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The Geography of Species Introduction in Louisiana

How did these alien species arrive? Pathways are the geographical features or patterns that species follow into new areas. Pathways may include shipping lanes, interstate rights-of-way, rivers, ocean currents, or transportation corridors. Media are the materials and physical objects on which the species “hitch a ride,” and may include ballast water, packing material, water trapped in used tires, or outboard motors. Pathways and media together are sometimes known as vectors. Portal is a term used here to describe the point of original introduction, which applies to both intentionally and accidentally introduced species. Once introduced, species may perish or persist in their new environment; those that persist may spread by (1) expansion diffusion, in which they expand contiguously into adjacent areas (for example, nutria); (2) by hierarchical diffusion, where they jump from place to place in a non-contiguous manner (for example, Formosan termites relocated by trucks hauling wood to new cities), or (3) by contagious diffusion (such as a virus, spreading from one to many). Considering only those species that have been accidentally introduced to our land, there are a number of common pathways of arrival and/or dispersion in Louisiana.

Port / Shipping Activity
A premier pathway of species introduction to the Gulf Coast is also one of the region’s most important industries. Ships from distant lands have been importing cargo to our shores since 1699; today, 6000 ocean-going vessels carry over 11,000,000 tons of cargo annually through the Port of New Orleans alone. The 2,340-mile-long Mississippi River accesses 14,500 miles of connecting waterways throughout the North American interior, and provides the entire world with access to that million square mile basin once it passes through southeastern Louisiana and joins the Gulf of Mexico. Species accidentally introduced by ocean-going vessels into ports in Louisiana, Texas, Alabama, and Mississippi have subsequently spread to surrounding areas. Among them:

  • Formosan termites originated in Asia and arrived probably first in Houston in the 1940s by means of wooden pallets used to stack freight.
  • Red and black imported fire ants arrived in Mobile from South America during 1910s-1940s by way of soil and shipping dunnage (packing materials).
  • Asian tiger mosquitoes arrived in Houston during the 1980s as larvae residing in water trapped in used tires.
  • Mediterranean geckos arrived to New Orleans as "stowaways."

Transportation Corridors
Trucks and trains, like ships, may relocate cargo bearing invasive species. The corridors they use also have been documented as pathways; for example, there is evidence that Formosan termites may spread by infesting consecutive ties along railroad tracks.

Waterways

Once introduced to a particular site, the labyrinth of natural and manmade waterways intersecting Louisiana often serve as pathways themselves, allowing aquatic species to proliferate throughout an entire drainage basin. The waters of the Mississippi have brought to Louisiana the zebra mussel, introduced via ballast water dumped in the Great Lakes region, and the rainbow smelt, stocked in northern lakes. Asian clams and Australian spotted jellyfish may spread by water currents, eddies, and phenomena such as the "dead zone" in the Gulf of Mexico.

Equipment / Object / Water Relocation

The relocation of equipment, oil rigs, and boats along terrestrial or marine transportation corridors may disperse aquatic plants, crustaceans, and other creatures over long distances to new habitats or into new drainage basins just a few miles from prior infested areas. The transportation of lumber, firewood, and railroad ties (used for landscaping) has spread Formosan termites to new areas. Dumping of water into waterways also accounts for species introduction. On a regional scale, this phenomenon has been observed in ballast water, as previously mentioned; at the local scale, the disposal of aquarium or bait water into the ecosystem has spread hydrilla, goldfish, dotted duckweed, and Asian clams.

Animal Routes

Native birds and animals are unlikely to introduce species from afar because of their relatively restricted ranges, but they may assist in spreading introduced species throughout the new habitat. Birds, for example, may have helped spread giant salvinia throughout the wetlands of southern Louisiana. When infected by the introduced viruses that cause encephalitis, crows and other birds do not necessarily die immediately, creating the possibility that infected birds' flight paths and migrations become pathways.

Monocultural Croplands
Vast expanses of a single agricultural or silvicultural species have served as pathways spreading pests into previously uninfested regions. Boll weevils diffused from Central America into Mexico and thence into the American South a century ago by infesting contiguous plantations across the cotton belt, causing billions of dollars in damage.

Ecological Niches
If an ecological niche opens up through the eradication of a native species, such as the wolf, that open niche may be viewed as a pathway if it attracts species from neighboring regions. Coyotes arrived to Louisiana from the American West in this manner. Open ecological niches may also help establish introduced species that otherwise would have perished.

Disturbed Ecosystems
Expansive areas of overgrazed grasslands, clear-cut forests, or other disturbed ecosystems enable invasive grasses and other plants to spread. Cogon grass spreads along stressed area paralleling interstates and other rights-of-way in southern Alabama and Mississippi, into Louisiana.

Pathways of the Future
In the near future, we may see new pathways of species introduction develop in Louisiana. Freshwater diversion projects designed to reverse salt-water intrusion and to slow wetlands erosion may carry zebra mussels and rainbow smelt into the wetlands. Global climate change may expand regions of subtropical or tropical climatic conditions, allowing exotic species to survive which otherwise would have perished. Deliberate introductions may increase as immigrant communities attempt to grow traditional foods locally. Globalization of economic activity, while not a pathway per se, will likely increase the rate at which pathways accidentally deliver new species to our shores, expand the geographical range from which these species come, and speed their spread once they are established.