The principles concerning populations are therefore important in understanding how to control pests. Natural systems are made up of many kinds of species mixed together, with each balancing and controlling the other. Man-made systems, however, such as agricultural fields or forest plantations, are usually large areas with just a single species. The pests which feed on our crops appreciate our providing them with so much food, and their populations multiply rapidly. A pigsty is an ideal nursery for flies, and the tender shoots in a vegetable garden could not be better for snails.
Some pests have natural enemies that keep their populations under control most of the time.
When the pest is numerous, there is more food for the enemy, which will multiply and reduce the pest population to normal levels. Unfortunately, the pests have often reached the islands while their enemies have not, so pest problems can be more frequent and more severe on islands than elsewhere. Controlling pests is a kind of population management. However, there are several ways to reduce pest populations to manageable levels. The simplest is to change the conditions so the pest has less food or has a harder time spreading.