Natural history is helping with the difficult task of restoring tropical dry forest in Costa Rica. Natural history also formed the foundation upon which modern ecology developed. Because ecological studies continue to be built upon a solid foundation of natural history, this chapter is devoted to the natural history of terrestrial biomes. Biomes are distinguished primarily by their predominant vegetation and are associated with particular climates. Uneven heating of the earth's spherical surface by the sun and the tilt of the earth on its axis combine to produce predictable latitudinal variation in climate. Because the earth is a sphere, the sun's rays are most concentrated at the latitude where the sun is directly overhead. This latitude changes with the seasons because the earth's axis of rotation is not perpendicular to its plane of orbit about the sun but is tilted approximately 23.5° away from the perpendicular. The sun is directly overhead at the tropic of Cancer, at 23.5° N latitude during the northern summer solstice. During the northern winter solstice the sun is directly overhead at the tropic of Capricorn, at 23.5° S latitude. The sun is directly overhead at the equator during the spring and autumnal equinoxes. During the northern summer the Northern Hemisphere is tilted toward the sun and receives more solar energy than the Southern Hemisphere. During the northern winter, the Northern Hemisphere is tilted away from the sun and the Southern Hemisphere receives more solar energy.
Heating of the earth's surface and atmosphere drives atmospheric circulation and influences global patterns of precipitation. As the sun heats air at the equator, it expands and rises, spreading northward and southward at high altitudes. This high-altitude air cools as it spreads toward the poles, eventually sinking back to the earth's surface. Rotation of the earth on its axis breaks up atmospheric circulation into six major cells, three in the Northern Hemisphere and three in the Southern Hemisphere. These three circulation cells correspond to the trade winds north and south of the equator, the westerlies between 30° and 60° N or S latitude, and the polar easterlies above 60° latitude. These prevailing winds do not blow directly south because of the Coriolis effect.
As air rises at the tropics it cools, and the water vapor it contains condenses and forms clouds. Precipitation from these clouds produces the abundant rains of the tropics. Dry air blowing across the lands at about 30° latitude produces the great deserts that ring the globe. When warm, moist air flowing toward the poles meets cold polar air it rises and cools, forming clouds that produce the precipitation associated with temperate environments. Complicated differences in average climate can be summarized using a climate diagram.Soil structure results from the long-term interaction of climate, organisms, topography, and parent mineral material. Terrestrial biomes are built upon a foundation of soil, a vertically stratified and complex mixture of living and nonliving material. Most terrestrial life depends on soil. Soil structure varies continuously in time and space. Soils are generally divided into O, A, B, and C horizons. The O horizon is made up of freshly fallen organic matter, including leaves, twigs, and other plant parts. The A horizon contains a mixture of mineral materials and organic matter derived from the O horizon. The B horizon contains clays, humus, and other materials that have been transported from the A horizon. The C horizon consists of weathered parent material.The geographic distribution of terrestrial biomes corresponds closely to variation in climate, especially prevailing temperature and precipitation. The major terrestrial biomes and climatic regimes are: Tropical rain forest: Warm; moist; low seasonality; infertile soils; exceptional biological diversity and intricate biological interactions. Tropical dry forest: Warm and cool seasons; seasonally dry; biologically rich; as threatened as tropical rain forest. Tropical savanna: Warm and cool seasons; pronounced dry and wet seasons; impermeable soil layers; fire important to maintaining dominance by grasses; still supports high numbers and diversity of large animals. Desert: Hot or cold; dry; unpredictable precipitation; low productivity but often high diversity; organisms well-adapted to climatic extremes. Mediterranean woodland and shrubland: Cool, moist winters; hot, dry summers; low to moderate soil fertility; organisms adapted to seasonal drought and periodic fires. Temperate grassland: Hot and cold seasons; peak rainfall coincides with growing season; droughts sometimes lasting several years; fertile soils; fire important to maintaining dominance by grasses; historically inhabited by roving bands of herbivores and predators. Temperate forest: Moderate, moist winters; warm, moist growing season; fertile soils; high productivity and biomass; dominated by deciduous trees where growing seasons are moist, winters are mild, and soils fertile; otherwise dominated by conifers. Boreal forest: Long, severe winters; climatic extremes; moderate precipitation; infertile soils; permafrost; occasional fire; extensive forest biome, dominated by conifers. Tundra: Cold; low precipitation; short, soggy summers; poorly developed soils; permafrost; dominated by low vegetation and a variety of animals adapted to long, cold winters; migratory animals, especially birds, make seasonal use. Mountains: Temperature, precipitation, soils, and organisms shift with elevation; mountains are climatic and biological islands.
