Introduction to Invasive Species

For the purposes of this document, invasive species refers to nonnative species that have been introduced, either intentionally or accidentally, into areas outside their natural ranges and that cause economic or environmental harm or impacts to human health. They are not a new phenomenon. Over the course of human history, over 50,000 nonnative species have been introduced into North America. Many of these species, such as wheat, rice, cattle, and poultry were introduced as sources of food and now provide more than 98 percent of the U.S. food system valued at approximately $800 billion per year (Pimentel et al. 2005). Other exotic species were introduced for landscape restoration, biological pest-control, sport, or pets.

The introduction of nonnative species has a long history in Georgia. When the English first entered what is now North Carolina, South Carolina and Georgia, they found nonnative peach trees growing wild and in Native American orchards. This is probably because Spanish or French colonists had introduced peaches into Florida in the sixteenth century. From there, Native Americans spread peaches northward where they eventually became naturalized (Crosby 2004). Over time, more nonnative species were introduced into the state. For example, Benjamin Franklin sent upland rice and Chinese tallow tree seeds to Georgia in 1772. While attempts were made to cultivate these rice grains, upland rice was not grown with any great success in Georgia until it was reintroduced by Thomas Jefferson twenty years later (Bell 1966).

While many significant benefits have resulted from these nonnative introductions, over time, accidental or intentional dispersion of some nonnative species into new environments has resulted in negative impacts to the ecological communities of infested areas, or to commercial, agricultural, aquacultural, or recreational activities dependent on these areas. These nonnative species are considered to be invasive species. For a nonnative organism to be considered an invasive species in the policy context, the negative effects that the organism causes or is likely to cause must outweigh any benefits it may provide (ISAC 2006).

Invasive species have historically played an important role in Georgia. Two species that exemplify the environmental and economic damage invasive species can have are Cryphonectria parasitica, the causative agent of chestnut blight, and the boll weevil (Anthonomus grandis). Before 1900, the American chestnut tree (Castanea dentata) made up as much as one-quarter of the tree species in the Appalachian forest. In Georgia, chestnuts were particularly prominent in the Cohutta and Blue Ridge mountains, but were also frequently found in the Ridge and Valley ecoregion, the Piedmont region, and on the Cumberland Plateau. As well as being environmentally important, chestnuts had many economic uses. Due to their large crowns, spring flowers, and edible nuts, chestnuts were extremely popular and widely planted as shade and ornamental trees. Chestnut was the preferred wood for log house and outbuilding construction and later utility and telegraph polls due to its resistance to decay. Because of its high tannin content, chestnut bark and heartwood supplied more than half of the tannin produced for the U.S. leather industry. As late as the mid-1940s, dead chestnut trees were still being hauled out of the North Georgia Mountains for shipment to tannin extraction factories. The chestnut blight fungus entered the U.S. through New York City on Japanese chestnut stock imported as orchard trees in the late 1800s. The infection spread south at the rate of 200 miles every ten years, reaching Georgia in the early 1930s. Nearly every mature chestnut tree in the species’ natural range (possibly three to four billion trees) was killed by chestnut blight by the 1940s. Today, American chestnut trees survive by resprouting from surviving root systems in the soil. However, they rarely mature or produce nuts before falling victim to the fungus (Merkle and Brown 1991). In 2006, a stand of 20- to 30-year-old American chestnut trees was found on Pine Mountain near Warm Springs, Georgia. These trees are able to produce flowers and nuts, and scientists are using their pollen in an attempt to breed chestnut blight resistant chestnuts (Merkle 2006).

The boll weevil is another invasive species that has had enormous effects on the state. The boll weevil is an insect that feeds on cotton buds and flowers, causing extensive damage to the plant. A native of Central America, the beetle entered Texas in 1892, and reached Thomasville, Georgia in 1915. Subsequently, state cotton production plunged rapidly from a historical high of 2.8 million bales in 1914 to 600,000 bales in 1923. Total state losses from boll weevil infestation were estimated at $40 million by 1919. In 1920, some parts of South Georgia lost 50 to 75 percent of their cotton crop to boll weevils. Georgia experienced the greatest decline in cotton acreage of any cotton-growing state by 1934, when cotton acreage dropped to 45 percent of the total land farmed in 1910-1914. Cotton production in the state continued to decline for another 50 years until 1983, when Georgia produced only 112,000 bales on 115,000 harvested acres. Boll weevil infestation was considered by some to be the biggest disturbance of Georgia’s economy since the end of the Civil War. In 1987, Georgia enrolled in the federal Boll Weevil Eradication Program, a cooperative effort involving USDA, state officials, and cotton growers. Consequently, the boll weevil was eradicated in the state by 1991. State cotton production has increased from 482 pounds per acre in the pre-eradication period (1971 to 1986) to 733 pounds per acre in the post-eradication period (1991 to 1995). In 1995, 2.0 million cotton bales were produced on 1.5 million harvested acres (59 percent more than in 1994 and the largest yield since 1919), with total revenues of about $270 million (the highest in Georgia’s history) (Haney et al. 1996). In Georgia elimination of the boll weevil has resulted in dramatic economic benefits, with average gross crop revenues increasing from $70 million per year prior to eradication to $400 million per year afterwards (USDA APHIS 2007).

Introduced nonnative species can presently be found throughout the state in each of its five major ecoregions: the Southwestern Appalachians/Ridge and Valley, Blue Ridge, Piedmont, Southeastern Plains, and Southern Coastal Plain. In the Southwestern Appalachians/Ridge and Valley, the red shiner (Cyprinella lutrensis) is suspected of having a serious impact on the native blue shiner (C. caerulea) through competition and hybridization. Invasive species of concern in this region include cogongrass (Imperata cylindrical), Japanese climbing fern (Lygodium japonicum), Japanese stilt grass (Microstegium vimineum), Chinese privet (Ligustrum sinense), Japanese honeysuckle (Lonicera japonica), oriental bittersweet (Celastrus orbiculatus), princesstree (Paulownia tomentosa), silvergrass (Miscanthus sinensis), and autumn olive (Elaeagnus umbellate). These species, along with kudzu (Pueraria montana), are also a concern in the Blue Ridge ecosystem as is the hemlock woolly adelgid (Adelges tsugae), which is causing significant losses of eastern hemlock as well as the few populations of Carolina hemlock in the region. Asian clams (Corbicula fluminea) and feral hogs (Sus scrofa) are examples of invasive animal species of concern in the Piedmont ecoregion. In addition, most river floodplains and valleys in the Piedmont are overrun with invasive plants such as Chinese privet and Japanese stilt grass. Hydrilla (Hydrilla verticillata), Japanese climbing fern, feral hogs, Chinese privet, cogongrass, and the Asian clam threaten habitats and species in the Southeastern Plains. Finally, the Southern Coastal Plain is facing significant negative impacts caused by flathead catfish (Pylodictis olivaris), feral hogs, Chinese tallow (Sapium sebiferum), tropical soda apple (Solanum viarum), water hyacinth (Eichhornia crassipes), alligatorweed (Alternanthera philoxeroides), parrotfeather (Myriophyllum aquaticum), giant reed (Arundo donax), and the channeled apple snail (Pomacea insularum) (GADNR 2005).

While most introduced species pose little threat to the environment, many do constitute a significant risk. Invasive species rank second only to habitat destruction as a threat to biodiversity. Almost half of the species in the U.S. that are at risk of extinction are negatively impacted by invasive species. Invasive species threaten biodiversity in several ways. They may cause or spread diseases, for example, or act as predators or parasites of native species. Invasive species also impact native species by out-competing them for food and natural resources and/or by altering habitat in such a way that native species can no longer flourish. Finally, invasive species may hybridize with local species so that within a few generations, few if any genetically pure native individuals remain. Of the 26 animal species in the U.S. that have gone extinct since being listed under the Endangered Species Act, at least three were wholly or partly lost because of hybridization with invaders (McGinley and Duffy 2008).

The introduction of nonnative species poses a profound threat to the state’s biodiversity. Georgia ranks sixth in the nation for overall biological diversity (4,004 species) and twelfth for number of endemic species (58 species). Nationally, Georgia ranks second in amphibian diversity (77 species), third in freshwater fish diversity (268 species), fifth in reptile diversity (83), seventh in vascular plant diversity (2,986 species), seventeenth in bird diversity (328), and eighteenth in mammal diversity (91). Unfortunately, Georgia also ranks eighth in the number of imperiled species (533 species), and fourth in number of known or suspected extinctions (24 species), due in part to the introduction and spread of nonnative species (Stein et al. 2000). Georgia currently has 239 native species that are in danger of immediate or foreseeable extinction in all or a significant portion of their range. Seventy-three additional native species occur rarely enough to need protection because of their scarcity (GADNR 2007, O.C.G.A. §27-3-130).

In addition to environmental harm, invasive species can have large economic impacts in the areas where they have become established (see Table 1). The costs associated with fire ants in the U.S., for example, have been estimated at $1 billion/year. In Texas, the agricultural economic losses caused by this ant are an estimated $90 million annually, and Texas spent at least $580 million in 2000 to control this insect (ISSG 2008). Nationally, invasive plant species cause a 12 percent reduction in agricultural crop yields, costing the industry $24 billion in lost crop production annually. In addition, about $3 billion a year in herbicides are used to protect U.S. crops from invasive plants (Pimentel et al. 2005). One recent study placed the U.S. benefit of controlling invasive aquatic plant species alone as being in the billions of dollars (Rockwell 2003). Hemlock woolly adelgid infestations in the eastern U.S. have cost $9 million for research and suppression as of 2007. Likewise, the cities of New York and Chicago have spent $180 million to eradicate the Asian longhorned beetle. Nationwide, infestation of this beetle could kill one third of urban trees valued at $669 billion (GA-ISTF 2007). Costs can be incurred through the loss of economic output, such as reductions in agriculture, aquaculture, fisheries, timber, and tourism, and also through the direct cost of combating and mitigating the impacts of the species (e.g., clogged waterways). Hydrilla, for example, blocks irrigation and drainage canals, enhances sedimentation in flood control reservoirs, interferes with public water supplies, impedes navigation, and generally restricts public water uses. At high densities, hydrilla also reduces productivity of recreational fisheries (OTA 1993).


Table 1. Estimated annual costs associated with some alien species introduction in the U.S. (x million of dollars) (Pimentel et al. 2005)


Category Losses and Damages Control and Management Costs Total
Aquatic weeds 10 100 110
Feral Pigs 800 0.5 800.5
Pigeons 1100 N/A 1100
Starlings 800 N/A 800
Imported fire ant 600 400 1000
Green crab 44 N/A 44
Crop pests 13,900 500 14,400
Forest pests 2100 N/A 2100
Asian clam 1000 N/A 1000
Forest plant pathogens 2100 N/A 2100
Crop plant pathogens 21,000 500 21,500
Livestock disease 14,000 N/A 14,000
Human disease N/A 7500 7500


While the economic costs of invasive species in Georgia have not been adequately determined, their impact could be immense. The state’s agriculture, forestry and tourism industries produce billions of dollars of revenue for Georgia each year. For example, the economic value of land-based agriculture exceeded $8.8 billion in 2002, making it Georgia’s single most productive industry. One out of five jobs in Georgia is related to agribusiness (Peabody 2003). Yet boll weevil eradication cost $99.3 million in Georgia, and continues to cost $4.5 million annually . Likewise, the invasive plant, tropical spiderwort, costs cotton farmers $1.2 million for extra herbicide annually (GA ISTF 2007). Within the agriculture industry, timber is the state’s highest-valued agricultural product. Forestry is also Georgia’s third-largest manufacturing sector, employing 11 percent of the manufacturing workforce at an annual payroll of $2.1 billion while contributing billions of dollars to Georgia’s economy (Daughdrill and Zickert 2001). However, sudden oak death threatens the state’s 9.8 million acres of oak forests (15.6 percent of the state’s trees) valued at $33 billion for timber, wildlife, tourism and urban forests (GFC 2007 ) In addition, Georgia spends in excess of $200,000 a year on detection surveys and suppression for gypsy moth, hemlock woolly adelgid, Phytophthora ramorum (causal agent related to sudden oak death), emerald ash borer, Sirex noctilio woodwasp, various exotic wood borers and bark beetles and cogongrass detection and suppression). Tourism contributes $26 billion to the economy and generates $1.12 billion in state and local tax revenues (Barry 2006). Tourists include hunters, anglers, campers, and wildlife observers whose activities depend on healthy, abundant natural resources.

Another challenge facing Georgia is the potential habitat expansion of invasive species due to climate change. The accumulation of greenhouse gases such as carbon dioxide, methane, and nitrogen oxides is causing many parts of the world to experience changes in annual temperature, weather patterns, and sea levels (IPCC 2007). Although scientists differ in their predictions for what temperature changes are occurring and what may occur in the future, some climate change models predict an increase in July heat indexes across the Southeast U.S. from 8-15oF to as high as 20oF (Glick 2007). Higher average temperatures may enable invasive species to take advantage of weakened ecosystems and out-compete native species. It is estimated that global warming will allow 48 percent of currently established invasive plants and animals to expand their northern distributions if temperatures warm. This can already be seen as warming winter temperatures permit species such as kudzu and garlic mustard to survive in areas much farther north than in the past. In addition, it is expected that global warming will contribute to more severe infestations and habitat damage from invasive insect species, including the gypsy moth (id.). Studies have also shown that increased carbon dioxide levels appear to stimulate the growth of invasive plants. Additionally, herbicides may be less effective as carbon dioxide increases in the atmosphere (Ziska and George 2004). Tests found common agricultural weeds like Canada thistle and quack grass to be more resistant to herbicides when grown in higher CO2 concentrations, making them harder to control. In addition, ragweed plants grown at elevated CO2 levels produce twice as much pollen as plants grown at lower levels. Studies have also shown that poison ivy grows more vigorously at higher levels of CO2 and produces a more virulent form of urushiol oil, the cause of allergic reactions in many people (Christopher 2008).

Because of the potential negative impacts that can result from invasive species, it has become an official public goal in the U.S. to reduce the environmental and economic damage done by harmful, nonnative species. This was articulated in the National Invasive Species Act of 1996 (NISA) (16 U.S.C §4701 et seq. 2006) which established a national objective of preventing new invasive species introductions and limiting the dispersal of existing invasive species in all of the states. In 1999, Executive Order 13112 (Order) established the National Invasive Species Council (NISC), which was charged with providing coordination, planning and overall leadership for federal invasive species programs and outreach to state, local and private partners. The Order also required the Secretary of the Interior to establish the Invasive Species Advisory Committee, a group of 30 nonfederal stakeholders from diverse constituencies (representing state, local and private concerns) around the nation, to advise NISC on invasive species issues. In addition, the Order called on NISC to prepare and issue a national plan to deal with invasive species. The National Invasive Species Management Plan was completed in 2001 and revised in 2008.

As a complement to this federal response, and in recognition of the complex natural resource management issues posed by invasive species, the Georgia Invasive Species Management Plan identifies and characterizes the scope of this problem in the state and outlines a coordinated set of actions towards the following eight objectives:

  1. Coordinate local, state, regional, federal and international activities and programs pertaining to invasive species in Georgia
  2. Control and manage the introduction and spread of invasive species in Georgia through education and outreach
  3. Prevent the establishment of invasive species populations in Georgia through early detection and rapid response programs
  4. Control or eradicate established invasive species in Georgia through cooperative management activities designed to minimize impacts to non-target species
  5. Monitor the distribution and impacts of invasive species in Georgia to determine management priorities
  6. Identify and implement needed research on impacts and control of invasive species in Georgia
  7. Prevent the introduction and spread of invasive species in Georgia through legislative and regulatory efforts
  8. Secure adequate long-term funding for invasive species programs in Georgia

The purpose of the invasive species management plan is to coordinate and support all invasive species efforts in the state in order to improve the efficacy of field actions, and open the doors to funding opportunities for the proposed actions.

Due to the multifaceted nature of invasive species issues, the Committee decided to define the problem by breaking it down by both pathway and species. Species were then prioritized and summarized in tabular form, and existing authorities and jurisdictions were researched and documented. Management actions were detailed according to eight objectives (listed above), and summarized in an implementation table. The plan concludes with a glossary, literature cited section, and appendices.

The draft invasive species management plan was made available for public review from January 16 to February 16, 2009, and one public meeting was held. Copies of the invasive species management plan were available in electronic form on the Wildlife Resources Division, Georgia Department of Natural Resources (WRD) website and were also distributed to interested groups and individuals.

Participants in the planning process included state and federal agencies, universities, trade associations, private industries, port authorities, non-governmental organizations, and research centers. Representatives of these entities, listed in Appendix H, were invited to serve on the Committee, which met six times throughout 2007-2008 to help assemble this plan. GADNR staff oversaw the planning process and assembly of the Committee, assisted by the Carl Vinson Institute of Government, UGA. Funding for this effort came from GADNR.