Distribution of Life

Terrestrial Biomes

Biomes are the major, easily recognizable associations between plants and animals.

Each biome has a unique climatic condition heavily influenced by rainfall (or availability of water) and to a lesser extent light, temperature and other abiotic factors. Each biome can be characterized by a particular plant species which predominates.

Biomes are subdivided into ecological communities, each with a unique association of plant and animal populations.

Biomes follow a characteristic distribution pattern north or south of the equator, or by elevation based on climatic factors. Generally traveling north or south of the equator one might encounter the following biomes:

Tropical Rain Forests

Most Tropical rain forests are typified by the following:
  • 250-450 cm of rainfall per year (100-175 inches)
  • Most occur on or near the equator
  • Rain fall is generally evenly distributed throughout the year
  • Trees which form strata or layers, the tallest being 30 to 45 meters high (100-170 feet tall)
  • Smaller trees, shrubs, and vines form a subcanopy and epiphytes (bromeliads and orchids) find a foothold on other plants.
    • Incredible array of plant and animal species utilizing every available niche -- biodiversity found in rain forests has provided mankind with numerous medicines and food sources.
    • Poor soil containing little organic matter because of many fungal and bacterial decomposers.

    Visit the National Geographic's page describing the tropical rain forest.


Deserts exhibit the following conditions:

Receive less than 25 cm (10 inches) of highly seasonal rainfall per year.

  • Rainfall is low where climatic or physical conditions cause cool dry air to descend over warmer regions. Moisture is held by the expanding air like a sponge and can no longer escape as rain.
    • Most deserts occur 30 degrees north or south of the equator.
    • Deserts have a large temperature shift between day and night.
    • Desert plants adapt to hot dry conditions in various ways. Called xerophytes desert plants store water in stems and roots and are armed with spines or thorns.

    Typical North American xerophytes are cactus while in Africa the euphorbs are common.

    Other adaptations include recessed stomata guarded by fine hairs, small leathery leaves, long dormant periods, and specialized photosynthetic cycles (CAM plants).

    Annual plants can flower rapidly and produce seeds which last many years waiting for adequate rainfall.

    Additional Information about deserts includes the following sites:

    Animals are adapted to desert conditions both physically and behaviorally. Many are nocturnal and live underground in humid burrows. Other animals hibernate in their burrows during the day while temperatures are high. This behavior is called estivation.

    Some animals such as the kangaroo rat have highly efficient kidneys and other behavioral and physiological mechanisms to retain water. (More on the kangaroo rat)


Modified Nov. 6, 2002