A survivorship curve is a

graph Graph may refer to: Mathematics *Graph (discrete mathematics), a structure made of vertices and edges **Graph theory, the study of such graphs and their properties *Graph (topology), a topological space resembling a graph in the sense of discre ...

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 Cohort or cohortes may refer to: * Cohort (educational group), a group of students working together through the same academic curriculum * Cohort (floating point), a set of different encodings of the same numerical value * Cohort (military unit ...

(a group of individuals of roughly the same age) based on a

life table In actuarial science and demography, a life table (also called a mortality table or actuarial table) is a table which shows, for each age, what the probability is that a person of that age will die before their next birthday ("probability of deat ...

. There are three generalized types of survivorship curves: * ''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, including

human Humans (''Homo sapiens'') are the most abundant and widespread species of primate, characterized by bipedalism and exceptional cognitive skills due to a large and complex brain. This has enabled the development of advanced tools, culture, ...

s and many other large mammals. * ''Type II'' or diagonal curves are an intermediate between Types I and III, where roughly constant

mortality rate Mortality rate, or death rate, is a measure of the number of deaths (in general, or due to a specific cause) in a particular population, scaled to the size of that population, per unit of time. Mortality rate is typically expressed in units of de ...

/survival probability is experienced regardless of age. Some

bird Birds are a group of warm-blooded vertebrates constituting the class Aves (), characterised by feathers, toothless beaked jaws, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a strong yet lightweigh ...

s and some

lizard Lizards are a widespread group of squamate reptiles, with over 7,000 species, ranging across all continents except Antarctica, as well as most oceanic island chains. The group is paraphyletic since it excludes the snakes and Amphisbaenia alt ...

s 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 (see

r/K selection theory In ecology, ''r''/''K'' selection theory relates to the selection of combinations of traits in an organism that trade off between quantity and quality of offspring. The focus on either an increased quantity of offspring at the expense of individ ...

). This includes most marine invertebrates. For example,

oyster Oyster is the common name for a number of different families of salt-water bivalve molluscs that live in marine or brackish habitats. In some species, the valves are highly calcified, and many are somewhat irregular in shape. Many, but not al ...

s 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 A Cartesian coordinate system (, ) in a plane is a coordinate system that specifies each point uniquely by a pair of numerical coordinates, which are the signed distances to the point from two fixed perpendicular oriented lines, measured in t ...

(generally with a

logarithmic scale A logarithmic scale (or log scale) is a way of displaying numerical data over a very wide range of values in a compact way—typically the largest numbers in the data are hundreds or even thousands of times larger than the smallest numbers. Such a ...

starting with 1000 individuals), while their age (often as a proportion of

maximum life span Maximum life span (or, for humans, maximum reported age at death) is a measure of the maximum amount of time one or more members of a population have been observed to survive between birth and death. The term can also denote an estimate of the m ...

) is plotted on the x-axis. In mathematical statistics, the

survival function The survival function is a function that gives the probability that a patient, device, or other object of interest will survive past a certain time. The survival function is also known as the survivor function or reliability function. The term ...

is one specific form of survivorship curve and plays a basic part in

survival analysis Survival analysis is a branch of statistics for analyzing the expected duration of time until one event occurs, such as death in biological organisms and failure in mechanical systems. This topic is called reliability theory or reliability analysi ...

. 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.

References

Population ecology {{ecology-stub

See also

References