Invasive Species Control

Restoration of Natural Conditions Following Anthropogenic Disturbance

Often the best time to attempt to control an invasive species is immediately after the first individual establishes, and before it sets seed. Attentive natural areas managers and restorationists often have stories of killing that first invading purple loosestrife plant in its first year of growth in a newly restored wetland. Some countries (e.g., New Zealand) request that citizens report the first sightings of various invasive species, so that the invasive can be removed before it infests the area. However, after the first individuals have established, invasive species are difficult if not impossible to eliminate. To control invasive species, natural disturbances have been used such as fire, flooding, manual removal, shading, substrate removal, herbicides and biocontrol. However, the success of implementing natural disturbance to remove invasives is situation depen dent. Fire may be a good means of controlling invasive shrubs in prairies in the midwestern United States; how ever, fire is not appropriate in Hawaii because indigenous species there did not evolve with fire.

Habitat Protection via Banning of Invasive Introductions

Many countries have laws to prevent the intentional intro duction of invasive species. In the United States, the introduction of alien species is regulated by the USDA Animal and Plant Health Inspection Service, and the movement of species across international boundaries into the US is highly regulated. Laws vary from country to country. A good source of information on the restrictions for the movement of organisms between countries is given on the website of The International Portal on Food Safety, Animal and Plant Health (http://www.ipfsaph.org/).

Biological Control

Biocontrol of invasive species sometimes can be achieved through the introduction of a parasite from the region of the invasive species' origin (Table 2), but the danger exists of introducing yet another invasive species, which could create harm to native species. One biocontrol suc cess story is in the control of Opuntia vulgaris in India, a species which was introduced from Brazil. A scale insect (Dactylopius ceylonicus) was released in 1795, and the insect completely controlled the cactus in India. However, in a similar attempt to control Opuntia on the island of Nevia in the Lesser Antilles, disaster occurred with wide reach ing and unintended consequences. In 1957, the cactus moth (Cactoblastis cactorum) was released to control Opuntia on Nevis, but the moth escaped to destroy popu lations of the rare O. spinosissima in the Torchwood Hammock Preserve in the Florida Keys. Having now reached the Gulf Coast and spread by hurricanes, the cactus moth is spreading across the region, and could eventually threaten populations of cactus in the south western United States.

Insects pests of L. salicaria have been introduced from Eurasia to control this invasive in North America. However, these insect species may attack related species of Lythrum native to North America. Furthermore, some studies suggest that the insects do not eliminate L. salicaria but only reduce the biomass of the species as compared experimentally inside and outside of insect exclosures. Other studies suggest that L. salicaria does not necessarily pose as severe a risk to wetland communities, as the release of the insect species. L. salicaria does not reduce

Table 2 Mechanisms for the control of invasive plant species

Method

Advantages

Disadvantages

Hand or machine cutting

Labor intensive although less so than some other

Most species grow back from

methods

underground parts

Root pulling

Removes underground parts of plant that may regrow

Disrupts the soil and may encourage reinvasion of the exotic species from seeds or plant fragments

Herbicide application

Easy to accomplish relative to more labor intensive methods

May kill native species

Handcutting with herbicide

Very effective removal mechanism, especially for

Uses herbicides, although in very small

application to cut shoots

woody species

amounts

(Bradley method)

Herbicide

Can be effective in the removal of certain species; less

Improper usage may harm the health of

labor intensive than other mechanisms, so that it can

users and/or the environment

be used to treat large tracts of land

Biocontrol (release of insects or

Can target the invading species only

Pests may be unpredictable; native

pathogens transferred from

species may be damaged

their continent of origin)

the species richness of other species, although the effect on the function of the wetland may be impacted.

The introduction of parasites to control invasive spe cies is not always very successful from the perspective of the survival of the introduced parasite. Sometimes the habitat requirements of the parasite are not met, so that these parasites become extinct shortly after introduction. Biocontrol can fail because the parasite cannot be main tained in the invasive environment.

Many argue that biocontrol is worth the risk that the parasite might damage the environment because the cost of doing nothing may be higher than any potential cost incurred or harm caused by the biocontrol agent. While the benefits of releasing biocontrol agents to the ecosys tem may be profound, the potential effects of harmful biocontrol agents are impossible to project. For example, who could have thought that a cactus moth released in the Lesser Antilles would be capable of making its own way across the Gulf Coast of North America.? Others use the same reasoning regarding the lack of ability to project the effects of biocontrol agents, to argue that the usage of biocontrol agents is never warranted.

Physical Control

The physical control of invasive species by digging, removal, and harvesting (Table 2) may or may not be feasible depending on the size of the invasive population, and number of available people to help with the removal (Box 1). Sometimes, the invasive species has built a large reserve of seeds in the soil, so that seedlings from germi nated seeds grow to replace the removed adults in the soil disturbed by the removal activities. Often, the invasion potential increases with the amount of disturbance created by the removal of invasive species, so care should be taken in physical removal programs not to disturb the sites. Especially in restoration projects, it is important to remove invasive species by hand, or the resulting vegetation will be unsatisfactory to meet the regulatory requirements of the restoration process. In such cases, the best approach is to remove invasive species immediately, before the invasives become widespread in the restoration area.

Invasive species can be snipped at the base by hand with a long handled clipper, while carefully avoiding native species (Bradley Method from Australia). The uncut native species are at an advantage since they are already estab lished, and often regrow to shade and reduce the invasive species after the treatment. This selective clipping method is very time consuming, but can have very good results in reducing invasive species in native plant communities.

Mechanical mowing with a tractor or by hand with a scythe can be an ideal way to reduce the amount of an invasive species and encourage native species, particularly if the mowing is done in certain seasons. For example, mowing in the late spring and early summer reduced the amount of Arrhenatherum elatius and increased the native

Box 1 Think globally, act locally: volunteer programs for invasive species control, and research data collection

Volunteers can help with the invasive species problem by volunteering their time to help in management and research projects. The control of invasive species can often be achieved by the removal of invasive species from natural communities, an effort greatly helped by teams of volunteers. The 'weed pulling organizations' are regionally coordinated, and these organizations often have websites with information for volunteers. A short search on the Internet can help volunteers locate an organization in their region engaged in weed removal activities. A few agencies for volunteers to check include The Nature Conservancy, California Invasive Plants Council, Hawaiian Ecosystems at Risk, Invasive Plant Association of Wisconsin, Midwest Invasive Plant Council, Southeast EPCC, and Texas Invasives.

Volunteers also can donate their time to help researchers collect data, e.g., the U.S.G.S. purple loosesetrife volunteer program. Volunteers in this program collect data on Lythrum salicaria around the world. See: http://www.nwrc.usgs.gov/special/purplel/

Danthonia californicum in a grassland. Harvesting by hand also can remove invasive species. Kudzu (P. lobata) is some times harvested for cattle forage, and the invasive Prosopis juliflora can be used for fuel wood (the famed mesquite barbecue charcoal). The downside of putting invasive spe cies to a 'good' use after their harvest is that the transfer of biomass elsewhere can spread seeds or disseminules.

Herbicide

Herbicides are widely used to remove invasive species, but the success of their application depends on the situation and the species involved. Herbicide eliminated E. crassipes (Figure 4) in Lake Hartbeespoort, South Africa, but only after four large scale aerial spraying episodes followed by spot spraying from boats. Sometimes herbicides work best in combination with other removal methods. For example, woody invasive species in grassland or forest situations can be controlled by a combination of hand cutting and appli cation of an appropriate herbicide to the cut stump. Without the herbicide, most woody species simply grow back from the stump after cutting. The bright side to this method is that it is an effective method of woody species removal, but at the same time releasing very little herbicide to the surrounding environment.

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