托福口语 Task 4

Today, we're going to discuss a fascinating concept in ecology known as 'keystone species.' A keystone species is one whose presence has a disproportionately large effect on its ecosystem, meaning that if it were removed, the entire ecosystem would change dramatically. Let me give you two classic examples. First, consider the sea otter. In kelp forest ecosystems along the Pacific coast, sea otters prey on sea urchins. Without otters, urchin populations explode and devour the kelp, destroying the habitat for fish and other marine life. When otters are present, they keep urchin numbers in check, allowing kelp forests to thrive. Second, think about the African elephant. In savanna ecosystems, elephants knock down trees and trample shrubs, which prevents the area from becoming dense woodland. This maintains open grasslands, which support grazing animals like zebras and wildebeests. Without elephants, woody plants would overtake the savanna, reducing the grassland and harming those herbivore populations. So, in both cases, the removal of a single species triggers a cascade of negative effects.