"To kill and to be killed, to grow and to die and get decomposed" is the law of nature. With the help of this law, nature maintains the population of each species of animals in sufficient numbers.
For example, consider a simple terrestrial food chain:
plants → grasshopper → frogs → snake → eagle
Suppose, there are no eagles in a community. In the absence of eagles, the population of snake will increase. Increased population of snake will eat more frogs. Gradually, the population of frog will decrease.
Decrease in the population of frog will lead to increase in the population of grasshopper. Now, grasshopper will eat more plants. Decrease in the population of plants would disturb the entire ecological balance.