APES 3.2 K-Selected / r-Selected Species

Enduring Understanding:

  • Populations change over time in reaction to a variety of factors

Learning Objective:

  • Identify differences between K- and r-selected species.

Essential Knowledge:

  • K-selected species tend to be large, have few offspring per reproduction event, live in stable environments, expend significant energy for each offspring, mature after many years of extended youth and parental care, have long life spans/life expectancy, and reproduce more than once in their lifetime.  Competition for resources in K-selected species’ habitats is usually relatively high.
  • r-selected species tend to be small, have many offspring, expend or invest minimal energy for each offspring, mature early, have short life spans, and may reproduce only once in their lifetime.  Competition for resources in r-selected species habitats is typically relatively low.
  • Biotic potential refers to the maximum reproductive rate of a population in ideal conditions.
  • Many species have reproductive strategies that are not uniquely r-selected or K-selected, or they change in different conditions at different times.
  • K-selected species are typically more adversely affected by invasive species than r-selected species, which are minimally affected by invasive species.  Most invasive species are r-selected species.

r-selected Species:

  • Have many small offspring
  • Give little to no protection/care for offspring
  • Most offspring die before reaching reproductive age
  • Early reproductive age
  • Small adults
  • Adapted to unstable climates and environmental conditions
  • High population growth rate (r)
  • Population size fluctuates wildly above and below carrying capacity (K)
  • Generalist niche
  • Low ability to compete
  • Early successional species

K-selected Species:

  • Fewer, larger offspring
  • High parental care and protection
  • Later reproductive age
  • Most offspring survive to reproductive age
  • Larger adults
  • Adapted to stable climate and environmental conditions
  • Lower population growth rate (r)
  • Population size fairly stable and usually close to carrying capacity (K)
  • Specialist niche
  • High ability to compete
  • Late successional species
  • J-shaped population curve displays exponential/unrestricted growth
    • Growth is accelerating, does not stop
  • S-shaped population curve displays logistic/restricted growth
    • Growth accelerates in beginning, reaches point of maximum growth, rate slows down and plateaus at carrying capacity

Survivorship Curves

  • Type I species (K-selected) have high survivorship when young and low survivorship when old.
  • Type II species have consistently declining survivorship as they age.
  • Type III (r-selected) have low survivorship when young and high survivorship when old.

Biotic Potential vs. Environmental Resistance

  • Biotic Potential

    • Reproductive rate
    • Ability to migrate (animals) or disperse (seeds)
    • Ability to invade new habitats
    • Defense mechanisms
    • Ability to cope with adverse conditions

  • Environmental Resistance

    • Lack of food or nutrients
    • Lack of water
    • Lack of suitable habitat
    • Adverse weather conditions
    • Predators
    • Disease
    • Parasites
    • Competitors

  • Factors that increase biotic potential

    • High birth rate
    • Generalized niche
    • Able to migrate
    • Sufficient food supply
    • Favorable environmental conditions
    • Few competitors
    • Resistance to disease
    • Ability to adapt

  • Factors that decrease biotic potential

    • Low birth rate
    • Specialized niche
    • Unable to migrate
    • Deficient food supply
    • Unfavorable environmental conditions
    • Many competitors
    • Few defense mechanisms against disease
    • Unable to adapt

  • To put it simply: Biotic potential is increased with adaptability and good conditions