Critical Factors

Animal or plant species do not exist in a vacuum. Almost any living thing has certain requirements that have to be met before they can survive in a given location. For instance,

Nutrition (food): Usually, the amount of food needed increases with the size of the plant or animal. The simpler organisms usually manufacture the basic food elements that they require. Living things which eat other plants or animals rely on what they eat to satisfy their nutrient requirements. In exchange for this dependency, they eliminate the need for the biological equipment required to manufacture these nutrients which open up other opportunities.
Suitable climate: Temperature, length of growing season and amount of rainfall affect what plants can grow there and, so indirectly, which animals can live there.
Shelter: Shelter may be required to protect an animal or its young from predators.
Water: Although some plants and animals can survive in very dry conditions, they do require at least periodic access to some source of water. This might be indirect, such as water derived by eating plants.
Oxygen: The atmosphere also provides a protective layer around the earth.
Amount of suitable habitat: Usually, the amount of living space needed will increase with size. Animals that prey on other animals need sufficient habitat to support enough of the kind of animal that they eat. Some grazing animals need lots of space to allow grasses to grow back in a previously visited area.
Getting there: As demonstrated by the introduction of European species into North America, sometimes plants or animals may not be present in a given habitat only because they had not arrived there yet. Once introduced, they may succeed better than the existing residents.

Critical Factors

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