A first step to understanding individual animals, and in turn populations of animals, is to understand the relationship they have with their environment. The environment in which an animal lives is referred to as its habitat. A habitat includes both biotic (living) and abiotic (non-living) compents of the animals environment. Abiotic components of an animal's environment include a huge range of characteristics, examples of which are:
- soil composition
- day length
Biotic components of an animal's environment include such things as:
- plant matter
- individuals of the same species
Animals require energy to support the processes of life: movement, foraging, digestion, reproduction, growth, work. Organisms can be categorized into one of the following groups:
- autotroph - an organism that obtains energy from sunlight (in the case of green plants) or inorganic compounds (in the case of sulfur bacteria)
- heterotroph - an organism that use organic materials as a source of energy
Animals are heterotrophs, obtaining their energy from the ingestion of other organisms.
When resources are scarce or environmental conditions limit the ability of animals to obtain food or go about their normal activies, animals' metabolic activity may decrease to conserve energy until better conditions prevail. The different types of metabolic dormancy or responses include:
- torpor - a time of decreased metabolism and reduced body temperature in daily activity cycles
- hibernation - a time of decreased metabolism and reduced body temperature that may last weeks or months
- winter sleep - periods of inactivity during which body temperature does not fall substantially and from which animals can be awakened and become active quickly
- aestivation - a period of inactivity in animals that must sustain extended periods of drying
Environmental characteristics (temperature, moisture, food availability, and so on) vary over time and location and animals are adapated to certain range of values for each characteristic. The range of an environmental characteristic to which an animal is adapted is called its tolerance range for that characteristic. Within an animal's tolerance range is an optimal range of values at which the animal is most successful. Sometimes, in response to prolonged change in environmental characteristic, animals' physiology adjusts to accomodate the change in its environment, and in doing so, its tolerance range shifts. This shift in tolerance range that the animal experiences in response to an changed environment, is called acclimation.