The Physical Template A Biomes

Global patterns of temperature and precipitation, reflecting the interaction among latitude, global atmospheric and oceanic circulation patterns, and topography, establish a regional template of physical conditions that support characteristic communities, called "biomes" (Fig. 2.1) (Finch and Trewartha 1949). Latitudinal gradients in temperature from Earth's equator to its poles define the tropical, subtropical, temperate, and arctic zones. Precipitation patterns overlay these temperature gradients. Warm, humid air rises in the tropics, drawing air from higher latitudes into this equatorial convergence zone. The rising air cools and condenses moisture, resulting in a band of high precipitation and tropical rainforests centered on the equator. The cooled, dried air flows away from the equatorial zone and warms as it descends in the "horse latitudes," centered around 30 degrees N and S. These latitudes are dominated by arid grassland and desert ecosystems because of high evaporation rates in warm, dry air. Airflow at these latitudes diverges to the equatorial convergence zone and to similar convergence zones at about 60 degrees N and S latitudes. Rising air at 60 degrees N and S latitudes creates bands of relatively high precipitation and low temperature that support boreal forests. These latitudinal gradients in climate restrict the distribution of organisms on the basis of their tolerance ranges for temperature and moisture. No individual species is capable of tolerating the entire range of tropical to arctic temperatures or desert to mesic moisture conditions.

Mountain ranges interact with oceanic and atmospheric circulation patterns to modify latitudinal patterns of temperature, and precipitation. Mountains force airflow upward, causing cooling, condensation, and precipitation on the windward side (Fig. 2.2). Drier air descends on the leeward side where it gains moisture through evaporation. This orographic effect leads to development of mesic environments on the windward side and arid environments on the leeward side of mountain ranges. Mountains are characterized by elevational gradients of temperature, moisture, and atmospheric conditions (e.g., lower elevations tend to be warmer and drier, whereas higher elevations are cooler and moister). Concentrations of oxygen and other gases decline with elevation so that species occurring at higher elevations must be capable of surviving at low gas concentrations. The montane gradient is much shorter than the corresponding latitudinal gradient, with the same temperature change occurring in a 1000-m difference in elevation or an 880-km difference in latitude. Hence, the range of habitat conditions that occur over a wide latitudinal gradient occurs on a smaller scale in montane areas.

The relatively distinct combinations of temperature and precipitation (MacMahon 1981) determine the assemblage of species capable of surviving and defining the characteristic community type (i.e., tundra, temperate deciduous forest, temperate coniferous forest, tropical rainforest, tropical dry forest, grassland, savanna, chaparral, and desert; Fig. 2.3). Representative terrestrial biomes and their seasonal patterns of temperature and precipitation are shown in Figs. 2.4 and 2.5.

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I I Ice and water I I Tundra I I Boreal forest

□ Temperate forest

□ Temperate grassland

□ Mountain ranges

Global distribution of the major terrestrial biomes. The distribution of biomes is affected by latitude, global atmospheric and oceanic circulation patterns, and major mountain ranges. Modified from Finch and Trewartha (1949) with permission from McGraw-Hill and E. Odum (1971) with permission from Saunders College Publishing.

Chaparral I I Desert

Tropical savanna

□ Tropical dry forest Tropical deciduous forest

□ Tropical rainforest

Habitat conditions in terrestrial biomes are influenced further by topographic relief, substrate structure and chemistry, and exposure to wind. For example, topographic relief creates gradients in solar exposure and soil drainage, as well as in temperature and moisture, providing local habitats for unique communities. Local differences in substrate structure and chemistry may limit the ability of many species of plants and animals, characteristic of the surrounding biome, to survive. Some soils (e.g., sandy loams) are more fertile or more conducive to excavation than others; serpentine soils and basalt flows require special adaptations for survival by plants and animals. Insects that live in windy areas, especially alpine tundra and oceanic islands, often are flightless as a result of selection against individuals blown away in flight. The resulting isolation of populations results in rapid speciation.

Aquatic biomes are formed by topographic depressions and gradients that create zones of standing or flowing water. Aquatic biomes vary in size, depth, flow rate, and marine influence (i.e., lakes, ponds, streams, rivers, estuaries, and tidal marshes; Fig. 2.6). Lotic habitats often show considerable gradation in temperature and solute concentrations with depth. Because water has high specific heat, water changes temperature slowly relative to air temperature. However, because water is most dense at 4°C, changes in density as temperature changes result in seasonal stratification of water temperature. Thermal stratification develops in the summer, as the surface of standing bodies of water warms and traps cooler, denser water below the thermocline (the zone of rapid temperature change), and

Orographic Effect
FIG. 2.2

Orographic effect of mountain ranges. Interruption of airflow and condensation of precipitation on the windward side (right) and clear sky on the leeward side (left) of Mt. Hood, Cascade Mountains, Oregon, United States. Please see extended permission list pg 569.

Tundra Parasitism

Arctic 8 alpine tundra Coniferous forest Deciduous forest Desert Grassland Tropical forest

200 300

Mean annual precipitation (cm)

Discrimination of geographic ranges of major terrestrial biomes on the basis of temperature and precipitation. From MacMahon (1981) with permission from Springer-Verlag. Please see extended permission list pg 569.

Shrubland Biome

| Examples of ecosystem structure in representative terrestrial biomes. A: tundra (alpine) (western United States), B: desert shrubland (southwestern United States), C: grassland (central United States), D: tropical savanna (note termite mounds in foreground; northern Australia), E: boreal forest (northwestern United States), F: temperate deciduous forest (southeastern United States), and G: tropical rainforest (northern Panamá).

| Examples of ecosystem structure in representative terrestrial biomes. A: tundra (alpine) (western United States), B: desert shrubland (southwestern United States), C: grassland (central United States), D: tropical savanna (note termite mounds in foreground; northern Australia), E: boreal forest (northwestern United States), F: temperate deciduous forest (southeastern United States), and G: tropical rainforest (northern Panamá).

again in the winter, as freezing water rises to the surface, trapping warmer and denser water below the ice. During fall and spring, changing surface temperatures result in mixing of water layers and movement of oxygen and nutrients throughout the water column. Hence, deeper zones in aquatic habitats show relatively little variation in temperature, allowing aquatic insects to continue development and activity throughout the year, even in temperate regions.

Habitat conditions in aquatic biomes are influenced further by substrate structure and chemistry; amount and chemistry of regional precipitation; and the characteristics of surrounding terrestrial communities, including conditions upstream. Substrate structure and chemistry determine flow characteristics (including turbulence), pH, and inputs of nutrients from sedimentary sources. Amount and chemistry of regional precipitation determine regularity of water flow and inputs of atmospheric gases and nutrients. Characteristics of surrounding communities determine the degree of exposure to sunlight and the character and condition of allocthonus inputs of organic matter and sediments.

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