Introduction

Invasive Species Defined

What's Being Done

---

 National Invasive Species Information Center

 

< Back

page 5 of 5

 

Submodule 8: Ecological Impacts

Forests and Invasive Species

Forests, covering about 40% of U.S. lands, are valuable natural resources used primarily for recreational purposes and timber. A major threat to forests is habitat destruction as land is cleared for building and agriculture. Forests are also being seriously strained by the invasion of non-indigenous species. These species include weeds and non-native trees, but the major threat comes from insects and pathogens such as fungi and viruses.

 

The USDA Forest Service has established the Exotic Forest Pest Information System for North America (ExFor), a database to provide information on exotic insects, mites and pathogenic organisms with potential to cause significant damage to North American forest resources. This database contains valuable background information on each pest, and serves as a resource for regulatory and forest protection agencies. In addition, ExFor ranks the relative importance of each pest, based on the following basic criteria:

 

• Potential to become established, based on factors such as host susceptibility and the likelihood of the pest to spread
• Potential to spread within North America following introduction by pathways such as wind dispersal and human intervention
• Potential to cause economic damage to high-value crops important to many industries
• Potential to cause environmental harm, especially the likelihood of affecting dominant or keystone species

 

The Forest Service also manages the Invasive Species Program, which oversees information on the management of non-indigenous organisms affecting the country’s forest resources. The goal of this program is “reduce, minimize, or eliminate the potential for introduction, establishment, spread, and impact of invasive species across all landscapes and ownerships”. A National Strategy was developed to integrate collaboration among different branches of the Forest Service to address the following concerns about invasive species:

 

• Prevention, focusing on education and outreach to reduce the chance of unintentional introduction; this is the most cost-effective control measure.
• Early Detection and Rapid Response (EDRR), requiring vigilant monitoring with well-trained personnel.
• Control and Management using integrated pest management principles in cooperation with other agencies and groups since invasive species do not respect boundaries.
• Restoration and Rehabilitation, to return degraded areas to proper ecological function with the goal of preventing reoccurrence of infestations

 

Within this program, the Forest Service also works with other agencies and groups (such as the U.S. Park Service and local weed management associations) to implement a “team approach” in the fight against the introduction and establishment of invasive species. The specific organisms currently of primary concern to this partnership are:

 

Insects – As trade around the world increases, the introduction of non-indigenous insects into the U.S. increases. The first line of defense is to keep these insects out of the country entirely. This requires understanding and then blocking the pathways and hosts through which the invasion would most likely occur. It is also important to have an idea as to which insects not yet introduced pose a potential risk to the U.S. When and if such insects are discovered in the U.S., it is critical to understand their biology in order to develop appropriate and effective control strategies for them.

 

Particularly worrisome invaders in U.S. forests include the following:

• The emerald ash borer (Agrilus planipennis), which was discovered in the Great Lakes region in 2002. These borers damage and kill ash trees, which are of major importance to the timber and landscape industry. People are now being urgently warned not to transport wood from ash trees for any use since the borers potential to spread outside the region is so high. This campaign is being coordinated by a coalition of government and private stakeholders.
• A more recent introduction is the Sirex woodwasp (Sirex noctilio), probably introduced in wood packing materials and now making its way into forested areas in regions near ports of entry. The wasp, native to Europe, Asia, and North Africa, has caused up to 80% pine tree mortality in other areas it has invaded. Approximately 6 months after infestation, pine tree foliage reddens as the tree dies. Fortunately, nematodes (Deladenus siricidicol) have been successfully used to infect wood wasp eggs and to create sterile females wasps, thereby regulating wasp populations.

 

Plants – Dozens of invasive plants pose threats to forest health. These plants can threaten species diversity, damage habitats for native species, and compromise soil integrity.

 

Forest nurseries provide much of the timber for products such as paper and lumber. Plants are particularly likely to invade such disturbed areas. Rapid growth of highly invasive plants such as kudzu (Pueraria lobata (syn. P. montana, P. thunbergiana)) throughout the eastern U.S. and mile-a-minute (Polygonum perfoliatum) throughout the Mid-Atlantic States is sobering examples. Both are able to grow quickly covering everything in their paths and retarding the growth of tree seedlings.

 

Control of weeds in forested regions can be very expensive. In pine forest used for the lumber industry in Arkansas, weed control costs are estimated to exceed $12 million annually. But the investment does produce returns: Pine forests yields given adequate weed control have been shown to produce 63% greater than yields from forests receiving no weed control.

 

In forests, the understory is usually populated with native plants that thrive on the low light filtered through the tree canopy. These plants contribute to proper nutrient cycling and provide habitat and food for native animals. When non-indigenous plants invade, they have a tendency to overtake native plants since they are more aggressive and can often out-compete the natives for resources. This, in turn, can lead to a monoculture with altered habitat, nutrient cycling, and fire frequency.

 

Pathogens – Since about one-third of U.S. land is forested, and since forests provide tremendous recreational, environmental, and economic services, much effort is put into keeping them dynamic and functional. Some invasive forest pathogens have successfully eradicated certain species of tress, completely altering entire regions.

 

The “perfect” forest pathogen is one that can infect quickly and furiously. It is highly virulent, can enter a tree at several different sites, and reproduces rapidly releasing millions of spores. It has no natural enemies in its new environment and its host tree has no natural resistance to it, since both trees and pathogen evolved separately. Historical examples of this in the U.S. include chestnut blight (Cryphonectria parasitica) in the Appalachian Mountains and Dutch elm disease (Ophiostoma ulmi) throughout North America. A more recent fungal disease introduction first identified on the west coast is sudden oak death (Phytophthora ramous). Initially found in oak and tanoak species, it infects many other plants including Rhododendron spp., huckleberry (Vaccinium ovatum), bay laurel (Umbellularia californica), and manzanita (Arctostaphylos manzanita); some of these plants are not killed by the disease but play a key role in harboring the fungus and promoting its spread.

 

 

< Back

page 5 of 5