The word biome is a scientific term used to describe a large community of plants and animals living together under the same environmental conditions. Some of the major biomes are the arctic tundra, desert, grassland, and rainforest. As the names of some of these biomes imply, each is typically defined by one or more of the main plant types that grow there. The plant types are determined by the environmental conditions found in the biome -- primarily climate (average yearly temperature and precipitation). Of course the climate of a region is determined largely by its geographic location.

Creating a model biome is a great way to better understand the importance of an area's environmental conditions, especially water and light availability, in determining the types of plants and animals that can exist there. The model also demonstrates how water cycles through the environment by the processes of evaporation, condensation, and precipitation.

Productivity in real-world biomes mirrors the trends shown in the models. Generally speaking, areas that receive high levels of precipitation are more productive than drier areas. Also, areas that receive more sunlight (which increases levels of photosynthesis), and are consequently warmer, are more productive than darker, cooler areas. (Nutrient levels, too, play a role in productivity, but vary more and are more difficult to predict than the other two factors.)

Around the globe, plants and animals have taken on evolutionary strategies that are particular to their biome's conditions, whether they be intense heat and solar radiation, little direct sunlight, or frequent flooding. Few biomes have an abundance of all three necessary resources -- water, light, and nutrients. At least one is nearly always in short supply. So plants and animals have adapted over generations to survive in spite of the shortages. Nowhere is this clearer than in the desert.

Although there is more than enough sunshine in the desert to sustain high levels of photosynthesis in any plant that can grow there, water is quite limited. For example, the Sonoran Desert in Arizona receives no more than about 15 days of rain per year -- not nearly enough for most plants. Over thousands of generations and millions of years, however, cacti and certain other types of plants have adapted to this harsh environment. They have evolved swollen, fleshy stems for storing water; spines that provide protection from thirsty animals; a waxy coating that makes their surfaces less porous; and stomata, the pores through which they take in carbon dioxide and release oxygen and water vapor, that stay tightly closed during the heat of the day. All of these adaptations enable cacti to conserve water, the desert's most precious commodity.