What causes species to replace one another in succession?

Question

What causes species to replace one another in succession?

Answer 1

If you are referring to ecological succession, it is a matter of one species being better "equipped" to outcompete another for the existing resources.  

For example, take an area that has been wiped out by a forest fire.  The pioneer species that move into the area are ones that are better able to survive in arid space with few resources.  Plants grow lower to the ground and animals are capable of creating their own protection or outrunning predators and finding food.  As these species become established, they actually at times create their own downfall.  As the ground becomes more fertile, plants that might need more nutrients to get established but then put down deeper roots and grow taller than the pioneer species are able to move in and outcompete the existing plants for the resources that are there.  The same goes for animals.  Maybe the pioneer animals can hide better but don't reproduce as quickly.  Once the new plants are established, there's more food and more shelter, so animals who spend less energy hiding and foraging and more reproducing move in and edge out the original species.