They carry blood which is low in oxygen and high in carbon dioxide into your lungs so that the carbon dioxide can be blown off, and more oxygen can be absorbed into the bloodstream. it is pumped all around your body to supply oxygen to cells and organs.
For succesion in ecosystems nothing remains the same or constaly changing. New specis, Rain and loss of animals can change an everment but only a Wildfire can change a ecosystem where nothing is left the same. So therefore the answer is Wildfires.