Survivorship Curves (College Board AP® Environmental Science): Study Guide

Survivorship curves for cohorts in a population

What is a survivorship curve?

• A survivorship curve graphically represents the pattern of survival within a population from birth to the maximum age reached by any individual

  • It tracks the proportion of individuals in a cohort (a group born at the same time) that survive to different age classes

Types of survivorship curves

Type I survivorship curve

• Characterized by high survival rates throughout most of the lifespan

• Reproduction occurs early in life

• Mortality increases with age (i.e. most deaths occur at the limit of biological lifespan)

• Examples: Humans, elephants, large mammals

Type II survivorship curve

• Shows a relatively constant mortality rate across all age classes (i.e. individuals have an equal chance of dying at any age)

• This results in a linear decline in survivorship

• Predation is the primary cause of death

• Typical of organisms that reach adult stages quickly

• Examples: Songbirds, small mammals like squirrels

Type III survivorship curve

• Indicates high mortality rates in early life

• High survival rates for those who reach adulthood

• Typical species that have great numbers of offspring and reproduce for most of their lifetime

• Examples: Insects, fish, and many plant and tree species

Survivorship curves for K-selected & r-selected species

• K-selected species:

  • Usually follow Type I or sometimes Type II survivorship curves

  • Their investment in parental care and individual offspring ensures low infant mortality rates and increases the likelihood of survival to adulthood

• r-selected species:

  • Usually follow a Type III survivorship curve

  • They produce a large number of offspring, but many die early due to limited parental care and high vulnerability to environmental factors