A diverse array of species, including plants, insects, and amphibians, have differing life stages that show different life forms, and thus show unique effects on population dynamics because of this life history. Ecological factors that limit one life stage (such as insect larvae) may have no effect on other age classes (such as the adult insect). Consequently, population dynamics of these species are less influenced by subtle changes in environmental conditions than are ^-selected species. This, in combination with high fecundity, results in population dynamics being far more density independent. For example, outbreaks of many insect pests depend upon an environment that provides (1) habitats to provide for growth and development of eggs and larvae and (2) habitats that allow adults to survive and reproduce. Habitat limitations for any life stage of the insect will result in populations being limited at relatively low levels regardless of how suitable environmental conditions are for the other life stages. However, even under suitable conditions, a minimum population size may be necessary to achieve a 'critical mass' and allow the population to rapidly increase numbers and extent of distribution. Such a dynamic is often described as a manifold or breakpoint in the species growth curve; if ecological conditions allow the species to grow beyond this breakpoint, the species can irrupt.
The environmental conditions necessary to trigger rapid population establishment and growth may be extremely different among life stages. At one extreme, the environmental effect that triggers the irruption of juveniles may be the destruction of the adult life stage. For example, fire-adapted shade-intolerant trees such as lodgepole pine require full sunlight, mineral soil, and heat (to open serotinous cones and free seeds) to germinate. These environmental conditions usually only occur following a stand-replacing fire, which kills the existing population of overstory lodgepole pine trees. This in turn creates the conditions necessary for rapid germination of seeds and widespread establishment of lodgepole seedlings. Without loss of the adult overstory, germination and establishment of the new cohort of lodgepole pine could not occur.
Conversely, habitat conditions necessary for rapid population growth can be similar for all life stages. Outbreaks of spruce budworms require extensive areas of mature spruce/fir forest to occur, as this forest structure provides necessary habitats for larvae to feed, molt, and pupate, as well as for adult moths to lay eggs. Thus, large-scale outbreaks may only be ended by extensive mortality of preferred host trees due to defoliation, which decreases available feeding habitat for larval instars and egg-laying habitat for adult moths. Consequently, numbers decrease to levels where their population size can be limited by natural predators, limited resources (mature balsam fir and white spruce forests), and weather, despite the ability of the adult moths to disperse widely and find most suitable trees.
See also: Adaptation; Age-Class Models.
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