Age Structure

Age structure reflects the proportions of individuals at different life stages. This variable is an important indicator of population status. Growing populations generally have larger proportions of individuals in younger age-classes, whereas declining populations usually have smaller proportions of individuals in these age classes. Stable populations usually have relatively more individuals in reproductive age-classes. However, populations with larger proportions of individuals in younger age-classes also may reflect low survivorship in these age classes, whereas populations with smaller proportions of individuals in younger age-classes may reflect high survivorship (see later in this chapter).

For most insect species, life spans are short (usually year) and revolve around seasonal patterns of temperature and rainfall. Oviposition usually is timed to ensure that feeding stages coincide with the most favorable seasons and that diapausing stages occur during unfavorable seasons (e.g., winter in temperate regions and dry season in tropical and arid regions). Adults usually die after reproducing. Although there are many exceptions, most temperate species have discrete, annual generations, whereas tropical species are more likely to have overlapping generations.

EASTERN SLOPES OF CASCADE RANGE

WESTERN SLOPES OF CASCADE RANGE

EASTERN SLOPES OF CASCADE RANGE

WESTERN SLOPES OF CASCADE RANGE

Current

Climate scenario i

Climate scenario

Current

Climate scenario i

Climate scenario

Alpine and forest zones: ■ Cold snow zone H Alpine

□ Mountain hemlock H Abies grandis

□ Ponderosa pine

Savanna and steppe zones: □ Juniper savanna H Sagebrush steppe

Alpine and forest zones: ■ Cold snow zone H Alpine

□ Mountain hemlock H Silver fir

H Western hemlock

Savanna and grassland zone: □ Oak savanna urn Grassland

Changes in the percentage area in major vegetation zones on the eastern (left) and western (right) slopes of the Cascade Range in Oregon as a result of temperature increases of 2.5°C and 5°C. Major changes are predicted in elevational boundaries and total area occupied by vegetation zones under these global climate change scenarios. Vegetation zones occupying higher elevations will decrease in area or disappear as a result of the smaller conical surface at higher elevations. Other species associated with vegetation zones also will become more or less abundant. From Franklin et al. (1992) with permission of Yale University Press.

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