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 Pathways of Invasion

Figure adapted from the Invasive Pathways Team: Final Report (2003)

 

Invasive species often enter their non-native environment through anthropogenic activities. There are two ways that a species is introduced through a man-made pathway: intentional transportation of species from one location to another, or unintentional transportation of species as a by-product of the movement of people and goods. There are three major categories of man-made invasion pathways, which include transportation, living industries, and miscellaneous activities. The discussion and diagram below (adapted from The Invasive Species Pathways Team: Final Report) depicts the three main categories of invasion pathways and their associated sub-pathways.

Figure adapted from the Invasive Pathways Team: Final Report (2003)
by the National Invasive Species Council and the United States Department of Agriculture.

 


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Living Industry Pathways

The living industry pathway describes invasion pathways associated with the transportation of living plants and animals. This pathway includes landscaping/horticulture, agriculture, aquaculture, aquarium/pet trade, and the live seafood trade.

Chinese Tallow distribution [Figure: Houston Advanced Research Center based on 2010 data from the USDA PLANTS Database] The landscaping/horticulture pathway is a significant source of invasion and includes the importation of trees and plants or plant parts for nurseries, landscaping, water gardens, etc. This pathway accounts for the thousands of plants that have escaped cultivation and also important, the pests and diseases that "hitchhike" on these ornamentals. An example of an ornamental plant that escaped cultivation is Chinese tallow (Triadica sebifera). This has become a major invader of the Central Southwest and Southeast. The map depicts the current range of Chinese tallow within the western Gulf Coast Region. [Figure: Houston Advanced Research Center based on 2010 data from the USDA PLANTS Database]

Agriculture as a pathway includes plants intentionally introduced as agricultural crops that later escape cultivation, as well as contaminated seed or hay used in agricultural production. Deep-rooted sedge (Cyperus entrerianus) is an example of an invasive species that was introduced via contaminated rice seed and spread through rice production.

The aquaculture pathway includes escapees from aquaculture facilities including fish and oysters, and the diseases, plants, and smaller organisms associated with aquaculture species. Pacific white shrimp (Litopenaeus vannamei) is an example of an aquaculture species that escaped and has established wild populations in Texas and along the East coast. Photo courtesy of NOAA, Hatchery on a shrimp farm[Figure below: Hatchery on a shrimp farm [Photo: NOAA] ]

The aquarium/pet trade is a
significant pathway for aquatic invasives. Fish and other pets such as reptiles, may be intentionally released into waterways by pet owners who can no longer take care of them.  Additionally, many species have escaped from personal or public aquariums.

The live seafood trade is a less common invasive pathway, but does occur via the intentional or unintentional release of live seafood and/or the plant or microorganisms associated with seafood. Asian clam (Corbicula fluminea) is an example of a pervasive species that is thought to have entered the US through the live seafood pathway.

Resources:

Alaska FWS, Invasive Species
 
Aquatic Nuisance Species Task Force

EPA, Pathways for Invasive Species
 
Invasive Pathways Team: Final Report

MIT, Pathways of non-native marine species

 

 

Miscellaneous Pathways

This pathway includes all other types of pathways not associated with transportation or the trade of live plants and animals. These include plant and animal research facilities, intentional release/stocking, government programs, biological control, and release for religious/cultural reasons.

Plant and animal research facilities are those public or private facilities that are associated with research and experimentation on animals and plants. One of the introduction pathways of blue tilapia (Oreochromis aureus), a species that is widely spread throughout the Gulf Coast , was release from a private research facility.

Intentional release/stocking
is a pathway most often associated with the intentional release of fish for sport fishing. There are over two dozen invasive species in the Gulf Coast region as a result of intentional release or stocking of fish by public and private entities.

The government programs pathway describes the intentional release or planting of species to aid in erosion control, wildlife habitat and food, or surface mine reclamation. This pathway includes some of the most pervasive plant invaders in the United States. Crown vetch (Coronilla varia), saltcedar (Tamarix chinensis), and autumn olive (Elaeagnus umbellata) are just a few examples of aggressive invaders that were introduced through this pathway.

The biological control pathway is the introduction of a species for the purpose of controlling an undesirable  species (native or exotic).The grass carp (Ctenopharyngodon idella) was introduced by public entities for the purpose of controlling aquatic plants, and now competes with native fish for food and spawning areas.

The release of species for religious/cultural reasons is not a common pathway; however, one important invader, kudzu (Pueraria montana var. lobata), was released in part for cultural reasons (also released as erosion control and livestock feed).

Nutria (Myocastor coypus) is an example of a species that has invaded through both biological control and intentional release/stocking. Nutria was originally brought to the US and raised for the fur industry until they were ultimately released into the wild when that industry failed. Nutria were then transported by the government to several southern states to help control invasive common water-hyacinth (Eichhornia crassipes) and alligator weed (Alternanthera philoxeroides). The map below  depicts the current range of nutria within the western Gulf Coast region.  [Figure: Houston Advanced Research Center based on 2010 data from the USGS Nonindigenous Aquatic Species Database]  Nutria range in CSWGCIN [Figure: Houston Advanced Research Center based on 2010 data from the USGS Nonindigenous Aquatic Species Database]

Resources:

Alaska FWS, Invasive Species

Aquatic Nuisance Species Task Force

EPA, Pathways for Invasive Species

Transportation Pathways

The transportation pathway is a category that describes all pathways related to the movement of goods and people. This pathway includes boat hull and trailer fouling, ship ballast water release, and transportation of cargo.

Boat hull and trailer fouling encompasses both commercial and recreational boats/equipment carrying both small organisms that have attached to the hard surfaces (e.g. zebra mussels, Dreissena polymorpha ) and aquatic plants that become entangled on propellers or trailers, and are carried from one body of water to another. Common water hyacinth (Eichhornia crassipes) is an example of an aquatic plant that has aggressively invaded southern waterways aided by boat hull and trailer fouling.

Zebra mussel range in CSWGCIN [Figure: Houston Advanced Research Center based on 2010 data from the USGS Nonindigenous Aquatic Species Database]Ship ballast water is another sub-pathway, in which water is carried within a ship's ballast tank from one water body to another. The ballast water, used to stabilize the ship at sea, is commonly taken in at one port as cargo and is offloaded and released as new cargo at another port, often hundreds or thousands of miles away. Ballast water serves as a pathway for transportation of small organisms. According to the U.S. Fish and Wildlife Service, over 21 billion gallons of foreign ballast water are released into US waters every year; and according to the National Oceanic and Atmospheric Administration, approximately 10,000 marine species are transported around the world in ballast water every day. The zebra mussel is an example of an aquatic invasive that was introduced to the United States via ballast water. The map to the left depicts the current range of the zebra mussel within the western Gulf Coast region.  [Figure: Houston Advanced Research Center based on 2010 data from the USGS Nonindigenous Aquatic Species Database]

 

Transportation of cargo encompasses all land/air/water vehicles (both private and commercial) that move goods from one location to another. Invasive species are often hidden within packing materials, luggage, or as stowaways in cargo holds. The red imported fire ant (Solenopsis invicta) is an example of a species that made its way into the Gulf Coast region via transportation of cargo.

Resources:


Alaska FWS, Invasive Species

Aquatic Nuisance Species Task Force

EPA, Pathways for Invasive Species

Invasive Pathways Team: Final Report

MIT, Pathways of non-native marine species

 

 Cost of Invasives

Taxa # of Non-indigenous Species in US Cost (millions of dollars)
Plants
25,000  
Purple loosestrife   45
Aquatic weeds   110
Crop weeds   27,000
Weeds in pastures   6000
Mammals
20  
Feral pigs   800.5
Fish
138 5400
Arthropods
4500  
Imported fire ant   1000
Formosan termite   1000
Gypsy moth   11
Crop pests   14,400
Forest pests   2100
Mollusks
88  
Zebra mussel   1000
Asian clam   1000
Microbes 20,000  
Forest plant pathogens   2100
    

There are currently 50,000 non-native species in the United States. Many of these alien species have a positive impact on our economy (i.e. wheat, rice, cattle, poultry, etc.), however, some species have become aggressively invasive in our ecosystems. These aggressive invaders cause significant economic damage due to crop loss, control efforts, recreation losses, damage to structures, etc. There are also significant environmental costs such as loss of biodiversity, habitat destruction, and loss of aesthetics. The table below (adapted from Pimentel et. al. [2004]) displays the number of non-indigenous species per taxa and the economic cost associated with select invasive species. Pimentel found that the United States incurs a total of approximately $120 billion/year in damages associated with invasive species.

Table excerpted from Pimentel et. al. (2004).

Resources:  Pimentel, D., Zuniga, R. and Morrison, D. 2004. Update on the environmental and economic costs associated with alien-invasive species in the United States. Ecological Economics. 52: 273-288

 


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