Asurvivorship curve is agraph 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 givencohort (a group of individuals of roughly the same age) based on alife table.

There are three generalized types of survivorship curves:^[1]

• Type I or convex 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, includinghumans and many other large mammals such aselephants. These are also known as K-selected species (see:r/K selection theory)
• Type II or diagonal curves are an intermediate between Types I and III, where roughly constantmortality rate/survival probability is experienced regardless of age. Somebirds and somelizards follow this pattern.
• Type III or concave curves have the greatest mortality (lowest age-specific survival) 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 (known as r-selected species). 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 they-axis (generally with alogarithmic scale starting with 1000 individuals), while their age (often as a proportion ofmaximum life span) is plotted on the x-axis.

In mathematical statistics, thesurvival function is one specific form of survivorship curve and plays a basic part insurvival analysis.

There are various reasons that a species exhibits their particular survivorship curve, but one contributor can be environmental factors that decrease survival. For example, an outside element that is nondiscriminatory in the ages that it affects (of a particular species) is likely to yield a Type II survivorship curve, in which the young and old are equally likely to be affected. On the other hand, an outside element that preferentially reduces the survival of young individuals is likely to yield a Type III curve. Finally, if an outside element only reduces the survival of organisms later in life, this is likely to yield a Type I curve.^[2]

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