The combination of evolution and events resulted in a massive number of different species emerging. While species evolved to use every possible energy source, they also evolved to take advantage of different events occurring. The result is the biodiversity we enjoy today.
If we go back to the concept of succession, we see the result. When the forest was destroyed by fire all the plants that evolved to grow on bare ground exploded into action. Their seeds had been lying in the soil since the last event, waiting for just this opportunity.
Then the next wave saw their chance and on it goes. Each wave of plants use the opportunity to restock their soil seed-bank. It is important to understand what is going on.
Throughout millions of years, plants grew accustomed to events occurring. For example a mob of grazing animals may move through a area at about the same time each year. They clear plants by eating them. They cultivate the soil by their movement and scratching the surface of the soil. They fertilise the soil with their excrement. In short they create an event a whole range of plants evolved to take advantage of.
One family of plants that evolved to take advantage of this event is the grasses. The new growth of a grass comes from the crown which is below the grazing line. Unlike a shrub where the growth is from higher up the plant where considerable damage is caused to the plant when it is pruned, a grass actually benefits from grazing.
Consider any energy source occurring in nature. You will find organisms that have evolved to utilise that energy source. Similarly consider an event that occurs regularly and you will find plants that evolved to utilise that event.
Now consider a plant that has evolved to utilise the event of annual heavy grazing. Suppose we have an annual grass that relies on a grazing event to trigger its seeds to germinate. It must grow through its cycle and deposit a new seed-bank for the next year.
Occasionally the annual migration of grazers did not occur. Those plants whose seeds could not last in the soil another year for the suitable event simply disappeared. They were deselected. Plants that had the ability for their seeds to last in the soil longer survived. They were selected for survival.