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第1段
1 .listen to part of a lecture in an ecology class.
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2 .P: One of our more interesting fields of research is invasion ecology, predicting and understanding the establishments, spread and impact of invasive species.
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3 .To understand invasion ecology, you must first understand the distinctions between native, non-native and invasive species.
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4 .A native species is found inside its historical range, wherever it evolved naturally.
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5 .A non-native species is found outside its native range.
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6 .It was brought there, intentionally or unintentionally.
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7 .An invasive species is a non-native species whose introduction to an area causes economic or environmental harm.
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8 .A classic case of an invasive species is a type of shellfish called zebra mussel.
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9 .This is a photo of a shellfish in someone\'s hand, and there\'s a cluster of smaller shellfish attached to it.
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10 .Those are zebra mussels.
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11 .The species is native to the Black Sea region of Eastern Europe, but in 1988 it suddenly appeared in the Great Lakes region of North America.
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12 .The species had reached this area in the ballast water of a commercial container ship, one of those gigantic ships that haul heavy cargo across the ocean.
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13 .Anyone familiar with ballast water? Daniel?
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14 .S: Yeah, ballast water is big ships have these huge water tanks at the bottom to balance the weight of all that stuff they\'re carrying on the top, you know, to keep the ship from tipping over.
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15 .P: Right, to prevent a ship from becoming top heavy, ocean water is pumped into the ballast tanks at the departure port and flushed out when the ship reaches its destination.
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16 .Unfortunately, a lot of small aquatic critters end up coming along for the ride and get released into the new habitat.
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17 .There they might just die.
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18 .Or if they\'re an opportunistic species that can exploit the possibilities of the new habitat, they might transfer successfully.
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19 .In some cases though, they\'re too successful.
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20 .The zebra mussel belongs in this latter category.
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21 .After being flushed out of a ballast tank, it took just two years to establish itself in all five of the Great Lakes and their river systems.
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22 .This rapid spread is typical of invasive species.
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23 .It\'s part of a set of characteristics we call r-selected.
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24 .Being r-selected means that that species reproduces early and often.
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25 .Rapid reproduction is part of its evolutionary strategy.
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26 .Now the zebra mussel starts laying eggs just one year after hatching, and each female produces more than a million eggs in a single spawning season.
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27 .If even a fraction of those eggs survive, that\'s still an exponential rate of population increase.
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28 .Another characteristic of r-selected species is that their populations disperse widely, very fast.
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29 .S: Disperse? But don\'t zebra mussels attach themselves to things?
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30 .P: Yes, but during the first month of their lives, during the larval stage, zebra mussels are free floaters.
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31 .They get carried along by water currents, or they can be transported, not only by cargo ships, but also by small recreational craft like speed boats, anything that can hold infested water.
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32 .But as I said, under the definition of invasive species, it\'s not enough just to spread rapidly.
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33 .The species must also cause economic or ecological harm.
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34 .The zebra mussel has done both.
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35 .Ecologically, it\'s decimated whole populations of native mussels by attaching to their shells and preventing them from functioning.
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36 .Also, zebra mussels are intense filter feeders.
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37 .They suck water into their body cavity and filter out specific types of microorganisms for their nourishment before expelling the water.
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38 .Not only does that alter the food chain directly, it also produces a dangerous increase in water clarity.
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39 .S: Water clarity is dangerous? I think that\'d be a good thing.
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40 .P: Not always.
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41 .Clearer water makes aquatic animals more visible to their predators.
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42 .That affects the food chain too.
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43 .Zebra Mussels are detrimental economically because they attach themselves by thousands to just about any artificial surface, which means they clog the intake and outflow pipes of water treatment plants, to take just one example.
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44 .So companies and municipalities have to budget money to remove the zebra mussels on a regular basis.
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45 .Yes, Daniel?
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46 .S: So, like, if I\'m walking through a marina and see zebra mussels attached to a speedboat, should I report it?
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47 .P: Sure, tell a local official.
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48 .There have been cases where zebra mussels were detected at inspection stations and removed before the boats were allowed to continue on their way.
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49 .And now that we\'re aware of the ballast water issue, governments have begun regulating when and where ships can take in and release that water, so we\'re making inroads finally.
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