Age Structure in Populations

The age structure of populations is driven primarily, but not exclusively, by age-specific mortality rates (Figure 1). In general, juveniles have relatively low but highly variable survival; prime-aged adults have high and relatively constant survival; and aged, senesced adults have lower and variable survival. Age-specific mortality rates determine both how long individuals in a population can potentially live as well as the proportions of individuals surviving into each successive age class.

For example, if the natural mortality rate of all age classes in a population is 0.10, then on average 90% of individuals in a given age class will survive the year and be recruited into the following age class. The result of incremental mortality through years is that populations tend to show a 'pyramid-shaped' age structure, with fewer individuals in each successive age class (Figure 1). Numbers of

In age-class-structured populations, the effects of age structure on population dynamics arise primarily because of varying age-specific mortality rates and differences in the viability of any individual cohort. Age structure also strongly influences population dynamics through effects on fertility and fecundity rates, timing and length of breeding seasons, population-level productivity, total population-level mortality, and sensitivity to population regulating mechanisms such as density dependence. In turn, age structure can be strongly influenced by actions such as harvesting and environmental variations such as severe weather events, age-specific predation or parasitism, or presence of disease.

Age-class effects are most definitively seen in populations that show three or more distinct age classes, including juveniles, prime-aged adults, and senesced adults. Although effects of population age structure can be seen in populations with two or fewer distinct age classes, they are generally less pronounced than with multiple age classes. Moreover, each cohort may be independently influenced by environmental conditions experienced by their mothers (maternal effects) or

Figure 1 Age structure of a North American elk (Census elaphus) population showing 'pyramid-shaped' age structure and differential survival into older age classes based on an adult survival rate (Sadult) of 0.90 (closed square) and 0.80 (open square). Note that more individuals survive into older age classes as survival rates increase. Mean age of the population is 6.1 and 4.4 years old at Sadult = 0.90 and 0.80, respectively, showing that population age structure declines (becomes younger) as population mortality rates increase. In this simulated population, no individuals survive beyond age 15.

individuals in each successive age class is dependent upon (1) numbers in the previous age class the previous year, (2) age-specific survival, and (3) possibly emigration and immigration. However, if additional mortality such as additive hunting harvest was placed on adults in a population, mortality rates for each age class would increase and the proportion of individuals in each successive age class would be lower (Figure 1). Similarly, mortality factors that preferentially act on juveniles, such as predation, can result in cases where a population is primarily comprised of older adults (Figure 2). Therefore, under most conditions the age structure of a population, and the effects of that age structure on population dynamics, are driven mostly by sex- and age-specific mortality rates. Because changes in age structure thus affect the numbers of individuals that will die or be born each year, the age structure of a population can have a marked effect on year to year changes in population size.

Management of populations in most cases involves managing population productivity, or production and survival of juveniles, because productivity typically accounts for the majority of variation in annual rate of population increase. The more productive a population, the greater the potential return, regardless ofwhether that return is a sustainable harvest or a high rate of population increase for recovery of a rare species. The effects of age structure on population dynamics are most obviously manifest in population productivity through effects on individual fertility, fecundity, and probability of raising a juvenile to weaning or recruitment (see the next section). However, age structure can also influence many other components of population dynamics including sensitivity to density-dependent effects (see the section titled 'Effects on population regulation') and consequently population-habitat relationships, and numbers of individuals that die annually due to differing age-specific mortality rates (see the section titled 'Effects of age-specific mortality').

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