If the presence or absence of a factor limits the growth of the ecosystems elements, it is called a limiting factor . There are several abiotic factors that limit ecosystem growth, including temperature, precipitation, sunlight, soil configuration, and soil nutrients
ATP is the energy currency in life. It’s the high energy molecule that stores the energy we need. Organisms get ATP by consuming food.
If you are conducting a study on the health of people who have been exposed vs not exposed, the method you should use to obtain data would be an observational study because it involves the pre-existing condition of the subjects.