Basic ideas - Predator -prey relationship.

Key Words:
Predator - prey - negative feedback - stable population - vertebrates.v.invertebrates - density dependent factors (Serengeti) - density independent factors (Tundra) - human interference (Kaibab Plateau example) - biological controls


Main points

  • If the number of prey animals increases, there is more food available for the predators and hence they are able to raise more offspring. This, in turn will cause the number of predators to rise after a short time lag.
  • In many invertebrate examples, this will cause a significant increase in the number of prey animals eaten, causing their population to drop. Reduced prey will then reduce the breeding success / survival of predators - causing the predator numbers to drop again (see graph below). This results in negative feedback and a stable population.
  • In vertebrates, the numbers of herbivores are rarely controlled by carnivores in this way - vertebrate predators mainly eat weak and sick animals, due to the presence of horns & hooves! In the Serengeti, wildebeest numbers are reduced by other, mainly density dependent, factors such as disease. In the tundra, lemming and snowshoe hare numbers drop for reasons of climate / food (density independent factors) BEFORE changes in predator numbers can have any significant effect.
  • The effect of human interference has caused environmental damage by the removal of predatory control of herbivore numbers- eg the introduction of rabbits to Australia, where there were insufficient resident predators - eg the removal of wolves and coyote from the Kaibab Plateau (just N of the Grand Canyon) caused a population explosion in mule deer which caused environmental degradation and reduction in the carrying capacity.
  • Vedalia ladybirds were introduced as a biological control to eat aphids that were a major pest of orange trees in California.
  • Predators are a density dependent factor.

    Predator prey graph

    This graph holds true mainly for relationships with invertebrate prey for example where the predator is a bird and the prey is an insect and the predator has no threat from the prey fighting back. Where the prey is a larger vertebrate (eg an antelope) the relationship os not so clear because an increase in healthy antelopes does not necessarily result in more lions... lions don't pick healthy, strong antelopes because of the risk of damage to the lion!



wildebeest have horns, this discourages lions from attacking any but the weakest.
Whilst vertebrate carnivores (eg coyote - above) are not the only control on the number of vertebrate herbivore (eg mule deer - below) they play an important part in population control. The removal of wolves and coyote from the Kaibab Plateau (Grand Canyon) resulted in mule deer populations rising beyond the carrying capacity of the ecosystem.
mule deer

Human interference through hunting can disrupt the normal homeostatic predator/prey relationship - as shown in the graph above (based on experience on the Kaibab Plateau).