Survival rates of age-structured species can vary among age classes. Generally, juveniles have low but extremely variable survival, whereas prime-aged adults have high and relatively constant survival. As individuals become older and move into the age classes that show reproductive senescence, survival rates typically decline. A review of survival studies from cervids found preweaning, postwean-ing, prime-aged adult female, and senescent adult female survival (CV) of 0.62 (0.25), 0.71 (0.28), 0.86 (0.09), and 0.79 (0.14), respectively, with similar patterns seen in bovids. Although fairly invariant with respect to large mammals, these patterns may be less pronounced in other taxa, such as rodents. Importantly, not only is survival ofjuveniles and older, senesced females lower than prime-aged females, but it is also two to three times more variable.
Increased mortality rates in senesced adults, when present, are due to a variety of factors, including decreased body mass, lower individual nutritional condition, wear of teeth, and decreased immunocompetence. For juveniles, smaller body size, lower energy reserves, greater energy requirements per unit body size, and inexperience all increase vulnerability relative to adults. Because of their sensitivity to environmental variation, juveniles are particularly affected by temporal variation in environmental conditions and hence the annual variation associated with survival rates (as well as fecundity; see below) is much higher than in adults. Lowered survival rates in senesced adults result in their lowered fecundity having less impact on overall population rate of increase than if survival remained comparable to prime-aged adults.
The sensitivity of juveniles to annual variation in resource availability can strongly affect population-level production and mortality. In many age-structured populations, if density or resource stress is low, juveniles may attain sexual maturity and breed earlier in life and productivity of young females may equal that of prime-aged adults. These effects are important when one considers proportional contributions to population rate of increase. Adult female survival has the greatest potential effect on rate of increase (i.e., the highest elasticity) but, as noted above, typically varies little annually. Conversely, juvenile fecundity and juvenile survival have much lower elasticity, but vary greatly annually. Consequently, the majority of the variation in annual changes in population size is a result of changes in juvenile survival and fecundity. Population age structure can thus exert a strong regulating effect on population rate of increase by dampening potential population declines when mortality of adults is increased as well as increasing population rates of increase during periods of high resource availability through density-dependent changes in survival and fecundity of juveniles. Despite their lower fecundity and survival as compared to prime-aged adults, juveniles can therefore be critical contributors to population-level productivity.
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