Wildlife Encounters - 9th - 12th grade
Lesson 2: Wildlife Populations
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Adaptations: Predator-Prey Relationships

Different species have different roles or niches in the ecosystem so that they do not compete for the same resources. Many species have niches where they serve as food for other species. Prey animals are species that are food for other animals known as predators. Many predators may even be prey for larger predators. Predator species that are not prey for any other animals are known as the top predator.

snow owlThe relationships between predator and prey animals make up the delicate balance that is part of an ecosystem. Changes in one population will result in changes in the other.

A well-studied predator-prey relationship is that of the snowy owl and the lemmings, a type of rodent, that are their prey. Lemming populations go through cycles of growth and decline. When lemming populations become too large, they consume all of the moss that is their food. Without the moss, the lemmings starve and the population crashes. This cycle repeats every three to four years. The changes in the lemming population affect the predators that rely on them for food. Snowy owls will only reproduce if lemming populations are large enough for the owls to have enough food to feed both themselves and their chicks.

The graph below shows that snowy owls on one island in Canada only nested in years with large numbers of lemmings.

snow owl chart Prey animals need predators to help keep their populations healthy too. In Illinois, all the top predators, such as the grey wolf, cougar, and American black bear, are extinct. Without these predators, the white tailed deer population has grown so large that there is not enough food for all the deer. The growing human population has made this problem worse by decreasing the amount of natural habitat for the deer. The Illinois state government allows human hunting of deer to help limit the population size and prevent death by starvation.

deer in snow