Population Dynamics

• Density-dependant factors reduce a population when numbers are high and allow the population to increase when numbers are low. Biotic interactions can act as density-dependant controls on the size of a population including predator-prey cycles, grazing, competition and parasitism.
• Population Growth

  Population growth depends on the birth and death rate. As long as there are sufficient resources available, there will be an increase in the population. Changes in population growth in response to changes in limiting factors can be modelled.
• Exponential Population Growth

  In the exponential growth model, a population increases over time regardless of resource limits or abiotic factors. It has a J-shaped curve. In nature, exponential growth is not sustainable as eventually a population will exceed resource availability and/or be affected by density-dependant factors.
• Exponential Curve Shape


  J
• Logistic Population Growth

  In the logistic growth model, a population may grow exponentially until it reaches the carrying capacity of its environment, then will slow. It has a S-shaped curve. In nature, logistic population growth is more sustainable than exponential, as resource availability and biotic factors ensure that a population growth rate exceeding carrying capacity is only temporary.
• Logistic Curve Shape


  S
• Population Oscillations

  As populations approach their carrying capacity, there will be insufficient resources to support more offspring. The population may temporarily exceed the carrying capacity and start to die off because of insufficient resources. The environment will then start to recover due to reduced population pressure, and after a period of time will be able to support increased population growth. This results in a series of population overshoots and crashes known as oscillations.
• Density-independent factors affect the size of a population independent of the population density. These factors tend to be natural disasters e.g. hurricanes, earthquakes.