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Part 1 - Zebra Mussels - Invasive Species Part 1 - Zebra Mussels - Invasive Species Read the passage below and answer the questions that follow What is a zebra mussel? The zebra mussel is a small shellfish that is originally from the Caspian Sea (near Iran and Russia). Although they are not native to North America, zebra mussels arrived here as an invasive species in the mid-1980’s, most likely in the ballast water of a ship. An invasive species is an organism that is non-native (or alien) to the ecosystem and whose introduction causes economic or environmental harm or harm to human health. Very quickly, zebra mussels became established in the Great Lakes. This invasion was first discovered in the Hudson River in 1991. At that time, there were very few, but by the end of 1992, zebra mussels had already spread throughout the river! How did the zebra mussel spread? Ships are designed to carry cargo such as oil, grains, and shipping containers. If a ship is travelling without cargo, or has dropped off some cargo in one port and is going to its next stop, ballast water may be taken in to keep the ship sinking deep enough in the water. When ballast water is taken into the ship, it may contain organisms (or their eggs or spores) that live in that area. When this water is released somewhere else, a species can spread and become an invasive species in a new area. This is how zebra mussels spread. They release egg and sperm into the water, where fertilization takes place. In 2 days, fertilized eggs develop into free-swimming larvae, which can be transported over long distances. Why is this a bad for the ecosystem? Zebra mussels are very efficient filter-feeders, which means that they filter food out of the water very quickly. They filter and eat small organisms that are in the water, such as phytoplankton and zooplankton. Numbers of zebra mussels can reach over 100,000 mussels per square meter. Because there are so many of them, they are able to filter all of the water in the freshwater part of the Hudson River every 2-4 days! Contrast that to the native population of mussels which filter the water only every 2-3 months. As a result of the zebra mussels’ huge appetite, populations of plankton in the Hudson River have fallen sharply. Both zooplankton and phytoplankton populations have declined. This is bad because Phytoplankton and zooplankton form the base of the aquatic food web and many animals depend on them for survival. This also results in less food available for fish, native mussel species, and other river animals. Many native mussel populations have decreased to the point of being threatened or endangered because they are out-competed by the zebra mussels. Other animals, including fish like shad and herring, have also suffered. Because phytoplankton levels have decreased, dissolved oxygen levels have decreased, which can have different negative effects on other species, as well. Below is the data for the numbers of zebra mussels in Lake Trasimeno, a body of water in Italy, where zebra mussels are not native. Studying what has happened to populations of zebra mussels in lakes around the world can help scientists predict what may happen in the United States. Notice that the number of zebra mussels increase over time Analysis 1. Based on the graph, what year would you suspect that zebra mussels were introduced into Lake Trasimeno? a. 1962 b. 1964 c. 1966 d. 1968 2. Describe what happens to the population from 1984 through 1988. a. Increases b. Decreases c. Stays the same d. Increases drastically and then decreases 3. The zebra mussel is a highly invasive species that was accidentally introduced to the Great Lakes region of the United States in the 1980s. The map below shows the distribution of zebra mussels in 2010. What is the primary reason the large-scale spread of zebra mussels concerns scientists? a. Zebra mussels are a freshwater species. b. Zebra mussels are a desired food for humans. c. Zebra mussels are eaten by some fish and birds. d. Zebra mussels outcompete native mussel species. 4. How did the zebra mussels arrive to North America? a. They were brought by plane to the Great Lakes b. They were carried in the ballast water of ships c. They were released by local restaurants into the water d. The mussels escaped from a nearby dentist office with Nemo 5. How do the zebra mussels impact the phytoplankton and zooplankton populations? a. They would increase b. They would decrease c. They would stay the same d. Increases drastically and then decreases 6. How do the zebra mussels impact the native mussel populations? a. They would increase b. They would decrease c. They would stay the same d. Increases drastically and then decreases 7. Ordinarily, people are happy to see a species being so successful in an ecosystem. Why is the success of the zebra mussels perceived as a negative thing? Part 2: Predicting Changes in Ecosystems Read each scenario card below to determine if each population is increasing, decreasing or staying the same 1. What will happen to the population of grasshoppers? a. The population will increase b. The population will decrease c. The population will stay the same 2. What will happen to the population of hawks? a. The population will increase b. The population will decrease c. The population will stay the same 3. What will happen to the population of garter snakes? a. The population will increase b. The population will decrease c. The population will stay the same 4. What will happen to the population of coyotes? a. The population will increase b. The population will decrease c. The population will stay the same 5. What will happen to the population of burrowing owls? a. The population will increase b. The population will decrease c. The population will stay the same 6. What will happen to the population of bison? a. The population will increase b. The population will decrease c. The population will stay the same 7. What will happen to the population of prairie dog? a. The population will increase b. The population will decrease c. The population will stay the same 8. What will happen to the population of deer mouse? a. The population will increase b. The population will decrease c. The population will stay the same 9. What will happen to the population of white -tailed deer? a. The population will increase b. The population will decrease c. The population will stay the same 10. What will happen to the population of population of pronghorn Antelope? a. The population will increase b. The population will decrease c. The population will stay the same Part 3: Biomagnification Biomagnification Toxic chemicals are released into the ecosystem from a variety of industrial, commercial and residential activities. Many human made chemicals and metals can be acutely poisonous in very small amounts. Many of these have the potential risk of causing cancer, birth defects, and genetic mutations from long-term exposure. Humans are exposed to toxic chemicals when they eat contaminated fish. These chemicals do not break down easily in the environment and they increase in concentration. The increase in concentration of a particular toxin, such as cadmium, PCB, or DDT, in animals high up the food chain is called biomagnification. Biomagnification is the buildup of toxic substances in a food chain. Let’s take mercury for example. In an ecosystem, phytoplankton may take up mercury, zooplankton eat the phytoplankton, taking in the toxin. A school of small fish will eat the zooplankton and larger fish will feed on the small school. Top predators like orcas, sharks, and humans will catch the larger fish and consume them. The outcome is a buildup of mercury in the tissues of the different organisms in the food chain. In some cases, this build up of toxic substances is so great that it can contaminate fish and render them unhealthy to eat. Top predators such as bald eagles have much higher levels of some toxins than the fish they eat. Detritus, along with small amounts of algae and aquatic plants, forms the base of the Hudson River food web and is eaten by small zooplankton, which is eaten by larger zooplankton, and so forth. If a toxin accumulates in the tissues of organisms, that toxin will move up the food web. The concentration in the tissues of organisms can be million times higher than the amount in the open water. Bioaccumulation is a natural process in all organisms, including us. When we eat, we accumulate vital nutrients such as vitamins and protein that we need in order to survive. Bioconcentration is used to describe the bioaccumulation of toxins or other harmful chemicals. Any compound, either natural or manmade, will be stored (or accumulate) in an organism if it is taken up faster than it is broken down (metabolized) or excreted. A pesticide called DDT is perhaps the most well-known cases of biomagnification. DDT was used in the United States as an agricultural pesticide from the late 1940s through the early 1970s (it was banned in 1972). It was such an effective insecticide that communities would drive trucks down the road, spraying DDT in a huge white cloud to kill mosquitos and other pests while neighborhood kids played in the plume. However, DDT caused top bird predators such as eagles, osprey, and pelicans to lay eggs with very thin eggshells that broken when the birds sat on their nests. The numbers of these birds dropped so dramatically, that once the Endangered Species Act was passed, bald eagles, osprey, and pelicans had nearly become extinct and were designated Endangered by the federal Environmental Protection Agency.
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