Pathways of Introduction

Invasive species are introduced to new environments in a number of different ways. Understanding the mechanism, or pathway, by which invasive species enter Georgia is important in order to prevent or minimize additional introductions. Species can be introduced either accidently or intentionally. Intentional introductions occur through deliberate human action. Many invasive plants, for example, were originally introduced as ornamentals or forage crops. Others, like kudzu, autumn olive, Japanese honeysuckle, and bicolor lespedeza, were introduced to control erosion or provide food for wildlife (Britton et al. 2004). Some nonnative fish species were imported intentionally to enhance sport fisheries or for aquaculture purposes, while other nonnative species were brought into the state for sale by pet and aquarium dealers. These species may become invasive when they are either intentionally released or escape into the wild and are able to establish viable populations.

Accidental introductions arrive through pathways such as horticulture, aquaculture, commerce, tourism, or travel. Invasive species can be unintentionally transported as hitchhikers by air, water, railways, or roads to new sites on people and their products. Invasive species can enter and spread throughout the state by being carried on construction and farm equipment, airplanes and helicopters, and boats and boat trailers. By not thoroughly inspecting, washing, or rinsing such vehicles, users can easily transport invasive species from one place to another. Contaminated plant nursery stock can also act as a vehicle to transport many invasive insect and pathogen species.

Accidental introductions of aquatic invasive species can occur through the transportation and release of live bait by fishermen and anglers. When live bait or the bait’s packing material is discarded, invasive species may inadvertently be introduced into a new water body. By lodging in nets and other equipment used to harvest baitfish, species can unintentionally be transported into non-infested waters. Stocked fish are another source of invasive species. Though the fish stocks themselves might not be invasive, the water used to transport them could be contaminated with invasive plants, invertebrates, or viruses. Fish stocking by unqualified individuals increases the chance of biological contamination, especially as these individuals are less likely to adhere to regulations and standards regarding transport methods and equipment.

Another prominent pathway for invasive species has been the trade in wood and wood products. In the U.S., 35 percent of all softwood consumed is imported, and up to 70 percent of all international cargo arrives supported by solid wood packing material (SWPM). This material is usually constructed of poor-quality wood, often from trees damaged or killed by pests. Bark inclusions increase the likelihood of the presence of insects, and boards with bark attached can be hidden in middle layers of products such as wooden spools, pallets, or wooden pieces attached directly to the cargo. The recent arrival in Georgia of the redbay ambrosia beetle (Xyleborus glabratus) in solid wood packing material has focused attention on this pathway (GFC 2004). Other non-native insects such as the emerald ash borer, Asian Longhorn Beetle, and Sirex noctilio woodwasp are all thought to have been introduced to North America via SWPM. The transportation of firewood from one place to another is another way invasive insect species are unintentionally moved to new environments. A person hauling infested firewood from home to campsite can move a species hundreds of miles in a single day. Wood from infested or dying trees is often is packed as firewood. The emerald ash borer is being spread from state to state by campers, as well as through the movement of plant nursery stock and green lumber such as ash logs with bark (Tirrell-Wysocki 2007).

Ballast water transport and hull fouling transfers by commercial ships are two pathways for invasive species introductions in Georgia. Ballast water is pumped into a ship’s hull to keep it stabilized and upright. This water is sometimes discharged at the receiving port when the cargo is being loaded or unloaded. Ballast water taken on in any port may include an abundance of live plants, animals, and pathogens not native to Georgia. The organic fouling community that grows on the hull, rudder, propellers, anchor, anchor chain or any other submerged structure of seagoing vessels that are not properly cleaned or maintained are also an important source of invasive species. This growth provides an opportunity for worldwide transport of fouling organisms, particularly on towed barges and other structures like mothballed ships and exploratory drilling platforms. (See Appendix F for a comprehensive outline of invasion pathways.)

Invasive Species Pathway Outline

This outline, adapted from the Invasive Species Pathways Report , lists and groups the pathways of introduction for invasive species. The pathways identified fall into three categories: transportation, living industry, and miscellaneous. For the “Organisms Transported” entry in the outline, the organisms have been listed with the following letter codes to avoid repetition. Some organisms may have been missed and some of the organism categories may be lumped together.

Key to Organisms Transported

ai = aquatic invertebrates (and larval stages) in = insects and similar invertebrates
adp = animal disease pathogens and parasites inv = other invertebrates (not insects)
ap = aquatic plants mbv = microbes, bacteria and viruses
av = aquatic plants pdp = plant disease pathogens
di = dinoflagellates ph = phytoplankton
dp = disease pathogens ps = plants and seeds
gm = gypsy moth si = snails and other invertebrates
hfo = hull fouling organisms tv = terrestrial invertebrates (insects and other arthropods)
v = vertebrates