Daily life in most environments, including the watery environments of salty oceans and freshwater streams, rivers, lakes, ponds, and wetlands is a balance between all the living things in the particular environment (ecosystem). Changing the mix of these living things can upset the ecosystem and have undesirable consequences.
Sometimes the change is accidental. A new species (a classification of related organisms that can reproduce) can happen to find its way into a new environment where the conditions include plenty of food, few enemies, and an ideal temperature. The change can also be deliberate, due to humans' attempts to control one undesirable species by adding another. Sometimes, the introduced species thrives in the environment and becomes the dominant species. This can reduce the biodiversity (the vast range and number of different species) of the environment, as the introduced species outcompetes other species. But, this is not always the case. Species that are already present can adapt to the introduced species, survive, and even thrive.
The problem of species introduction
Introducing one organism into a new environment can often lead to disastrous results. For example, rice farmers in the Philippines have had their crops chewed up by the water-loving golden apple snail. The appetite of this snail, which was not native (natural to an environment) to the area, but which has taken over, has cost the farmers an estimated $1 billion. Some water-loving plants can also disturb the biodiversity of an area. For example, the water hyacinth and water lettuce are two plants (normally considered weeds) that grow well in water. Their growth crowds out other plants that are being grown for food. In Africa, about $60 million is spent each year battling these two plants.
A century ago, the problem of species introduction was more of a local issue. An undesirable plant could have flowed downstream in the river to come ashore and take root, for example. Spread of organisms could occur over a wider area, such as on logs drifting in an ocean current. Natural barriers, however, such as oceans, differing climates, and soil conditions often prevented a new species from widespread drift. The world remained mostly distinctive in its distribution of life from place to place (think of all the exotic creatures that are found in Australia and nowhere else).
In today's world of rapid international travel, the spread of unwanted species occurs all over the globe. When a foreign species enters a new environment, there may be few other creatures that can outgrow it or find it a good source of food. So there may be little to stop the explosive growth of the new species.
A major cause of species introduction: ballast water
Water is the perfect container to transport many organisms from one environment to another. Ocean-going ships use water to weigh down and balance the weight of the ship. This water (ballast) is typically pumped into a ship in one harbor after the ship unloads its cargo. When cargo is loaded at a new harbor, the ballast water is pumped out to reduce the weight of the ship, which reduces the amount of fuel needed to power the ship through the ocean water. This practice of filling up with water in one harbor and dumping the water in another harbor means that water, and the living things in that water, can be moved all over the world from one environment to an entirely different environment.
Huge amounts of ballast water can be shifted from place to place. Big ocean-going tankers and cargo ships can hold millions of pounds of ballast water. In the water, a wide variety of creatures (anything small enough to make it through the pumps) can travel with the ship. Scientists have estimated that a typical load of ballast water can carry at least 7,000 different species of microorganisms, plants, and other living things! While all do not survive the journey, many species do survive.
The spread of species in ballast water is now recognized by agencies such as the United Nations as being a threat to the health of the oceans, freshwaters, and to the economy of many countries. Unlike an oil spill that can be quickly detected and whose damage can be at least partly reversed, the spread of species in water is often invisible until the problem is already difficult to treat. Sometimes, an introduced species reproduces so successfully in its new environment that the resulting problems are irreversible.
Examples of species introduction
Studies have shown that ballast water can transport a type of bacteria called Vibrio cholera from place to place. If this microorganism sounds familiar, it is because of its last name. Vibrio cholera is the cause of cholera, a serious disease of the intestines that can cause loss of body fluids in the form of diarrhea and lead to death in severe cases. Cholera is still a big problem in some areas of the world, such as India. People around the world are susceptible to the bacteria (the bacteria can cause disease in everyone), but are not exposed to the bacteria, as it is not found in local waters in numbers large enough to cause disease. When Vibrio cholera is introduced and reproduces in new waters, a cholera outbreak among the local population can result.
Of course, ballast water is not the sole means of transport of the bacterium. In the developing world, where the bacterium exists naturally in the warm water, cholera remains a big problem. The disease can also be spread through contaminated food such as fish and shellfish, and via feces.
Zebra Mussels in the Great Lakes
Zebra mussels are a type of shellfish related to clams and oysters that were normally found in the Caspian Sea area of Asia. Scientists hypothesize that in the mid 1980s some of the mussels got into ballast water of a ship that sailed from that area of the world to a port in the Great Lakes. That port was likely the city of Detroit, Michigan, since the zebra mussel was first discovered nearby in 1988 in Lake St. Clair (a relatively small lake that is located in between Lake Erie and Lake Huron).
In the new environment, where there were no natural competitors to control the numbers of the zebra mussels, their population exploded very quickly. In about a decade, the mussels spread to all of the Great Lakes, as well as into waterways that connect to the lakes.
A big reason for the spread of zebra mussels is their remarkable ability to latch onto surfaces. They can attach so tightly that they can clog up the intake of water into pipes and can slow down the rotation of ship propellers.
The huge numbers of zebra mussels in the Great Lakes has reduced the numbers of microscopic plants and animals that are the main food source of other creatures. As well, the mussels can produce a toxin (poison) that sickens fish and wildlife that eats them. The result has been a drastic and undesirable change in the balance of life in the Great Lakes. Numbers of some native species have dwindled, while the zebra mussel continue to thrive.
Another example of troublesome species introduction involves the Japanese shore crab. While the nature of its introduction is not known, the outcome of its presence has been disturbing for the natural population in the coastal waters. This species of crab is rapidly growing in numbers along the Atlantic coast of the United States around Cape Cod, Massachusetts. This is bad news for the crabs and other creatures that naturally live in the area, as they must now compete with a growing new population for the same amount of available food.
Brian Hoyle, Ph.D.
For More Information
McNeely, Jeffrey, ed. Great Reshuffling: Human Dimensions of Invasive Alien Species. Gland, Switzerland: IUCN, 2001.
Mooney, Harold A, and Richard J. Hobbs, eds. Invasive Species in a Changing World. Washington, DC: Island Press, 2000.
"Alien Species." Secretariat of the Convention on Biological Diversity, United Nations Environment Programme.http://www.biodiv.org/programmes/cross-cutting/alien (accessed on September 8, 2004).
"Ships' Ballast Water and Invasive Marine Species." UN Atlas of the Oceans.http://www.oceansatlas.org/servlet/CDSServlet?status=ND0yMzU0Jjc9ZW4mNjE9KiY2NT1rb3M~ (accessed on September 8, 2004).