A population is all the organisms that both belong to the same group or species and live in the same geographical area.All of the species in an ecosystem.
Several terms can be used to describe a population in nature.Three important characteristics of a population are its geographic distribution, density, and growth rate. Geographic distribution, or range, is a term that describes the area inhabited by a population. The range can vary in size from a few cubic centimeters occupied by bacteria in a rotting apple to the millions of square kilometers occupied by migrating whales in the Pacific Ocean.
A population can grow when its birthrate is greater than its death rate. If the birthrate equals the death rate, the population stays more or less the same size. If the death rate is greater than the birthrate, the population shrinks. Sea otter populations grew when hunting stopped, because their death rate dropped. Those same otter populations are shrinking now because killer whales have raised the death rate of otters again. Immigration (im-uh-GRAY-shun), the movement of individuals into an area, is another factor that can cause a population to grow. Emigration (em-uh-GRAY-shun), the movement of individuals out of an area, can cause a population to decrease in size. Wildlife biologists studying changes in populations of animals such as grizzly bears and wolves must consider immigration and emigration. For example, emigration can occur when young animals approaching maturity leave the area where they were born, find mates, and establish new territories. A shortage of food in one area may also lead to emigration. On the other hand, populations can increase by immigration as animals in search of mates or food arrive from outside.