A Conflict of Species The Zebra Mussel and its Effects on the NorthAmerican Ecosystem as a Result of the Post-Columbian Invasion
by
Kevin C. KearneyIntroduction
Grade schools teach their children that "In 1492, Columbus sailed the ocean blue." True enough, but little mention is given of the countless changes in the North American ecosystem due to the wealth of non-indigenous species introduced since that time. The North American continent was considered virgin territory by present-day revisionist historians, "an untouched world, a prelapsarian Eden of astonishing plentitude" (Sale 268). However, at the time of its discovery, this land was viewed as a wasteland that was in need of settlement in order to reach its full potential. Alexis de Tocqueville wrote that this continent was a "desert land" prior to settlement by the European peoples (Lord and Burke 22). Somewhere in between lies the truth, although Kirkpatrick Sale is regarded as somewhat closer. In any case, this land was about to undergo countless changes with the coming of countless new inhabitants - inhabitants that would change the face of not only this continent, but the world as well. There are many different ecosystems that exist on this planet, ranging from the Sahara Desert to the Arctic tundra; the Amazon rainforests to the deciduous forests of the eastern United states. In each ecosystem, there are systems of checks and delicate balances between every living thing within that ecosystem. Intricate food webs are a staple of every ecosystem in the world. One organism is consumed by another, which is then preyed upon by others, and those predators under pressure from still others, and so on. If one part of that web is removed or otherwise disturbed, or a new, more voracious predator is introduced to a particular ecosystem, then the delicate balance of that ecosystem is damaged, often beyond repair due to the prevailing conditions. Herein lies the prevailing environmental problem since time immemorial - living things are constantly moving on to new (and often better) environments and changing them forever. Many species were introduced to the New World from the Old, and vice versa. Horses were brought over by Spanish settlers in Mexico. The natives thought that a man on a horse was an entirely new creature they had never seen before. Wild horses now run free on the midwestern American plains, as well as other places such as Assateague Island, Virginia. European settlers brought many diseases that Native Americans had no tolerance for and therefore could not battle with their different immune systems. Millions of natives died due to the germs and bacteria that cause these diseases. Kudzu is an ornamental plant that is indigenous to China and the Orient that was introduced to the southeastern United States. This plant is now seen overgrowing buildings, telephone poles, and even junkpiles. As for the Old World, it received its fair share of new species. Potatoes were introduced to Europe, and corn was soon grown in Africa. This "exchange" that was prompted by Christopher Columbus' drive to find a new trading route to the Far East turned out to be a "global swap of animals, plants, people, ailments, and ideas" (Lord and Burke 33) that is still going on today. These kinds of exchanges and introductions will continue indefinitely into the future, as long as man travels the globe and drag along nature behind him. Kirkpatrick Sale said it best when he wrote that the Columbian Discovery, enabled the vast redistribution of life-forms, purposely and accidentally, that has changed the biota of the earth more thoroughly than at any time since the end of the Permian Period, in effect joining the continents of the earth that were separated so many geological eons ago and thereby causing the extinction, alteration, and even creation of species at a speed and on a scale never before experienced (Sale 4).When confronted with the notion that if it had not been for Columbus, he would not be where he is, Fidel Castro replied, "Columbus brought good things as well as bad" (Schlesinger 18). Leave it to someone like Castro to put things in perspective for a perspectives class. The list of species exchanges between continents could go on forever, as could their potential consequences. "Invasions of animals and plants...will continue far as the next Millennium and possibly for thousands of years beyond it" (Milne and Milne 3). However, the purpose of this report is to show how one species introduced from the Old World to the New can disrupt the delicate biological, ecological, and even economical balances in a certain ecosystem. This report focuses on the zebra mussel Dreissena polymorpha, a bivalve mollusc native to eastern Europe that was introduced to the northeastern United States - particularly the Great Lakes region - relatively recently. This small animal reproduces at an astonishing rate and can aggregate in groups of thousands in the right environmental conditions. It is now recognized as a definite problem by most environmental action groups and research is currently underway in order to find out ways to limit this animal's spread or eliminate it from its newfound home altogether. The Zebra Mussel Dreissena polymorpha
Some Basic Biological Background of MusselsMussels are invertebrate animals belonging to the Phylum Mollusca. This quite large taxonomic group of animals includes such creatures as clams, snails, and even octopi. Most molluscs possess a hard (in some cases, soft or only semi-hard) shell made up of biodeposited calcium carbonate (Pechenik 219). Mussels belong in particular to the Class Bivalvia, Subclass Lamellibranchia. Bivalve molluscs are characterized by two adjoining and hinged shells enclosing the animal, two siphons for pumping in and pumping out water, the absence of a head and sensory structures, and a sedentary lifestyle (Pechenik 241-242). Bivalves are filter feeders, meaning they strain microscopic plants (phytoplankton) and animals (zooplankton) from the surrounding water (Pechenik 243). The majority of bivalves belong to the lamellibranch subclass, with most species inhabiting the marine environment and a few species living in freshwater. These animals have plate-like gill structures that water passes over from the incoming siphon for oxygen exchange (Pechenik 246). Mussels differ from other bivalve molluscs in that they attach themselves to hard surfaces and usually don't let go. They can attach themselves to rocks, wood, metal, and even shells of other molluscs (other mussels included). These animals accomplish this feat via the formation of byssus threads - proteinaceous strings as strong as nylon thread that are formed by the secretion of a gland in the animal's foot which hardens when it comes in contact with water (Sumich 217-218). These threads allow the mussel's attachment to virtually any hard surface - a fact which will have economic effects seen later. Mussels reproduce sexually by the synchronous release of eggs and sperm into the watery environment. Upon fertilization, a larval form known as a veliger develops. Veligers are characterized by being microscopic miniatures of their parents, with a ciliated structure (velum) that allows the larvae to swim about as part of the zooplankton (Pechenik 268). After three to four weeks, the veliger settles on a hard substrate and begins to develop into an adult, attaching itself with its byssus threads (Sumich 218). Zebra Mussels In Particular
Origins
It is believed that zebra mussels were first introduced to North America in the mid to late 1980's. They were first discovered in Lake St. Clair, near Detroit, Michigan, in 1988. After one year these animals could be found attached to almost every hard surface in Lake Erie (Reuter 1). These thumbnail-size creatures with their distinct black and white striped pattern are native to the Caspian Sea area of eastern Europe and westernmost Russia. Canals built during the early 19th century throughout Europe easily facilitated the spread of the zebra mussel all through Europe and Britain by the 1830's (Reuter 2). Transoceanic shipping is believed to be the culprit responsible for the spread of Dreissena polymorpha and its close relative D. bugensis, the quagga mussel, to North America. Cargo ships in Lake St. Clair discharged ballast water that was taken on in Europe that may have contained adult or larval forms (or both) of these mussels (Reuter 2). Today's faster ships making shorter voyages back and forth on the Atlantic facilitate safe transport of these animals to their newly conquered homelands. Zebra mussels have now spread to all of the Great Lakes and waterways in 18 of the United States and two Canadian provinces, including the St. Lawrence Seaway and the Hudson, Illinois, Mississippi, Ohio, Arkansas, and Tennessee rivers (Reuter 1). Biological and Ecological Effects The staggering rate at which zebra mussels reproduce can be seen as a cause for their rapid spread in North America. Egg release by female mussels is temperature dependent, where optimum range for spawning is between 54o Fahrenheit (12o Celsius) to 68o F (20o C). Over the course of a season in which temperatures are in this range, a mature female zebra mussel can produce up to one million eggs (Reuter 2). As mentioned before, mussels like Dreissena attach themselves to hard surfaces near the end of their veliger larval stage. With only about a third of the larvae surviving to maturity, beds of zebra mussels in Lake Erie still reach 30,000 to 70,000 mussels per square meter (Reuter 2). These animals reach a maximum size of four to five inches (although most are much smaller), and their lifespan is about four to six years (Reuter 3). The disruption of the aquatic food web is an important concern when dealing with zebra mussel. These filter feeders are capable, when adult, of filtering nearly all particulate matter - zooplankton and phytoplankton in particular - out of one or more liters of water per day (Reuter 3). The phytoplankton make up the base for the food web in this ecosystem, providing a food source for the zooplankton, which in turn are a food source for larval and juvenile animals like fishes and amphibians. Effects on the fisheries in the Great Lakes are still under intense study, and studies of the full effect of the zebra mussel's competition for resources are as yet inconclusive (Reuter 3). This prolific filtering of water by the zebra mussel has a two-pronged effect. First, it has had a dramatic effect on water clarity. In the 1970's, water clarity in Lake Erie was 3 feet, now, after colonization by the mussel, it has improved to 10 - 17 feet (Reuter 3). Second, the mussels are prone to bioaccumulation of organic pollutants, such as PCB's. Studies show that the zebra mussel can accumulate these poisons in their tissues more that 300,000 times the concentration of the compounds in the environment (Reuter 3). So predators of the zebra mussel will have these high concentrations of toxins in their bodies, and if they happen to be fish - the kind we like to eat - the consequences could be very risky. Zebra mussels are known to attach themselves to North American native mussels and crowd them out of their own habitat. Severely reduced populations of native mussels have been observed in Lake Erie and Lake St. Clair (Reuter 3). Economic Impacts
The zebra mussels affinity for hard substrates makes it a blight for industries surrounding the waters that it inhabits. Intake structures such as those for sewage treatment and power plants often become subject to fouling by these aquatic versions of locusts, resulting in reductions of pumping capabilities and shutdowns (Reuter 4). These animals also can attach themselves to both industrial and recreational boats, making cleanup costs increase dramatically. It has been estimated that the total cleanup costs for power and sewage plants will reach $5 billion by the year 2000 (Reuter 5). Conclusions
Control Measures
Various methods of population control are currently being studied in an attempt to stem the tide - literally - of zebra mussel spreading. While it appears that this animal has made a permanent fixture of itself in the Great Lakes and neighboring ecosystems, methods such as disrupting the reproductive process, chemical toxins, microorganisms such as bacteria, are all under constant study to try and prevent this animal from spreading further. So far, the greatest success has come from natural predators, although the birds and fish that prey on the mussel don't eat as much of this invader as we would hope. In Germany, diving ducks and coots consumed 97% of zebra mussels in the Rhine River over one winter. While back here, birds that feed on these mussels include scaup, canvasbacks, and squaws, but they don't feed on these mussels to the extent seen in Germany. Mallard ducks have also been observed foraging for zebra mussels on shore. Sheepshead and yellow perch are among the fish that prey on zebra mussels; the sheepshead in particular is an excellent predator, feeding substantially on these bivalves (Reuter 4). Future Considerations
Currently there are guidelines for boatsman and ship captains that, ideally, should help stop the spread of zebra mussels to other lakes and rivers in the U.S. They include removing visible vegetation, flushing the bilge pump, cooling systems, and live wells, drying the boat for at least 48 hours after contact with infested waters, and so on (Reuter 5). The Environmental Protection Agency is pushing Congress to pass a law that requires overseas tankers and cargo ships to change ballast water in mid-voyage, to prevent any further unwanted introductions from other continents. As for the future, who can really say which species belong where and which deserve to survive? The North American continent and the world itself have been changing with the times ever since time began. Evolution and natural selection - as defined by Charles Darwin - are not simply limited to living things, but also to where those living things live; their habitats, their ecosystems. Of course, some of the blame must be put on the shoulders of Christopher Columbus and the Europeans who brought about these changes. "Ecohubris" is still viewed as the number one crime committed by the early settlers (Schlesinger 25). But one cannot fault the Admiral of the Ocean Sea for bringing about the inevitable. North America and the world were changing long before there ever was a Columbus. Ecosystems adapt to new inhabitants, change, and survive. This is the way it is and always will be. The zebra mussel may be a thorn in our sides now, but in time the pain will pass. Literature Cited
Lord, Lewis, and Sarah Burke. "America Before Columbus." U.S. News and World Report. September, 1992. pp 22-37.
Milne, Lorus J. and Margery. Ecology Out of Joint: New Environments and Why They Happen. New York: Charles Scribner's Sons Publishing, 1977.
Pechenik, Jan A. Biology of the Invertebrates. Boston: Wm. C. Brown Publishers, 1996.
Reuter, Jeffrey M. Aquatic Nuisances: The Zebra Mussel in North America. Online Resource: http://www.osc.edu/OhioSeagrant/osgrant/com/nuisances/zm/fs045.html. Updated April 1994.
Sale, Kirkpatrick. The Conquest of Paradise: Christopher Columbus and the Colombian Legacy. New York: Penguin Books, 1991.
Schlesinger, Arthur Jr. "Was America A Mistake?" Newsweek. September, 1992. pp 16-27.
Sumich, James L. An Introduction to the Biology of Marine Life. Boston: Wm. C. Brown Publishing, 1996.