Compensatory Mortality in Age ClassStructured Populations

The age structure of a population can also help buffer populations from changes in the mortality patterns of specific age classes. A primary but poorly understood aspect of population-level mortality patterns is the concept of compensatory mortality, which postulates that an increase in any mortality factor(s) (such as harvest) can result in declines in other mortality factors, such that the total mortality rate either does not change or increases less than if the additional mortality was completely additive to existing mortality (Figure 5). Because adult

Year

Figure 5 Dynamics of females in a North American elk (Cervus elaphus) population showing effects of partial compensation in juvenile survival rate for changes in adult survival (compensatory mortality). Curves show population growth when Sadult = 0.85 and Sjuvenile = 0.50 (a); growth when Sad ult is decreased to 0.75 by some additional mortality factor such as harvesting and Sjuvenile remains unchanged (♦); and partially compensatory growth when Sadult is decreased to 0.75 but Sjuvenile increases to 0.60 in compensation (•).

Year

Figure 5 Dynamics of females in a North American elk (Cervus elaphus) population showing effects of partial compensation in juvenile survival rate for changes in adult survival (compensatory mortality). Curves show population growth when Sadult = 0.85 and Sjuvenile = 0.50 (a); growth when Sad ult is decreased to 0.75 by some additional mortality factor such as harvesting and Sjuvenile remains unchanged (♦); and partially compensatory growth when Sadult is decreased to 0.75 but Sjuvenile increases to 0.60 in compensation (•).

survival in most age-structured populations is very high and shows little variation, little opportunity exists for mortality to be compensatory among adults in a population simply because adult mortality rates are typically at or near the chronic minimum, that is, there is no excess mortality to 'trade off among mortality sources. However, age structuring of a population presents a mechanism whereby mortality at the population level can be compensatory even if additive within certain or most age classes (i.e., adults) through compensatory responses in juvenile survival (Figure 5).

Because juveniles are smaller and have a higher metabolic rate, they require greater resources per unit of body mass than do adults, are more susceptible to density-stress and other resource limitations than adults, and thus are more vulnerable to most causes of mortality. Consequently, any reduction in population size disproportionately benefits juveniles because they are more resource stressed than adults in any given set of environmental circumstances. Therefore, even if mortality is completely additive on adults, the reduction in numbers of adults may result in a density-dependent increase in juvenile survival. Hence, age structuring within a population provides a mechanism whereby compensatory effects can act to dampen changes in population size due to mortality factors such as harvesting. In this manner, age structuring helps to regulate populations, buffering populations from large annual changes in population size potentially brought on by age-selective mortality factors such as harvesting (Figure 5).

Solar Power

Solar Power

Start Saving On Your Electricity Bills Using The Power of the Sun And Other Natural Resources!

Get My Free Ebook


Post a comment