การฟัง TOEFL

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Listen to part of a lecture in an environmental science class. Professor: Today we're going to talk about a fascinating example of ecosystem engineering—beavers. Now, most animals adapt to their environment, but beavers actively modify it to suit their needs. By building dams across streams, they create ponds that provide protection from predators and easy access to food. These ponds also have a ripple effect on the surrounding ecosystem. For instance, as the water spreads, it kills trees that can't tolerate flooded roots, creating standing dead wood that attracts insects and woodpeckers. Over time, the pond may fill with sediment and become a meadow, which supports different species like grasses and grazing animals. Interestingly, this process can repeat over decades, as beavers abandon a site and later return to rebuild after the meadow forms. So, beavers don't just live in their habitat—they actually create a dynamic landscape that benefits a wide range of organisms, from fish to birds to mammals. This makes them a keystone species, meaning their presence has a disproportionately large effect on the ecosystem.