… there's no place like home (in the words of the poem by John Howard Payne). Although the environments inhabited by some organisms seem humble to us, to these organisms they are ‘home’. Extreme habitats abound on Earth. They seem extreme to us because of high temperatures (hot springs, hydrothermal vents, hot deserts), low temperatures (polar regions, alpine environments, winter temperate environments, cold deserts), lack of water (deserts), high pressures (ocean depths), acidic or alkaline conditions (acid mine waste, the stomach, soda lakes), high salt concentrations (salt lakes) and lack of oxygen (decomposing organic material, estuarine muds, vertebrate intestine). Other extreme situations include toxic chemicals and exposure to ultraviolet radiation. Many extreme habitats challenge the organisms that live there with some combination of these stresses. Let us look at the characteristics of some of these extreme habitats and the adaptations which enable organisms to call them home.

DESERTS

Not all deserts are hot and sandy. A desert is defined by its lack of water rather than its temperature. This does not mean that there is no water but that rainfalls, and other inputs of water, are irregular and infrequent. Deserts cover one-third of the Earth's land surface (Figure 2.1). This includes semi-arid areas (annual precipitation of less than 600 millimetres), arid areas (less than 200 millimetres) and hyper-arid areas (less than 25 millimetres). Deserts form in the leeward side of mountain ranges, in inland areas which are remote from oceans and where dry stable air masses form, resisting convective currents which would bring rain.