Answer:
Reproduction.
Explanation:
At the cellular level indicate that the living organisms are made up of the smallest building blocks called cell. It refer to unicellular organisms i.e organism with one cell.
At the cellular level, membranes; for the whole organism, the skin is Reproduction because a single parent cell divides into two cells (offsprings), two divides into four cells and so on. It is called cell division and cell reproduction because a single cell skin divides to produce many cells.
Through the things they eat
One big reason for this is because individuals of the same species often have to fight over shared resources (this means shared food and shared environment).
Because they fight over a limited amount of space and food, competition between individuals within one species can be present and very tough.
On the other hand, having members of different species also possibly means different territories and different food sources. A lion has no quarrel with dolphins, while it can have a lot of fights with lions from different prides (large group of lions).
Because science is always changing and every time you cam come up with a new outcome.