Age structuring in a population can help regulate numbers through a variety of processes noted above, usually associated with density-dependent changes in juvenile survival and fecundity rates in response to resource stress. These mechanisms allow age-class-structured populations to exhibit much higher levels of population productivity during periods of resource abundance because of earlier sexual maturity and greater fecundity of juvenile females. Further, increases in juvenile survival rates can compensate for decreases in adult survival to some degree, making the population more resilient to additional mortality such as associated with harvesting (Figure 5). The mechanism behind this response is also density dependence. Thus, age structuring in a population, through variation in age-specific survival and fecundity rates, can contribute significantly to density dependence as a regulating mechanism of populations. Typically, the more age-structured a population, the greater the number of potential density-dependent responses available (i.e., increases in juvenile survival or fecundity; increases in young adult (yearling) survival or fecundity; increases in fecundity of older, senesced females, etc.) and hence the more sensitive density dependence becomes as a means of population regulation; consequently, the more ^-selected a species, the greater the effects of density dependence on the population dynamics of that species.
Extreme age structuring in a species may also limit the sensitivity of density dependence as a population-regulating mechanism. Megaherbivores such as the African elephant can potentially have a dramatic effect on the vegetation of their habitat through foraging. The long life (>50 years) and large number of juvenile (pre-reproductive) age classes (>12) contributes significantly to the potential of these species to impact vegetation communities through herbivory. Even when reproduction is shut down due to density effects, there are still <12 cohorts of juvenile elephants that will grow and increase in individual mass, adding to the existing adult biomass. However, larger elephants consume greater amounts of plant biomass, and it is the total biomass of elephants that determine herbivory levels more so than total numbers. Because adult biomass declines only very slowly due to extremely high survival rates (which allow the long life span) and juveniles continue to grow, total elephant biomass may actually increase for up to 12 years after density dependence has shut down reproduction in the population due to resource (food) stress. Thus, plant communities can be impacted by feeding above their sustainable levels; the increase in total elephant biomass continues well after the capacity of plant communities to support elephants is exhausted. Consequently, elephants can potentially overutilize their habitats because the extreme age structuring in the population makes density dependence a relatively insensitive means of balancing population biomass with available resources.
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