduce the weed species' range and density that future control work could be carried out under the Park's normal budget for resource management. Now that the program is coming to an end, what is the prospect?

Chemical, mechanical, and muscle-powered control efforts are expensive, particularly in terms of time and person-power. Furthermore, they do little to prevent reinfection from seed sources outside the cleared area. (Here we see the true danger of those species promoted as food for wildlife.) For these reasons, biocontrol appears attractive.

Unfortunately, complications arise with regard to this approach, too. Because they have close relatives in North America, such damaging invaders as the honeysuckles, bittersweet, and Ampelopsis, regretfully, are unlikely to be suitable for biocontrol. Privets are major invaders in Great Smoky Mountains National Park and indeed throughout eastern deciduous forests. However, they are mainstays of the nursery trade and thus such an effort is virtually certain to meet objections by that industry.

On a more hopeful note, Bernd Blossey of Cornell University is interested in pursuing biocontrols for one of the most troublesome of the ground-level herb, garlic mustard (Alliaria petiolata; = A. officinalis). Others are exploring the idea of a program aimed at tearthumb or mile-a-minute vine (Tracaulaon (Polygonum) perfoliatum), a species causing trouble in the Mid-Atlantic region that may spread to this area. Scientists need to discuss priorities species for an expanded biocontrol effort aimed at pest plants of the eastern deciduous forests.

Finally, all involved with research and management in the SAMAB region should increase their efforts to inform their agencies, Congress, the media, and the public about the damage being caused by biological pollution and the options for combating these threats.

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