A survivorship curve summarizes the pattern of survival in a population. Patterns of survival can be determined either by following a cohort of individuals of similar age to produce a cohort life table or by determining the age at death of a large number of individuals or the age distribution of a population to produce a static life table. Life tables can be used to draw survivorship curves, which generally fall into one of three categories: (1) type I survivorship, in which there is low mortality among the young but high mortality among older individuals; (2) type II survivorship, in which there is a fairly constant probability of mortality throughout life; and (3) type III survivorship, in which there is high mortality among the young and low mortality among older individuals.The age distribution of a population reflects its history of survival, reproduction, and potential for future growth. Age distributions indicate periods of successful reproduction, high and low survival, and whether the older individuals in a population are replacing themselves or if the population is declining. Population age structure may be highly complicated in variable environments, such as that of the Galápagos Islands. Populations in highly variable environments may reproduce episodically.A life table combined with a fecundity schedule can be used to estimate net reproductive rate (Ro), geometric rate of increase (λ), generation time (T), and per capita rate of increase (r). Because these population parameters form the core of population dynamics, it is important to understand their derivation as well as their biological meaning. Net reproductive rate, Ro, the average number of offspring left by an individual in a population, is calculated by multiplying age-specific survivorship rates, lx, times age-specific birthrates, mx, and summing the results:
The per capita rate of increase may be positive, zero, or negative depending on whether a population is growing, stable, or declining.Dispersal can increase or decrease local population densities. The contribution of dispersal to local population density and dynamics is demonstrated by studies of expanding populations of species such as Africanized bees in the Americas and collared doves in Europe. Climate changes can induce massive changes in the ranges of species. As availability of prey changes, predators may disperse, which increases and decreases their local population densities. Stream organisms actively migrating upstream or drifting downstream increase densities of stationary and migrating populations by immigrating and decrease them by emigrating.
Ecologists are using the effects of pollutants on population dynamics to predict the potential ecological impact of these pollutants on populations. Good candidates for indicators of pollution are those aspects of population dynamics that are sensitive to environmental variation. Based on its environmental sensitivity, per capita rate of increase, r, appears to be an excellent predictor of the impact of a wide range of potential pollutants. The results of this research suggest that population processes and the mechanisms underlying variation in those processes can be used as sensitive predictors of the ecological effects of environmental change.
