Competing species differ in at least one way in their use of a limitingresource. This may involve character displacement between thecompeting species, in which some aspect of anatomy, or behavior, or physiologyis different and allows for a difference in the use of the limiting resource. In other words, the ecological niche , defined as the total rangeof biological and physical conditions within which a species can survive andreproduce, must be different in some way. This is called the CompetitiveExclusion Principle. Environmental variability increases the number of competing species thatcan coexist, by constantly shifting the competitive advantage from one speciesto the next.
Predation dictates the flow of energythrough and ecosystem.
This class of interaction also includes pathogens and parasites. They are instead part of a complex interactingcommunity. Keystone predators are an example of that complexity. Changes incommunity structure that follow a disturbance are collectively called succession . This sequence can be more complicated than it looks.