Aquatic invasive species are plants and animals that evolved in one location and are introduced through a variety of means into another location.
Species have always used the oceans to move about the planet. By swimming or hitching a ride on a log, leaf, or coconut, organisms have found new worlds in which to thrive. But until recently, this process has been moderate, limited by the currents and the winds.
Since humans first took to the seas, though, intrepid stowaways have had ever expanding vehicles for dispersing themselves both faster and farther. The result is an increasing number of ocean ecosystems, primarily near shorelines, that are being compromised or wiped out by non-native species.
What Are They?
An invasive, or non-native, aquatic species can be any organism that exists somewhere in or near water where it doesn't belong. When an alien species like this arrives in a new location, several things can happen: It can find its new habitat unwelcoming and die off; it can survive with little environmental impact; or it can take over, harming the naturally existing wildlife in a variety of ways.
Invasive species that thrive usually do so because their new habitat lacks natural predators to control their population. They do damage mainly by consuming native species, competing with them for food or space, or introducing disease.
One infamous example is the zebra mussel, accidentally introduced by a cargo ship into the North American Great Lakes from the Black Sea in 1988. The tiny mollusk multiplied uncontrollably, starving out many of the Great Lakes' native mussel populations and interfering with human structures from factory intake pipes to ship rudders. They've now spread from Canada to Mexico and are considered a major nuisance species. Hundreds of millions of dollars are spent annually to control their numbers.
How Do They Get There?
Most marine invasive species stow away in ship ballast. Large boats have tanks in their hulls that are filled with seawater to counterbalance cargo weight. Boats draw in water at their loading port, in some cases more than 20 million gallons (75 million liters). When the ship arrives at its destination, it releases the ballast—along with whatever species happen to be inside, from schools of fish to microscopic organisms. And these days, there's plenty of opportunity for hitchhiking species. Some 45,000 cargo ships move more than 10 billion tons of ballast water around the world each year.
Invasive species also hitch rides on the outside of ship hulls and on the millions of tons of plastics and other trash that floats around the globe in ocean currents.
Pets acquired through the aquarium and exotic pet trade—and then released—can become invasive species, as can escapees from aquaculture farms.
And ongoing sea temperature rise caused by global warming is allowing non-native species to populate ocean habitats that were once too cold to be hospitable.
Combating Invasives
To combat invasive species, governments are focusing on how they handle ship ballast. New regulations in a handful of countries require ships to exchange their ballast water while out at sea or treat it to kill stowaway species before they are released.
The oceans are so vast and deep that until fairly recently, it was widely assumed that no matter how much trash and chemicals humans dumped into them, the effects would be negligible. Proponents of dumping in the oceans even had a catchphrase: "The solution to pollution is dilution."
Today, we need look no further than the New Jersey-size dead zone that forms each summer in the Mississippi River Delta, or the thousand-mile-wide swath of decomposing plastic in the northern Pacific Ocean to see that this "dilution" policy has helped place a once flourishing ocean ecosystem on the brink of collapse.
Pollution's Many Forms
There is evidence that the oceans have suffered at the hands of mankind for millennia, as far back as Roman times. But recent studies show that degradation, particularly of shoreline areas, has accelerated dramatically in the past three centuries as industrial discharge and runoff from farms and coastal cities has increased.
Pollution is the introduction of harmful contaminants that are outside the norm for a given ecosystem. Common man-made pollutants that reach the ocean include pesticides, herbicides, chemical fertilizers, detergents, oil, sewage, plastics, and other solids. Many of these pollutants collect at the ocean's depths, where they are consumed by small marine organisms and introduced into the global food chain. Scientists are even discovering that pharmaceuticals ingested by humans but not fully processed by our bodies are eventually ending up in the fish we eat.
Many ocean pollutants are released into the environment far upstream from coastlines. Nitrogen-rich fertilizers applied by farmers inland, for example, end up in local streams, rivers, and groundwater and are eventually deposited in estuaries, bays, and deltas. These excess nutrients can spawn massive blooms of algae that rob the water of oxygen, leaving areas where little or no marine life can exist. Scientists have counted some 400 such dead zones around the world.
Solid waste like bags, foam, and other items dumped into the oceans from land or by ships at sea are frequently consumed, with often fatal effects, by marine mammals, fish, and birds that mistake it for food. Discarded fishing nets drift for years, ensnaring fish and mammals. In certain regions, ocean currents corral trillions of decomposing plastic items and other trash into gigantic, swirling garbage patches. One in the North Pacific, known as the Pacific Trash Vortex, is estimated to be the size of Texas. A new, massive patch was discovered in the Atlantic Ocean in early 2010.
Noise Pollution
Pollution is not always physical. In large bodies of water, sound waves can carry undiminished for miles. The increased presence of loud or persistent sounds from ships, sonar devices, oil rigs, and even from natural sources like earthquakes can disrupt the migration, communication, hunting, and reproduction patterns of many marine animals, particularly aquatic mammals like whales and dolphins.
End of the "Dilution" Era
Humans are beginning to see the shortsightedness of the "dilution" philosophy. Many national laws as well as international protocols now forbid dumping of harmful materials into the ocean, although enforcement can often be spotty. Marine sanctuaries are being created to maintain pristine ocean ecosystems. And isolated efforts to restore estuaries and bays have met with some success.
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