3.阅读：trophic cascade

定义: a phenomenon that the predators’ population decrease causes the preys’ population to increase quickly and the eco-system will be destroyed.

听力

e.g. In an area of North America, an animal species feeds on deer and other small animals. Soon roads are built in this area and the traffic there is much busier so that animal species leaves there. And consequently, the deer population increases a lot. Naturally, the cotton trees which are the food resources of the deer decreases soon. And the birds that live in the cotton tree begin to lessen. Finally, the ecosystem undergoes an irreversible change.

4.阅读 Academic term: Aposematic behavior

Definition: which relies on the memory of the would-be predator, is a defense mechanism referring to the exaggerated movements prey do to indicate that they are unpalatable and evade predators

听力

Example：Some caterpillars have evolved countermeasures and eat the leaves of toxic plants. In addition to being unaffected by the poison, they sequester it in their body and make them highly toxic to predators, such as birds that will get sick after swallowing these caterpillars. After noticing that their predators are approaching, the caterpillars will keep still and do some exaggerated movements like shaking their heads up and down, which are deemed as the signals to warn off the birds and remind them of the truth that they would get sick after eating  these preys. Every time these birds see the caterpillars shaking their heads up and down, which associates with toxicity and sickness, will endeavor to avoid a repetition of the experience, back off and go to eat other species.