L C Bender, US Geological Survey, Las Cruces, NM, USA © 2008 Elsevier B.V. All rights reserved.
Age Structure Effects in Age-Class-Structured Species Further Reading Age Structure Effects in Stage-Structured Species
Age structure in populations can take a variety of forms and thus have a variety of effects on population dynamics. Species can be comprised of multiple age classes in a single-stage life cycle (hereafter, age-class structuring), such as in most birds and mammals. Species can also show multistage life cycles, where different stages in the life cycle are analogous to age classes (hereafter, stage structuring). Such life cycles are common in invertebrates, amphibians, and plants. Species can also show both types of age structuring; typically, this involves a stage-structured life cycle where the longest-lived stage (usually the adult or reproductive stage) also shows two or more distinct age classes. For example, trees have both multiple life stages (seed, plant) and age classes within the plant stage (i.e., seedling, sapling, mature tree). Both age-class structuring and stage structuring in populations can affect population dynamics, albeit typically at different scales. Stage-structured populations are commonly an adaptation to habitats that show extreme temporal or spatial heterogeneity in resource availability. As such, stage-structured populations generally have one or more life stages which can persist through long periods of unfavorable environmental conditions or are capable of rapid long-distance dispersal. When favorable conditions return, these species can rapidly increase in numbers and distribution ('irrupt') because of their high reproductive potential. This allows these species to quickly colonize or occupy all available ephemeral habitats. Such species are usually defined by very high fecundity and display typical r-oriented 'boom or bust' population dynamics in one or more life stages. Examples of such stage-structured populations include many irruptive insect 'pests' such as the eastern spruce budworm (Choristoneura fumiferana).
In contrast, age-class-structured populations are comprised of two or more age classes of individuals born at the same time, or cohorts, in a single-stage life cycle. Often age-class structuring can be complex, such as in long-lived vertebrates which can have many age classes, including newborns and multiple distinct subadult and adult age classes. In contrast to stage-structured species which show irruptive population dynamics to take advantage of ephemeral habitats, age structure in age-class-structured species generally affects population dynamics through more subtle changes in annual population growth rate in response to annual variation in habitat quality and population density. Because of differing sensitivity to habitat conditions, this variation can affect survival and fecundity rates of different age classes unequally, resulting in often subtle effects on population rates of increase and compensatory effects among age classes. However, the effects of age-class structuring on population dynamics are extremely varied and numerous, and go well beyond simply differences in age-specific survival or fecundity. Such species are typically far more ^-selected and show population dynamics more typical of ^-selected species such as a general trend towards density-dependent regulation. Extreme examples include megaherbivores such as African elephants (Loxodonta africana) and black rhinoceros (Diceros bicornis).
Some species show both age-class structuring and stage structuring, and their population dynamics show aspects of each. Usually, survival and fecundity is dependent upon age and phenotypic quality as in age-class-structured species, but individual reproductive output can be enormous and one or more life stages can persist through long periods of dormancy to allow rapid colonization or recolonization of habitats. Classic examples include fire-adapted tree species with serotinous seed cones, such as lodgepole pine (Pinus contorta).
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