As is shown by the two graphs,
the U.S. population soared from 2 million to 250 million from 1800 to 1990.
In contrast, the number of wildlife species no longer existing shot up,
adding up to 70 over the same period of time.
It is obvious that the extinction of wildlife resulted from the population explosion.
We may cite a variety of reasons for this situation.
First of all, the destruction of ecological balance plays a pushing role.
Due to population problems,
pollution and desertification are becoming increasingly serious.
Furthermore, a great number of species were killed for food, for money and even for fun.
Even worse, with the rapid growth of human population,
inhabitable areas were expanding so fast that few areas were left for wild animals.
To sum up, it is imperative for us to take effective measures to protect wildlife.
Firstly, the U.S. government should pour more investment into wildlife protection
in a bid to create a better environment.
Secondly, the birth rate in the U.S. must be put under control
as the dramatically expanding population gives rise to serious environmental problems.