What is the struggle for use of limited resources among Organisms

Competition

Explanation:

Competition is the scientific term for the struggle for use of limited resources. The limited resources is the reason we don't experience significant overpopulation in animal ecosystems. It's also the reason why biological fitness, or ability to reproduce, is so important - the more resources (food, water, shelter, safety) you have as an animal, the more you can reproduce, and this makes you more "competitive."

There is a constant struggle for survival among organisms within a population because they are all competing for the same resources.