U.S. Fish & Wildlife Service

Invasive Species Pathways

Invasive species “pathways” are the means by which they are moved from one location to another. Natural pathways could include means such as wind or water currents. Other pathways can be enhanced by, or even entirely created through, human activity. Sometimes this is done intentionally, other times quite unintentionally.

Other Pathways:

Available for download: documentary DVD produced as part of the National Invasive Species Threat Campaign with support from a lot of different organizations including the U.S. Fish and Wildlife Service.

Defending Favorite Places: how hunters and anglers can stop the spread of invasive species

Playing Smart Against Invasive Species: How to Enjoy and Protect the Great Outdoors

Global travel
Invasive Species can travel globally

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Let’s take as an example a weedy species that has a pretty flower and hooked seeds. The seeds of that flower may attach to the fur of a rabbit and spread naturally within the valley that is home to that rabbit. The same seed could have instead attached to the pants or boots of a hiker who then carries it over the mountain to a new valley and thus enhances its spread. Or, at trails end, the hiker may fail to brush off his gear and then board a plane for his home that is far away in a place that this seed may never have reached naturally. Finally, since this plant had such a pretty flower, that same hiker might remember it and upon his return home decide to go to the internet and order a packet of its seeds to plant in his garden. Each of these represent different pathways, natural or human-mediated, enhanced or wholly artificial, intentional or unintentional, that affect how rapidly and to what places a species is moved. If the species thrives in its new home, the whole cycle of opportunities for spread is begun again from this new location. Pathways are thus simply the “modes of transportation” for how species move about.

Some individual species present unique or urgent challenges and offer unique opportunities for controlling their spread. However, there are literally millions of species around the globe that come into contact with humans and are thus potential travelers along these pathways. Even if it was technically possible and society wanted to, it would be unimaginably expensive to develop unique methods for preventing the human-assisted spread of each and every individual species. A much more practical approach is to focus on these pathways and develop methods to reduce the risk of spreading all species that have “access” to these pathways.

For aquatic species, particularly coastal species, one of the most significant pathways for the movement of species from one area of the globe to another is transportation by ships – either on the outside (hull) of the ship or in “ballast water” carried within the ship. Large ships take water (and all of the organisms living in that water) onboard into various internal compartments to provide stability and trim, or ballast, while they are at sea. That water is often taken in at one port and then pumped back out once they arrive in a new port to take on a load of goods. The result is that a stew of living organisms is transported and released in places these species would never have made it on there own.

Among the thousands of species transferred around the globe in this way, organisms such as the zebra mussel in the Great Lakes, toxic dinoflagellates in Australia, and cholera along the Gulf Coast of the United States have caused serious ecological, economic and human health impacts. Ships discharge over 21 billion gallons of foreign ballast water, along with all of its living non-native stowaways, into U.S. bays, estuaries, lakes and rivers each year. This is a huge source of potential aquatic invaders.

Aquaculture: Many non-native species have been intentionally introduced for aquacultural purposes – for example, oysters, Atlantic salmon, tilapia, carp, and others. Many cultured species have caused little harm and produced significant economic benefits. However, others have escaped containment and become a source of potential harm to native species and their habitats. Sometimes the larger concern is with smaller, undetected species that can come along as unknown and unwanted riders with the intended aquacultural species. For example, Pacific oysters were introduced and remain an important seafood product; but their introduction also brought along with them the oyster drill, a harmful pest to both commercial production and native oysters.

Aquariums & Water Gardens: Aquarium pet dealers and water garden suppliers may offer hundreds or even thousands of different species or varieties for sale. Often the vast majority of these are not native to the location in which they are purchased. Sometimes you even get more than you bargained for! It is not uncommon to order one species and have it come with other, potentially more problematic, species mixed in with the shipment. If these non-native species stay in their aquarium or water garden, or are returned to the dealer or otherwise safely disposed of when no longer wanted, this may not present a problem. However, many invasive plants and nearly half of all non-native fish that have become established in U.S. waters are the result of escape from commercial or personal holding facilities or disposals from personal aquariums or water gardens. Hobbyists tired of their pets or unable to move them may think it is humane to dump them into nearby waters, but this is not safe and usually illegal. Released pets can be subject to the diseases and parasites of native species or may spread their own exotic diseases to native species. Many released pets will be eaten by fish, birds or snakes. Some, like pirahna, can be dangerous to humans; others will eat or compete with native species and damage natural ecosystems.

If you have a no longer wanted aquarium species, here are some ideas for what to do protect the natural environment while caring about your pet. Instead of releasing it:

  • Return it to a pet shop to trade/sell
  • Give it to another hobbyist friend
  • Donate it to a school or other public facility (e.g., hospital)
  • If none of these will work, you can put it to sleep at home by
    putting it in a container of water and freezing it. Cold has a
    natural numbing effect and is considered a humane way to put
    it to sleep. If you’d rather not do this, your vet may be able to help.

Recreational or Commercial Watercraft: Small boats, float planes, jet skis, and their trailers, or any other recreational or commercial gear that is regularly in contact with water can move non-native species around, often without us even realizing it. Aquatic plants, in particular, are easily transported when plant fragments get tangled on boat propellers, trailers and other gear. An excellent source of additional information on how to be a clean boater (or pilot) and help stop aquatic hitchhikers can be found at Protect Your Waters web site.

Bait: The use of live bait may or may not be legal in your area. Always check the regulations. If live bait is legal, it is essential that any “left over” bait not be released into the wild. Some bait species, for example certain chub and minnow species, can reach explosive population levels when released into new waters and eventually overwhelm native sport fish species.

Fish Stocking: Many fish species, particularly those considered to be valuable for sport or commercial fisheries, have been stocked in waters to which they were not native. If careful studies are done on both the potential benefits and potential risks of such introductions, and a well-reasoned protocol is followed, these introductions can provide significant social benefits. However, some ill-conceived introductions have caused a great deal of ecological harm. These harmful introductions, often by well-intentioned but poorly informed individuals not associated with the proper authorities, are nearly always illegal. An unfortunate example of this in Alaska is the introduction of northern pike into southcentral waters, including the extraordinarily important Kenai River drainage.

Live Seafood: This pathway is similar in many respects to the “Aquariums and Water Gardens” one discussed above. Both the live seafood itself (e.g., lobster, tilapia, crabs) and the materials in which some live seafood is shipped (e.g., seawater, moist algae) can cause problems if they are allowed to escape confinement or are disposed of improperly.

Biological Control: One of the reasons that some non-native species can become invasive is that the natural controls (e.g., competitors, predators, and pathogens) that kept them in check in their native habitat are not present in their new homes. Sometimes these natural biological control agents from their native habitats can be brought over to control them in their new home as well. If the agent is truly selective and harms only the non-native species that it is intended to control, this can work well. However, sometimes the new introduction can cause other problems of its own and compound the situation by causing additional harm to native species or their habitats.

One important way to reduce the chances of introducing harmful invaders is to use an approach called Hazard Analysis and Critical Control Point, or HACCP (pronounced hassip). HACCP planning is essentially a process for taking a close look at each step along the pathway, identifying which ones present the greatest risks and the best chance to reduce those risks (i.e., identifying “critical control points”), and then taking specific actions at those points to reduce risks in an accountable, tractable manner. Many Alaskans in the seafood industry may already be familiar with HACCP planning from a different perspective. Its original use was as a seafood safety technique for reducing the risk of introducing biological contaminants and protect product quality.