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Origin of Life Tables
About a century ago, life tables developed when insurance companies needed to calculate the cost of life insurance premiums. They needed to know the number of years individuals could be expected to live, and what proportion of the population would be alive at all ages. Life tables--which follow a cohort (a group of individuals of the same age) from birth until death--constituted the solution. However, since the tables contained lots of data, interpretation became difficult. Graphing the data in the form of survivorship curves made the information easier to interpret. Today, the technique works well for population ecologists. Three types of survivorship curves exemplify three different strategies various organisms utilize in order to maximize fecundity, or reproductive success. The y-axis plots the number of living individuals and the x-axis plots their ages. Ecologists can use separate lines for male, female and morphological (or physical) characteristics.
Type I: Convex
The line of the Type I survivorship curve begins high on the graph and continues across all young and middle-age groups. It indicates a low death rate of young and middle-aged individuals. A large proportion of individuals of each age survive to the next age; thus, a large proportion live to old age. The curve drops steeply as individuals reach old age, because mortality becomes high at old age. This type of curve is associated with humans and other large mammals, that produce few offspring during their lifetimes. The young are dependent on parental care for an extended period of time.
Type II: Linear
Type II survivorship curves exhibit a constant, downward slope. The same proportion of the population survives at all ages. Type II is the intermediate between Type I and Type III curves, indicating a constant death and birth rate. Type II is associated with many small mammals, various invertebrates, some reptiles and annual plants. Young are dependent upon parental care, but for a shorter period of time.
Type III: Concave
Type III is the opposite of Type I. The curve begins with an immediate, steep, downward slope, and levels off while individuals are still young. In these populations, mortality remains high among the very young. The shallow slope continues through old age. This type of survivorship curve indicates that if individuals survive their youth, they will likely survive to old age. Type III curves are associated with fish and other organisms that produce thousands of young, but provide little to no parental care.