Animals use aggression as a technique for gaining control over necessities... that are scarce or are likely to become so... They intensify their threats and attack with increasing frequency as the population around them grow. As a result the behavior itself induces members of the population to spread out in space, raises the death rate, and lowers the birth rate. In such cases aggression is said to be a "density-dependent factor" in controlling population growth.

On Human Nature (1978) - Ch.5 Aggression