Population Density - Biological Stress
You are aware of what might follow an uncontrolled growth of human population. There will be competition for common consumables and for space. But .this is not merely a human problem. Too thick population densities of plants can result in the competition for common nutrients, water and photosynthetically active radiation. The effect can be a self inhibition or inhibition of other plant species growing in the same environment. That is one of the reasons why weeds are removed from croplands.