- 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
- 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.
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!