What are three factors in a decrease of population?

Pollution, not enough resources to sustain to population, death rate, and immigration.

Explanation:

Pollution can damage habitats, and if there is to much pollution, the habitat can be unsuitable for the animals to live in.

If there are not enough resources, like not enough grass for a herbivore to eat, then the herbivores would die off, making there not enough to eat for the carnivores.

As I said before, if there are not enough resources, animals will die off, and that is death rate. The more animals die, the higher the death rate is, and the higher the death rate is, the lower the population gets.

When animals immigrate, they leave their original habitat and go to a new one. This can also lower population because if the animals leave, then there are less animals in that habitat, and that will also decrease the population.