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Survivorship curves and K-/r-selection

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Review your understanding of survivorship curves, K-selected species, and r-selected species in this free article aligned to AP standards.

Key points

  • Survivorship curves are graphs that show the proportion of a population that survives from one age to the next. These curves represent age-specific mortality in a group of organisms.
  • To generate survivorship curves, ecologists typically collect age-specific survival rates for organisms within a cohort. A cohort is a group of individuals from the same population who are born at the same time. Survivorship data from a cohort can be representative of the larger population.
  • In general, scientists divide survivorship curves into three types based on their shapes:
    • Type I survivorship curves represent populations whose organisms tend to survive beyond their young and middle-ages and die when they become elderly. These organisms usually have small numbers of offspring and provide lots of parental care to make sure those offspring survive.
    • Type II survivorship curves represent populations with a constant proportion of individuals dying at each age interval. Similar to species that exhibit Type 1 survivorship, these organisms usually have relatively few offspring and provide them with lots of parental care.
    • Type III survivorship curves represent populations that have a high death rate among the young, but a relatively low death rate for those who survive into middle and old age. These organisms usually have lots of offspring at once, but don't provide them with much parental care.
  • Type I and Type II curves represent typical survivorship patterns for K-selected species. Type III curves represent typical survivorship patterns for r-selected species.
    • K-selected species have traits that are advantageous at high population densities. These species tend to have long lives, thrive under stable conditions, be large in size, produce few offspring, reproduce more than once in a lifetime, and provide lots of parental care to their offspring.
    • r-selected species have traits that maximize reproductive success in uncrowded environments. These species are often found in disturbed environments, or other areas where competition is low. They tend to have short life spans, be small in size, mature early, have many offspring, and they may reproduce only once per lifetime.
  • Most real populations show some mix of Types I, II, and III survivorship patterns. For example, juveniles of some species may display Type III survivorship, while adults of the same species display Type II survivorship.
A graph plots the number of surviving organisms against the percentage of maximum life span. The y-axis is labeled Number of organisms surviving (log scale). It is labeled 1 at the bottom, 10 in the center, and 100 at the top. The x-axis is labeled Percentage of maximum life span. It is labeled 50 in the middle and 100 at the right end. A line labeled type I starts on the y-axis at 100 individuals, slowly decreases to the right, then quickly decreases to the bottom of the graph. A line labeled type II starts on the y-axis at 100 individuals and follows a straight diagonal path to the bottom right of the graph. A line labeled type III starts on the y-axis at 100 individuals, quickly decreases to the right, then gradually decreases until it reaches the bottom right of the graph.
Type I, Type II, and Type III survivorship curves.

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