Predator-prey relationship considerations
In natural ecosystems, organisms tend to eat a range of foods and therefore fluctuations in popualtion size are less severe.
Other reasons for cyclic fluctuations:
> Climatic factors
Periodic population crashes are important in evolution as they create a selection pressure. This means only the individuals that are able to escape predators, withstand disease or climatic changes will survive to reproduce. The population therefore evolves to be better adapted to the prevailing conditions.