Sometimes, they go extinct. Extinctions can be associated with external factors as changes in sea level or the fall of an asteroid. Eventually, the players in a community can be associated in an unlikely chain of events.
Interactions can be extremely complex and involve apparently unrelated species.
The chain involved rabbits, certain type of grasses, a species of ants and the caterpillars of the blue butterfly. Population fluctuations are a classical problem in theoretical ecology. These models have been explored in deep. The so called commnity matrix is the basic subject of all these studies. Let be the fraction of non-zero elements in and let be the variance of the set .