For the human species which is the major concern on the issue of overpopulation

Depletion and destruction of natural resources is the major concern on the issue of overpopulation.

Explanation:

It is like an elephant in a room where no one would want to talk about. Nowadays people are concerned more about the climate change which is definitely important but the root cause has to be understood. Here the root cause implies the effect of substantial increase in human population. The population in 1960 is going to be doubled soon with the increase in head count. The negative impacts like human emissions through vehicles, industries have already started showing their effects on the planet. Global warming is on the rise to deplete ozone layer, melting the glaciers. Everything is interdependent making it a high time to address the current issues.  Unless family planning norms are strictly followed, this issue will never get subsided. Humans existing and reducing the negative effects will not work, but reducing head counts only will slowly solve the issue. All the natural resources will get used up soon making it difficult for the future generations to survive.

Each side has to be less then the sum of the other two sides.a, b, c, and e work.