One of the factors that determine the distribution of life forms is the availability of the resources necessary for life.

   In the desert, for example, the limiting factor is water: there are areas of bare earth between the plants. Because there are few plants, animals are small and many of them are cold blooded: insects and reptiles survive on very little food, though a few specially adapted mammals, such as the kangaroo rat, wait out the heat in burrows and search for seeds at night.