For centuries, as humans have traveled around the globe, they have at the same time, both intentionally and unintentionally, transported plants and animals along with them. Humans are responsible for introducing non-indigenous species for agriculture, horticulture, aquaculture or as domestic pets. Other species arrive unintentionally, through ships’ ballast water, attached to ships, or as stow-aways on shipments of goods including agricultural and aquacultural products, or timber. Early ocean-going ships were made of wood and their hulls were colonized by a large variety of local “fouling organisms”, which were then transported to other ports around the world where they could colonize. These ships often carried ballast in the form of rocks or sand to stabilize the ship; this ballast probably transported many organisms to new locales. Twentieth-century ships are painted with anti-fouling paints to prevent the settlement of fouling organisms, but use water as ballast. Millions of gallons of water, along with the small organisms living in it, are taken up into the ship at one port and released in another. Millions of planktonic organisms including larvae can be contained in the ballast water. When the water is taken up, some sediments are sucked into the ballast tanks as well. The sediment accumulates in the bottom of ballast tanks, where whole benthic communities can be transported around the world. Some fouling organisms still hitchhike around the world attached to nooks and crannies of ships.
When non-native species arrive at a new location, most of them do not survive long enough to become established, but if the new environment is favorable, with few predators and parasites, the new immigrant may flourish. There is usually a lag period of several years between the establishment of an exotic species and its rapid population growth. Scientists think that stressed or disturbed environments are more susceptible to invasion. It is possible that human-caused changes to natural communities (e.g., reduction in diversity or other change in habitat) make them more vulnerable to invasive species.
Ecological and economic consequences of marine invasive species
While most foreign species are harmless, there are many examples of plants and animals that have been moved to new areas where they have caused ecological and or economic problems. These troublemakers are called “invasive” species. Non-indigenous species can become a problem if they cause a decrease in populations of native species, a decline in native species diversity, alteration of habitat, or changes in nutrient dynamics or productivity. Invasive species can cause major economic problems as well (e.g., zebra mussels that clog intake pipes for water systems in the Great Lakes). As international trade and shipping continue to increase, more species will be transported around the world. The great increase in the amount of commercial traffic is moving ballast water all around the world, increasing the transfer of species from one part of the world to another. Although some species are well-known to be invasive in new areas, we cannot predict which species will become a problem, or when or where.
Invasive species have been recognized globally as a major threat to biological diversity, as well as to agriculture and other human interests. The number of invasive species and their impact continues to increase at an alarming rate. Biological invasions have caused extensive ecological changes in salt marshes. Over 300 species of non-native invertebrates have established themselves in North America as a result of human introduction. The rate of introduction has been accelerating in recent decades, probably due to increased shipping and faster trans-oceanic trips.
Controlling the spread of marine invasive species
It is, of course, more cost-effective to prevent invasive species from establishing themselves than it is to try to eradicate them once they have become established. A plan to prevent future introductions should include understanding the pathways, or vectors, through which introductions take place. This realization has led to some steps to reduce the major vector of transport, ballast water. Ships are now required to exchange ballast water before reaching a destination port, releasing organisms from the port of origin in the middle of the ocean, where they are unlikely to survive. The ships then take in ocean water, with its collection of planktonic organisms that will be released in the destination port. Oceanic plankton are not likely to survive long and establish themselves in the variable environmental conditions in a port. While promising, this method is not always a viable option, as ballast water exchange cannot be done in stormy seas, since it would destabilize the vessel. Thus, efforts are underway to develop treatment techniques to kill the organisms in ballast water. Another approach is to evaluate invasive species that have become established to see if they can be contained, if not eradicated. Such is the case with invasive marsh plants on the eastern coast of the U.S., where Phragmites removal is common, and on the West Coast, where Spartina removal is common.
Despite these efforts, it is likely that continued changes in the environment of origin and in the recipient environment will alter both species availability for transport and the degree of susceptibility to invasions, such that they will continue to occur in the 21st century. In addition to human-caused modifications in the local environment, climate change, in particular, will interact with species arrivals in new areas to modify ecosystem functions and biological diversity.
- Ruiz, G.M., Fofonoff, P.W., Carlton, J.T., et al., 2000. Invasion of coastal marine communities in North America: Apparent patterns, processes, and biases. Annual Review of Ecology and Systematics 31: 481-531 2000