Comment by ndr

The term of art for everything that includes those factors is "carrying capacity" [0].

It's not as much that the fertility necessarily drops, but that under some conditions the death rate equals or overtakes the birth rate.

And the system is dynamic. So conditions changes, as in some of your examples, even just because the population grows. EG scarcer resources.

[0] https://en.wikipedia.org/wiki/Carrying_capacity