The organisms that live on the earth are not scattered about the surface of the planet in a random pattern. Each type of organism is found in some places and not in others. The distribution patterns are determined by the environmental conditions existing in different parts of the world and by the ability of organisms to adapt to and survive in those conditions. The pattern of environmental or climatic conditions is not random, either. The location of major climatic types can be traced to the location of the continents and the major circulation patterns that occur in the oceans and the atmosphere. The circulation patterns themselves result from the shape and tilt of the earth, and the unequal distribution of solar energy during the earth's rotations and revolutions. Extremely diverse habitats exist on the earth – for example, oceans, fresh water, and terrestrial habitats. Terrestrial habitats can differ greatly in their climates, having vastly different temperatures, amounts of precipitation, and amounts of sunlight. Each different habitat has its own assemblage of organisms that lives in it; the terrestrial assemblages are known as biomes. As we examine the different biomes in this chapter, we are dealing with ecology at a global level, on a much larger scale than individual populations, or communities, or even ecosystems.