Hobbies And Interests
Births
Births increase a population's growth rate through reproduction. Factors that positively affect birthrate include food abundance, suitable habitat for raising young and availability of healthy mating partners. Some pregnant animals, such as rabbits, can spontaneously abort if food supplies run low so the female rabbit can save her reproductive energy for another litter. In areas of habitat loss, mating partners may have difficulty finding one another as in the case of pandas.
Deaths
Mortality rate defines how many deaths contribute to a decrease in population size. Death within a population is caused by factors such as disease, age, predation and starvation. For example, the Black Death in Medieval Europe was a disease that eliminated nearly two-thirds of Europe's human population.
Immigration
Populations can increase when members from another group move in to the population. This can be seen in some herding animals where the groups merge together. Wolves who leave their own pack and join another are examples of immigration. Humans often immigrate to another population in search of a better life.
Emigration
Emigration decreases a population when individuals leave the population for reasons other than death. Emigration can occur when populations get too large and need to split off to have enough resources. During the Potato Famine, a large group of Irish emigrated from Ireland to avoid starvation. Emigration can also happen when individuals are kicked out of the population. Wolf packs may chase an individual wolf out of the pack who doesn't behave. This ostracized wolf may or may not join another pack.