Aquatic Invasive Species
Thousands of freshwater, estuarine, and marine species have been dispersed or transplanted across the globe by humans. These aquatic invasive species arrive in the ballast or on the hulls of ships, through the movement of shellfish and bait, by the opening of new channels or canals, through intentional release, and other vectors. Once established, they can change ecosystems, reduce native biodiversity. and impact local economies. Aquatic communities are becoming increasingly homogenized as a result.
In recent years, the introduction rate of plants, animals, and protists and other microorganisms has been accelerating. The invasion rate of freshwater cladocera, small crustaceans such as the water flea Daphnia, is now 50,000 times higher than the background level before humans played a dominant role in species transport. In San Francisco Bay, a new aquatic species becomes established every 14 weeks. (Before 1960, the rate was approximately once every 55 weeks.) This acceleration is likely because of a rise in propagule pressure, the number of individuals released in a particular area, and human disturbance to aquatic systems. Propagule pressure has risen as a result of increased shipping traffic and aquacultural activities. Humans have also changed aquatic systems through eutrophication (the increase of nutrients such as nitrogen and phosphorus), the removal of top predators, and other modifications.
San Francisco Bay is one of the most invaded bodies of water and also one of the best studied. There are now over 230 exotic species in the bay, and in some communities, nonindigenous species make up more than 95% of the biomass and total abundance of organisms. New species have come in through ship fouling, ballast water, intentional transplants, or by hitchhiking along with bait packaging. Hydrozoans, amphipods, isopods, copepods, mollusks, crustaceans, and algae have all become established in the bay in recent decades. Surprisingly, the establishment of an introduced species, rather than impeding further invasions, appears to facilitate the settlement of other nonindigenous species. Interactions among species can accelerate the impacts to native ecosystems leading to an invasional meltdown.
San Francisco Bay is well studied, but not alone. From the Great Lakes to Tokyo Bay and the coast of Tasmania, species have been transported around the globe since humans first took to the seas. In fact, there are probably few if any marine, coastal, or inland water systems that have not been impacted by aquatic invasives. At least 70 alien species have been found in every estuary that has been surveyed in the continental US. The number is likely much higher, as many species may have been transported in ancient times before there was detailed information on aquatic flora and fauna.
Ballast water and other vectors
A vector provides the means of transport from a species' native range to its new environment. Ships and boats have probably been the primary vectors for moving aquatic invasive species. Historically, organisms may have attached themselves to the hulls of vessels or been transported through dry ballast, such as rocks and sand. In recent years, ballast water has become a major vector. On any given day, thousands of species are transported around the world via ships. Post-transport ballast water contains high densities of both holoplankton, organisms such as dinoflagellates and jellyfish that spend their entire life as plankton, and meroplankton, the temporary larval stages of crustaceans, worms, and fish. Because ballast tanks may hold millions of liters of water, numerous individuals can be introduced in a single event.
The construction of canals can remove dispersal barriers, allowing species to cross between water systems that may have been isolated for millions of years. The completion of the Suez Canal in 1869, for example, led to the Mediterranean incursion of several species for the Red Sea. These Lessepsian migrants, named after Ferdinand de Lesseps, the French diplomat responsible for the construction of the canal, show no signs of reduced genetic diversity in their new range, indicating that individuals regularly cross the canal.
Many fish species, such as the Nile perch (Lates niloticus) in Lake Victoria and Asian swamp eels (Monopterus albus) in the US, have been intentionally released as food sources. Others like the walleye (Stizostedion vitreum) and rainbow trout (Oncorhynchus mykiss) have been moved to stock waterways for sport fishing. Additional important vectors include releases from aquariums, both intentional and accidental, and commercial aquaculture, especially shellfish. Oyster transfer is considered the primary vector for the spread of invasive macroalgae in the Mediterranean and is probably the source for many invasive animals as well. Algal packing material used for shipping live seafood and bait may contain juvenile crabs, snails, mussels, and other organisms. Often discarded near shore, live seaweed appears to be emerging as an important vector in the United States.
Ecological and Economic Consequences
Aquatic invasions can lead to profound ecological changes. New species may be competitors, disturbers, consumers, or prey. They can cause local extinctions through competitive exclusion, niche displacement, hybridization and introgression with native species, and predation. The seaweeds Caulerpa taxifolia and C. racemosa overgrow seagrasses, creating monocultures that have been described as biological deserts. Filter feeders and species that act as ecosystem engineers, creating and modifying habitats, can have big impacts on biodiversity and ecosystem function. The Japanese eelgrass (Zostera japonica) for example, converts tidal flats, important foraging grounds for shorebirds, to eelgrass beds and alters nutrient fluxes in its invasive range in the Pacific Northwest.
A species' arrival may be spectacular, as in the case of zebra mussels (Dreissena polymorpha) in the Great Lakes or the comb jelly Mnemiopsis leidyi in Europe, or it may go unnoticed in the absence of molecular tools and careful monitoring. Along the coast of California, the decline of the native bay mussel, Mytilus trossulus, was undetected until genetic assays revealed that a nonnative species, M. galloprovincialis, had displaced it from much of its range. The two species now exist in a highly dynamic state, following temperature and salinity gradients in a pattern that includes hybrid zones. After several aquacultural introductions, all three species of blue mussels (M. galloprovincialis, M. trossulus, and M. edulis) have now hybridized in Puget Sound in the Pacific Northwest of the United States.
Human-mediated invasions cost billions of dollars in damages each year, impacting commercial and recreational fisheries, ecosystem services, human health, and native biodiversity. One study estimates the impact of all invasive species, including terrestrial and microbial species, as more than $314 billion per year for six countries (US, UK, Australia, South Africa, India, and Brazil). In the US, environmental losses from invasive mussels are $1.3 billion per year, and $1 billion per year for invading fish. After Mnemiopsis became established in the Black Sea, the collapse of the anchovy fishery cost harvesters about $250 million per year.
Invasions cause ecological change, but human alteration of the environment can also facilitate the establishment of nonnative species. Overfishing, habitat destruction, climate change, eutrophication, and pollution can alter aquatic ecosystems, making them more vulnerable to invasion. Wastewater discharge and bottom trawling in the Mediterranean impacted native seagrasses, helping an invasive strain of the tropical algae Caulerpa taxifolia colonize more than 130 square kilometers of seafloor in six countries in less than 20 years. Rapid expansion of the algal clone continues. In the Gulf of Maine, climate change and sustained overfishing may be acting synergistically, creating an environment that favors invasive species such as the green alga Codium fragile and the bryozoan Membranipora membranacea.
Genetics of invasions
The traditional scenario of a biological invasion is a single successful inoculation and colonization event, followed by the establishment of a self-sustaining population and spread of the species. The startling rise in the success of aquatic invasive species has occurred despite this apparent 'genetic paradox.' Small founding populations of introduced species are expected to have genetic variation that is lower than that of native populations as a result of bottlenecks. Lessons learned from conservation genetics lead us to expect that such arrivals would be subject to high risk of inbreeding and extinction. Drift and founder events should also limit the ability of such populations to adapt. How then do these species become established, expand their invasive range and respond to novel environmental conditions? In many cases, introduced species don't have to overcome these effects. The discharge from the ballast of a large ship can release numerous individuals in a single event. Preliminary bottlenecks can be overcome by gene flow from multiple source populations, a pattern that has been found in the European green crab (Carcinus maenas) in North America, the Eurasian spiny waterflea in the Great Lakes, and numerous species that have undergone genetic analysis. Genetic variation is not essential for invasion success. Some species invade as clones. A single American lineage of D. pulex replaced a diverse assemblage of genotypes in Africa. In 75 years, this new lineage has become dominant despite the presence of resting native egg banks and competition from native D. pulex and ten additional daphnid species.
Beyond the ability to determine variation and infer propagule pressure, genetic tools can be used to detect, identify, and monitor invasive species. Because morphological identification of larvae and eggs can be particularly challenging, genetic techniques such as DNA barcoding, which employs variation in DNA sequences to identify particular taxa, offer promise to help uncover cryptic invasions. Molecular studies may also prove helpful in the postinvasion control of aquatic introduction. An understanding of genetics will be critical in assessing proposed control efforts of introduced species using genetic engineering or parasites and pathogens.
The prevention of invasions requires the management of vectors from the point of origin to arrival. Effective quarantine regulations are essential. More than 50 national and international laws and regulations are in place to restrict the transport of nonnative species. Yet few of these carry stiff penalties for noncompliance. To date, Australia and New Zealand have some of the most proactive approaches to preventing, eradicating, and controlling aquatic invasive species.
Ballast-water exchange is generally considered to be one of the most effective ways to reduce the number of potential propagules crossing the oceans. In the United States, the highly publicized spread of the zebra mussel in the Great Lakes in 1988 helped pass the Nonindigenous Aquatic Nuisance Prevention and Control Act of 1990. The law was expanded and renamed the National Invasive Species Act, or NISA, in 1996. Ships entering the Great Lakes are now required to exchange their freshwater ballast with salt water before entering the lakes. Yet for many coastal areas, ballast-water exchange remains voluntary and the efficacy of the program unknown.
Government policy to prevent new species introductions is more cost effective than control after establishment. In addition to ballast-water exchange, fouling management programs have been proposed to help reduce the spread of organisms from communities that foul the hulls of ships. Public education about the risks of releasing aquatic plants and pets and the accidental distribution of exotics in bait and seafood algal packaging materials is also essential in curbing the damage from these vectors.
Once an aquatic invasive species has been established, physical removal has rarely been successful. More than 30,000 Northern Pacific seastars (Asterias amurensis) were removed from the shallow waters of southern Tasmania in 1993. In 2001, there were more than 140 million individuals in the area. Yet there are a few success stories. About 1,600,000 native turban snails (Tegula funebralis) were taken by hand from the rocky intertidal zone of southern California to prevent the spread of an introduced South African worm that infected the mollusks. No infections were detected after the snails were removed and screens were installed at the suspected source, an abalone aquacultural facility.
Eradication is less costly than prolonged control programs, and it is most feasible in the early stages of invasion when distribution is limited. Risk assessments, including the probability that a species will establish and the harm that it is likely to cause, can be used to prioritize management actions. Introduced species that can facilitate later invasions are of special concern. The arrival of the European green crab in San Francisco Bay, for example, resulted in the aggressive spread of a previously rare introduced bivalve in the western US.
In some cases, biocontrol is a possibility. The use of parasites, species-specific predators, or disease can help reduce the impact of invasive species in freshwater and marine ecosystems, although there are risks involved with introducing new species into ecosystems. The development of new fisheries has been proposed to reduce populations of edible invasives, such as the European green crab and common periwinkle (Littorina littorea) in northeastern North America, although cultural barriers and concerns about creating a sustainable market for the exotics remain.
References and Further Reading
- Braby, C. E., and G. N. Somero. 2006. Ecological gradients and relative abundance of native (Mytilus trossulus) and invasive (Mytilus galloprovincialis) blue mussels in the California hybrid zone. Marine Biology 148:1249-1262.
- Byers, J. E. 2002. Impact of non-indigenous species on natives enhanced by anthropogenic alteration of selection regimes. Oikos 97:449–458.
- Carlton, J. T. 2001. Introduced Species in U.S. Coastal Waters: Environmental Impacts and Management Priorities. Pew Oceans Commission, Arlington, VA.
- Cohen, A. N., and J. T. Carlton. 1998. Accelerating invasion rate in a highly invaded estuary. Science 279:555-558.
- Lockwood, J. L., and M. L. McKinney. 2001. Biotic Homogenization. Kluwer Academic, New York. ISBN: 0306465426
- Mooney, H. A., and E. E. Cleland. 2001. The evolutionary impact of invasive species. Proceedings of the National Academy of Sciences of the United States of America 98:5446-5451.
- Mergeay, J., D. Verschuren, and L. De Meester. 2006. Invasion of an asexual American water flea clone throughout Africa and rapid displacement of a native sibling species. Proceedings of the Royal Society London B Biological Sciences 273:2839–2844
- Occhipinti-Ambrogi, A., and D. Savini. 2003. Biological invasions as a component of global change in stressed marine ecosystems. Marine Pollution Bulletin 46:542-551.
- Pimentel, D., S. McNair, J. Janecka, et al. 2001. Economic and environmental threats of alien plant, animal, and microbe invasions. Agriculture, Ecosystems, and Environment 84:1-20.
- Roman J. We shall eat them on the beaches. New Scientist. September 10, 2005.
- Roman, J., and J. Darling. 2007. Paradox lost: genetic variation and the success of aquatic invasions. Trends in Ecology and Evolution 22:454-464.
- Simberloff, D. and B. Von Holle. 1999. Positive interactions of nonindigenous species: invasional meltdown? Biological Invasions 1:21-32.
- USGS. 2007. Nonindigenous aquatic species.
- USGS. 2007. Nonindigenous Aquatic Nuisance Prevention and Control Act of 1990.
- Williams, S. L and E. D. Grosholz. The invasive species challenge in estuarine and coastal environments: marrying management and science. Estuaries and Coasts 31:3-20.