<span>A. If one essential nutrient within an ecosystem runs out, primary productivity will be limited.
A specific nutrient is need for every organism in the ecosystem. If this particular nutrient is diminished within the ecosystem then cellular processes will decreases and a great number of specie will die. </span>
There are so many examples for that in different areas, like TPBi experiment carried out in our lab recently.Here's one link: http://www.alfa-chemistry.com/tpbi-cas-192198-85-9-item-282866.htm
D- The digestive system breaks down your food and your circulatory system takes it throughout your body