The ability of a population to persist and to avoid extinction. The viability of a population will increase or decrease in response to changes in the rates of birth, death, and growth of individuals. In natural populations, these rates are not stable, but undergo fluctuations due to external forces such as hurricanes and introduced species, and internal forces such as competition and genetic composition. Such factors can drive populations to extinction if they are severe or if several detrimental events occur before the population can recover. The concept of population viability gained importance with the National Forest Management Act of 1976, which charged the U.S. Forest Service with maintaining “viable populations” of all species of vertebrates found on its properties.