Limiting Population Size (SL IB Biology)

• The maximum number of individuals of a species that an ecosystem can support is known as its carrying capacity
  • Carrying capacity is represented by the letter K
• While every individual within a species population has the theoretical potential to reproduce and have offspring that will contribute to population growth, in reality there are many factors that prevent every individual in a population from surviving and reproducing

• This means that the population size of each species is limited, i.e. the ecosystem has a carrying capacity for that species
• The graph below shows the population growth of a population of lions
  • The point at which the graph starts to flatten out is the carrying capacity of this population
  • At this point the environmental factors that stop all individuals from surviving and reproducing mean that the population can no longer increase

Carrying capacity graph

Carrying capacity is reached when the growth of a population starts to level off

Factors affecting carrying capacity

• Abiotic factors involve the non-living parts of an ecosystem, e.g.
  
  • Light availability
    • A lack of light will reduce the carrying capacity for a plant population as it will limit photosynthesis
  • Temperature
    • Low or high temperatures will influence carrying capacity as this will affect the rate at which the reactions of metabolism can occur
  • Soil mineral availability
    • Low mineral availability will reduce carrying capacity as it will affect the ability of plant populations to build biological molecules such as proteins and chlorophyll

• Biotic factors involve the living parts of an ecosystem, e.g.
  • Competition for resources
    
    • A lack of resources will limit the carrying capacity of an ecosystem
    • Energy that an individual puts towards competing for resources will not be available for growth and reproduction, so this will reduce carrying capacity
  • Predation
    • Energy that an individual puts towards avoiding predators will not be available for growth and reproduction, so this will reduce carrying capacity
  • Disease
    • Energy that an individual puts towards fighting off disease will not be available for growth and reproduction, so this will reduce carrying capacity

• Population density can be defined as follows:

The number of individuals present per unit area of habitat

• Factors that influence the size of a population can be density-dependent or density-independent
  • Density-dependent factors have a different effect at different population densities, e.g.
    • Pathogens and pests will spread faster through a dense population, and so will have a greater effect on the population size
    • Dense populations will be in increased competition for resources, so competition will have an increased effect at high densities
    • Predators will be attracted to dense populations of prey organisms, so predation will have an increased effect at high densities
  • Density-independent factors have the same effect on a population at any population density, e.g.
    • A natural disaster, such as a flood, is equally likely to affect populations of different densities
• Density-dependent factors tend to act to keep a population at or below its carrying capacity; this is a negative feedback effect
• A negative feedback system acts to keep conditions within narrow limits; if conditions stray too far from an ideal value then negative feedback causes a return to that value, e.g.
  • If a population increases above its carrying capacity then density-dependent factors, such as spread of disease, or competition for food, cause a reduction in survival and reproduction, resulting in a decrease in the population size
  • If a population drops below its carrying capacity then the same density-dependent factors will lead to increased survival and reproduction, and there will be an increase in population size
• When population size is controlled by negative feedback it will fluctuate around its carrying capacity

Density-dependent factors & population size graph

Density-dependent factors tend to act to keep a population at or below its carrying capacity; this is a negative feedback effect

• A population that is controlled by positive feedback will respond to a change in population size by continuing to change in the same direction, e.g.
  • An increase in population size means that more individuals are present, leading to increased reproduction, and a further increase in population size; this can continue until a density-dependent factor, such as competition for resources, starts to limit population growth