Every species, including humans, are generally always suffering the effects of overbreeding. When a species is suffering the effects of overbreeding, births cause the death of children. Every species breeds faster than required to maintain their numbers. The ones that did not, have gone extinct. All have a finite environment, and sufficient time, thus all species are at the limit of what the environment can provide. This includes humans too. There are exceptions, but they are only temporary.

When a species invades a new territory, or some disease wipes out large numbers, the survivors will be below the limit and while their numbers grow back to the limit they are not suffering the effects of overbreeding. Humans are unique in that we have the ability to discover and use better technologies to increase the capacity of our environment.

We have a world economy which means that there is only one environment, the whole Earth. Inexpensive shipping allows the inhabitants of what seems like a separate environment to use the production of another environment. The fact that you can eat a coconut in Seattle and that people live on Antarctica shows this. The average number of children for the whole population is too high which creates the horrible high child mortality found in developing countries. In addition, our ignorance of the fact that averaging too many babies causes the death of children, results in the associated dreadfully low adult life expectancy in those same countries.

A birth anywhere, to rich or poor, contributes to the death of a child somewhere.

We are using resources faster than they can renew, for example coal, uranium, oil, draining aquifers, and removing forests. These resources are essential for food production. If we do not consume those resources faster than they renew, we cannot plant, fertilize, harvest, package, distribute, and store our food in sufficient quantities to feed the current population. This means that to the best of our knowledge we are acting in a way that will cause premature death in the future when those resources become scarce.

Imagine an island where the inhabitants chop down trees to make boats to fish for food. The trees are essential for survival because without them they cannot get the fish to survive. As the population of the island grows, they will need to chop down more trees each year because they need to make fishing trips proportional to the number of people on the island, and the boats wear out. When the number of trees chopped down each year exceeds the growth rate of the trees, the island is overpopulated. The inhabitants will not feel the effects of the overpopulation until the trees are depleted to the point where they cannot make enough boats.

If the inhabitants recognize this, and not only bring their average number of children below two but also target a population level that the island can sustain, they might avoid the horror of the pending population collapse.

This is not a hypothetical scenario. It happened on Easter Island. The population crashed from more than 20,000 down to less than 2,000. The inhabitants were wealthy enough to make those huge stone faces before the collapse, but not after the collapse. We can only imagine the suffering and horror as food production failed. (See Collapse: How societies choose to fail or succeed and Guns, Germs, and Steel both by Jared Diamond.)

We might be able to escape the same horrible population crash that Easter Island suffered if we manage to figure out how to live without using resources faster than they can renew before the resources become scarce. For example, we might be able to figure out how to use solar energy more directly and thus supply enough energy to feed the current population. We might not be able to do that, so we must bring our numbers down to where we are not consuming resources faster than they renew by limiting our births.

Other species have no choice but to suffer the effects of overbreeding. They breed as fast as they can. Humans, in contrast, have discovered modern birth control, and this knowledge. This knowledge is the essential ingredient. Everyone must know it.