The essential resource an organism needs for life that is in shortest supply in their environment. In environmental science, this usually means the resource (e.g., food supply, water supply, air to breathe) that is limiting the growth of an organism or population within a given ecosystem. For example, the number of fish that can survive in an aquarium depends on the amount of food, dissolved oxygen, and space available. If there is enough space and oxygen, but not enough food, the fish population will decrease. The individual organisms will only be able to survive as long as their basic needs for life are met. The factor that is in least supply is limiting the growth of the organism and/or population.