Respiration uses oxygen to break down the plant material, releasing the stored energy.
CEP- competitive exclusion principle. The CEP puts strict limits on the number of different species we should expect to find. The answer is very complicated, but essentially it comes down to the fact that competitive exclusion depends on a set of conditions that must be met before it can occur. Earth's environemnts would have to be an identical flat plain with no differences in temperature, moisture, etc. Droughts shift the competitive balance from species that need lots of water to those that don't, and back again.