Population Growth

Natural populations may stay the same size from year to year. But a population can grow rapidly, as sea otter populations did when they were first protected from hunting. Populations can also decrease in size, as otter populations are doing now because of predation by killer whales. But just how do interacting factors such as these influence population growth?

  Three factors can affect population size: the number of births, the number of deaths, and the number of individuals that enter or leave the population. Simply put, a population will increase or decrease in size depending on how many individuals are added to it or removed from it.

Generally, populations grow if more individuals are born than die in any period of time. For some organisms, such as penguins, being born may actually mean hatching. Plants can add new individuals as seeds sprout and begin to grow.

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.