In every ecosystem around the world, there are invasive species, whether they are animals or plants. Some species may be more harmful than others, but they are nevertheless detrimental to an ecosystem and the native species in them. Two specific invasive species that harm North American ecosystems are the zebra mussel and the tree of heaven.
The zebra mussel, also known as Dreissena polymorpha, is a small freshwater mussel. It belongs to the Dreissenidae family, which consists of small freshwater mussels, including the zebra mussel and the closely related quagga mussel. It gets its name from the zebra-like stripes on its shell, although not every single mussel will have this striping. The zebra mussel was first discovered in lakes and rivers of Russia, including the Ural and Volga rivers, as well as in the Black and Caspian Seas. They have since established themselves in regions including Great Britain, Italy, Sweden, and the Great Lakes.
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Zebra mussels are not that large, as they only grow to about the size of a fingernail, but their maximum length is about 2 inches. However, since they bunch up and attach to each other, a cluster of zebra mussels may be heavy enough to weigh down navigational buoys (paraphrased from nas.er.usgs.gov/queries/FactSheet.aspx?speciesID=5.). Zebra mussels usually attach to either hard or soft surfaces underwater, both natural and manmade. Legally, it is classified as a prohibited invasive species, which means it is illegal to import or transport zebra mussels without a permit for a certain use. Scientifically, it is in the Mollusca phylum, the Myida order, and the Dreissenidae family. In its genus, Dreissena, only two known subspecies are listed: the zebra and quagga mussels.
Zebra mussels are filter feeders, primarily filtering large amounts of detritus and plankton from the water. The maximum amount of water each mussel can filter is about one liter a day. However, due to the large number of zebra mussels that may occupy an area, this may mean filtering all the water in a lake in one day. Since the zebra mussels are filtering the plankton and algae that are also necessary for other native species, they may cause a decline in those species because they are not getting enough food to eat. There is another danger that comes with the zebra mussels diet. When the mussel feeds, it must open its shell. Their shells are very sharp and are known for cutting feet, so it is advised that people wear water shoes before entering bodies of water prevalent with zebra mussels.
It is very easy and quick for zebra mussels to reproduce, shown by their colossal population in numerous bodies of water across the world. The average lifespan of a zebra mussel is six to seven years, and a female starts to reproduce at about two years of age. More than 40,000 eggs can be produced in one reproductive cycle, with up to one million a season (paraphrased from www.fws.gov/midwest/endangered/clams/zebra.html.). Once an egg is fertilized, it develops into a microscopic free-living larva, called a veliger, and usually drifts in the water for several weeks until it settles onto a hard surface. Only about two percent of zebra mussels reach adulthood.
Zebra mussels mainly affect aquatic ecosystems, including lakes, rivers, and river basins. One specific ecosystem that has been invaded by zebra mussels in Lake Erie. Zebra mussels were most likely accidentally introduced in the mid-1980s from the ballast water in cargo ships that traveled from Europe to ports on the Great Lakes. Since they reproduce so quickly, their vast population has had many effects, mainly negative ones, on ecosystems like Lake Erie. Zebra mussels attach to any hard surface, including docks, pipes, water intake structures, and even boat engines. These are all very costly to repair, and increase the chances for injuries like cuts on their shells as previously mentioned. Zebra mussels may also completely cover other mussels, making it weak and causing a decline in the native mussel's population, to a point where some are listed as endangered. "As they spread, zebra mussels threaten the extinction of at least 30 freshwater mussels. Losses of crayfish and snails have been implicated by zebra mussel colonization." (www.seagrant.umn.edu/ais/zebramussels_threaten.). Likewise, the fact that zebra mussels must filter so much water in a given area reduces the amount of food for other species in an environment. It also increases water clarity as well as the growth of aquatic plants.
Some methods have been developed for the removal of zebra mussels in an ecosystem. It is always recommended for boat owners to clean plants and animals from any aquatic equipment including docks and anchors, as well as draining water-related equipment before leaving the water. This is to prevent any person from possibly spreading zebra mussels. It is also common to use pesticides in closed systems like power plants. Physical barriers have also been created to prevent zebra mussels from attaching to surfaces, as well as being scraped off. However, there is no known specific method of elimination once they have been established in a body of water.