Migration of parts of one population to another area can disturb equilibrium because different ecosystems can have very different conditions. Populations adapt to the conditions they are under over time through natural selection. Introducing members of a different population with different adaptations can provide advantages and disadvantages for both the portion of the migrating population as well as the population to which the migrants enter. As the populations begin to interbreed, the genetic equilibrium may shift as natural selection favors particular traits. Even a small migration of a single species can have effects on all species in a particular ecosystem as they are all part of the food web and predator/prey relationships.