Organisms need air, food, water and space to survive. Most organisms also need shelter. A place where an organism can get all of these life needs is its habitat. The Earth has great variability in climate, topography and accessibility of water. Despite this variability, just about every part of our planet is a habitat for some living thing because living things have evolved great variability in body structures and behaviors.

A habitat can be as large as continents (for the birds that migrate from South America to North America) or entire oceans (for migrating fish, sea turtles and whales) or as small as a moist piece of bread (for fungus) or your intestines (for the bacteria that help you digest your food). Put a whale on land or a bird in the ocean, though, and suddenly they are in big trouble. It is not a suitable habitat for them because they are not built to get air, food and water from this type of environment.

Even in environments that are limited in certain crucial life needs such as air or water, organisms evolve methods to find enough of those things to make that environment their habitat. For example there is much more oxygen in the atmosphere than in the oceans and yet the oceans are teeming with life. This is because the animals that live in the ocean have adaptations that allow them to pull the air they need to survive right out of the water (for example, fish and their gills). Another example is the deserts that have very little water in them but still support life. When camels, inhabitants of Arabian and Asian deserts, find water, they will drink as much as twenty gallons at a time and then will store this water in the fat in their humps. They can then go for weeks without water living off their hump stores. Cacti have shallow roots that extend a great distance from the plant itself and allow the cactus to collect a lot of water during the brief rainy periods. The water can then be stored in the thick stems of the cactus for the dry times.

Because animals are so dependent on having habitat available, we now know that habitat loss is the main reason that animals go extinct. Habitat loss can occur because of natural processes such as climate changes or volcanic eruptions. In recent centuries, though, it is the expansion of human populations that has lead to cataclysmic habitat loss across the planet. Not just urban development, but human activities such as farming, logging and mining take away the space other organisms need to survive. Pollution contaminates water and air which is another loss of habitat for organisms that depend on clean air and water. Organisms with specialized habitat needs, such as wood storks or shortnose sturgeon, begin to die out, while organisms who make urban areas their habitat, such as pigeons, squirrels and cockroaches, thrive. The changes we make in the environment drastically reduce the habitat for other species.

All living things need food, water, air and space, but they all have different methods of getting them. For this reason, every place on Earth is potentially the habitat of some living thing as long as at least small amounts of each of those essential things are available there. By preserving habitats, we help preserve our fellow organisms.