15.2: Biomes are large ecosystems that occur around the world, each determined by temperature and rainfall.

Dense vegetation surrounds you. Above you is a canopy of evergreen trees, 30–40 meters (100–130 feet) tall. Climbing vines hang from virtually all the trees. And dozens or even hundreds of species of insects are flourishing around you. Even if you’ve never been there, the description of a tropical rain forest is easy to recognize. But where exactly would you be if you were in this scenario? It could be South America or Africa or Southeast Asia. The species are different, but the general pattern of life forms is the same. The same holds for arctic tundra: whether you were in northern Asia or North America, the view would be similar. These are examples of the largest of the earth’s ecosystems, the biomes.

Biomes cover huge geographic areas of water or land—the deserts that stretch almost all the way across the northern part of Africa, for example. Terrestrial (land) biomes are defined and usually described by the predominant types of plant life in the area. But looking at a map of the world’s terrestrial biomes, it is clear that they are mostly determined by the weather. Specifically, when defining terrestrial biomes, we ask four questions about the weather:

• 1. What is the average temperature?
• 2. What is the average rainfall (or other precipitation)?
• 3. Is the temperature relatively constant or does it vary seasonally?
• 4. Is the rainfall relatively constant or does it vary seasonally?

For example, where it is always moist and the temperature does not vary across the seasons, tropical rain forests develop. And where it is hot but with strong seasonality that brings a “wet” season and a “dry” season, savannas or tropical seasonal forests tend to develop. At the other end of the spectrum, in dry areas with a hot season and a cold season, temperate grasslands or deserts develop. FIGURE 15-3 shows examples of the nine chief terrestrial biomes; all are determined, in large part, by the precipitation and temperature levels.

Aquatic biomes are defined a bit differently, usually based on physical features such as salinity, water movement, and depth. Chief among these environments are (1) lakes and ponds, with non-flowing fresh water; (2) rivers and streams, with flowing fresh water; (3) estuaries and wetlands, where salt water and fresh water mix in a shallow region characterized by exceptionally high productivity; (4) open oceans, with deep salt water; and (5) coral reefs, highly diverse and productive regions in shallow oceans (FIGURE 15-4).

If the terrestrial biomes of the world are determined by the temperature and rainfall amounts and seasonality, what determines those features? In other words, what makes the weather? We investigate next how the geography and landscape of the planet—the shape of the earth and its orientation to the sun, and patterns of ocean circulation—cause the specific patterns of weather that create the different climate zones and the biomes characteristic of each. Then we’ll see how energy and chemicals are made available for life to flourish in these biomes.