Plants and animals usually do not live apart, as in many ways they are important to each other. Most green plants can make their own food. The food of other living things, even the food of meat-eating animals, comes directly or indirectly from green plants. For example, some people eat mutton (羊肉); mutton comes from a goat; the goats eat grass. These steps, or links, make up what is called a food chain. Without green plants, all other living things would in time die of starvation.
Plants help animals in other ways too. They furnish homes and shelter for many animals. Many birds, as you know, build their nests in trees. Deer and many other animals use bushes for shelter. And even man depends much upon plants for his shelter. Animals repay some of this debt by helping plants. You may be aware that many insects and some birds do important work for plants by pollinating (授粉) them. You must have also learned how animals often help plants by spreading seeds.
What is waste material for one of these kinds of life is often valuable substance for the other. For example, animals give off carbon dioxide gas when they breathe. When plants make food, they absorb this gas from the air, using the carbon and releasing some of the oxygen back into the air. The oxygen that is released, then, is again supplied to animals. Thus the cycle continues, animals helping plants, and plants helping animals.
All plants and animals have their enemies. Insects eat plants; birds eat insects; other animals kill birds. But here too, animals such as birds and bats help the plants by destroying harmful insects. Many plants and animals are helpful to some living things but are harmful to others. Then there are other rules in nature that act as checks and balances. For example, if there are more animals in a certain area than there is food to support them, some of the animals must migrate or starve. In either case, the number of animals will be reduced until the balance between animals and the available food supply is restored.
These helps and hindrances (障碍) are constantly going on in the plant and animal world to achieve a balance. As long as one living thing is dependent on another, whenever the scales are tipped (使倾斜), nature takes steps to balance the scales again.
 (Note: Answer the questions or complete the statements in NO MORE THAN TEN WORDS.)
What does the author mean by saying “Plants and animals usually do not live apart”?
Besides providing food and shelter for animals, plants can also ______ to help animals to live.
“Checks and balances” refers to ______ in the world of animals and plants.
What will be the final result if some animals migrate or starve for lack of food?