​The Niche

​Every species has its own tolerance, or a range of conditions under which it can grow and reproduce. A species’ tolerance determines its habitat, the place where it lives.

​A niche consists of all the physical and biological conditions in which a species lives and the way the species obtains what it needs to survive and reproduce.

An organism’s niche must contain all of the resources an organism needs to survive. A resource is any necessity of life, such as water, nutrients, light, food, or space.

​Competition

​Competition occurs when organisms try to use the same limited resources.

Direct competition between species often results in one species dying out. This is the basis of the competitive exclusion principle. This principle states that no two species can occupy exactly the same niche in exactly the same habitat at the same time.

Competition helps to determine the number and type of species in a community.

​Predation, Herbivory, & Keystone Species

​Predator-prey and herbivore-plant interactions help shape communities.

Predation occurs when one organism (the predator) captures and eats another (the prey).

Herbivory is an interaction that occurs when an animal (the herbivore) feeds on producers (such as plants).

Sometimes changes in the population of a single species, often called a keystone species, can cause dramatic changes in the structure of a community.

​Symbiosis

​Symbiosis occurs when two species live closely together in one of three ways: mutualism, commensalism, or parasitism.

In mutualism, both species benefit from the relation ship.

In parasitism, one species benefits by living in or on the other and the other is harmed.

In commensalism, one species benefits and the other is neither helped nor harmed.