Survivorship curve

File:Survivorship Curves.jpg

An illustration of the three idealized types of curve (logarithmic scale)

A survivorship curve is a graph showing the number or proportion of individuals surviving to each age for a given species or group (e.g. males or females). Survivorship curves can be constructed for a given cohort (a group of individuals of roughly the same age) based on a life table.

There are three generalized types of survivorship curves:

• Type I survivorship curves are characterized by high age-specific survival probability in early and middle life, followed by a rapid decline in survival in later life. They are typical of species that produce few offspring but care for them well, including humans and many other large mammals.
• Type II curves are an intermediate between Types I and III, where roughly constant mortality rate/survival probability is experienced regardless of age. Some birds and some lizards follow this pattern.
• In Type III curves, the greatest mortality (lowest age-specific survival) is experienced early in life, with relatively low rates of death (high probability of survival) for those surviving this bottleneck. This type of curve is characteristic of species that produce a large number of offspring (see r/K selection theory). This includes most marine invertebrates. For example, oysters produce millions of eggs, but most larvae die from predation or other causes; those that survive long enough to produce a hard shell live relatively long.

The number or proportion of organisms surviving to any age is plotted on the y-axis, generally with a logarithmic scale starting with 1000 individuals, while their age, often as a proportion of maximum life span, is plotted on the x-axis.

In mathematical statistics, the survival function is one specific form of survivorship curve and plays a basic part in survival analysis.

See also

• Actuarial science
• Life history theory
• Population ecology