Carrying Capacity (College Board AP® Environmental Science): Study Guide

Carrying capacity of populations

What is carrying capacity (K)?

• Carrying capacity refers to the maximum number of individuals of a population that an ecosystem can sustainably support over time

  • It is the maximum stable population size of a species

• Carrying capacity is determined by the availability of resources such as food, water, and shelter

• Every individual within a species population has the potential to reproduce and have offspring that will contribute to population growth

  • In reality, however, there are many abiotic and biotic factors that prevent every individual in a population from making it to adulthood and reproducing

• This ensures the population size of each species is limited at some point (i.e. the carrying capacity of that species is reached)

  • This is why no single species has a population size that dominates all other species populations on Earth

  • The exception is humans (we have managed to overcome many of the environmental factors that could potentially limit the population growth of our species)

• The graph below shows the population growth of a population of lions

  • The point at which the graph flattens out (plateaus) is the carrying capacity of this population

Impacts of population overshoot

What is population overshoot?

• Population overshoot occurs when a population temporarily exceeds the carrying capacity of its environment

  • This leads to resource depletion as demand exceeds supply

Environmental impacts of overshoot

• Depletion of essential resources such as water, food, and habitat

• Increased environmental degradation, such as soil erosion or deforestation, caused by overuse of land

  • Example: Overgrazing by livestock can strip vegetation, leading to desertification

Ecological effects of overshoot

• Population dieback:

  • Dieback is a significant reduction in population size

  • Dieback can range from moderate declines to catastrophic crashes

  • Dieback occurs due to

    • Increased competition and conflict over scarce resources

    • Famine

    • Increased vulnerability to disease as dense populations facilitate the spread of pathogens, such as diseases and parasites, leading to increased mortality rates

Population growth curves

• Population growth can either be exponential or limited by carrying capacity

  • If there are no limiting factors, population growth follows a J-curve (exponential growth)

  • When limiting factors start to have an effect, population growth slows and stabilizes around carrying capacity, and the curve becomes S-shaped (logistic growth)

J-curves

• For some populations, when population growth is plotted against time, a J-curve is produced

  • A J-curve describes the growth pattern of a population in an environment with unlimited resources

• Common in populations experiencing temporary favorable conditions

• The J-curve has three distinct phases:

1. Lag phase:

   • The initial growth is slow when the population is small

2. Exponential growth phase:

   • Population growth accelerates exponentially as the number of individuals increases

   • The curve takes a J-shape due to exponential growth, as resources are not limiting the growth of the population

   • The population will continue to grow until a limiting factor such as disease or predation occurs

3. Crash phase:

   • At this point, if there has been a significant population overshoot (if the population has increased far beyond the natural carrying capacity), there may be a sudden decrease in the population, known as a population crash or dieback

S-curves

• For most populations, when population growth is plotted against time, an S-population curve is produced

  • An S-population curve describes the growth pattern of a population in a resource-limited environment

• The S-population curve has four distinct phases:

1. Lag phase:

   • The initial growth is slow when the population is small

2. Exponential growth phase:

   • With low or reduced limiting factors, the population expands exponentially into the habitat

3. Transitional phase:

   • As the population grows, there is increased competition between individuals for the same limiting factors or resources

   • This competition results in a lower rate of population increase

4. Plateau phase:

   • The population reaches its carrying capacity and fluctuates around a set point determined by the limiting factors

   • Changes in limiting factors cause the population size to increase and decrease (these increases and decreases around the carrying capacity are controlled by negative feedback mechanisms)