Unit Eight. The Living Environment
Temperatures are higher in tropical ecosystems for a simple reason: More sunlight per unit area falls on tropical latitudes (see figure 36.11). Solar radiation is most intense when the sun is directly overhead, and this occurs only in the tropics, where sunlight strikes the equator perpendicularly. Temperature also varies with elevation, with higher altitudes becoming progressively colder. At any given latitude, air temperature falls about 6°C for every 1,000-meter increase in elevation. The ecological consequences of temperature varying with elevation are the same as temperature varying with latitude. Figure 36.12 illustrates this principle comparing changes in ecosystems that occur with increasing latitudes in North America with the ecosystem changes that occur with increasing elevation at the tropics. A 1,000-meter increase in elevation on a mountain in southern Mexico (figure 36.12b) results in a temperature drop equal to that of an 880-kilometer increase in latitude on the North American continent (figure 36.12a). This is why the “timberline” (the elevation above which trees do not grow) occurs at progressively lower elevations as one moves farther from the equator.
Figure 36.12. How elevation affects ecosystems.
The same land ecosystems that normally occur as latitude increases north and south of the equator at sea level (a) can occur in the tropics as elevation increases (b).
When a moving body of air encounters a mountain, it is forced upward, and as it is cooled at higher elevations, the air’s moisture-holding capacity decreases, producing the rain you see on the windward side of the mountains shown below—the side from which the wind is blowing. Thus, moisture-laden winds from the Pacific Ocean rise and are cooled when they encounter the Sierra Nevada mountains. As the winds cool, their moisture-holding capacity decreases and precipitation occurs.
The effect on the other side of the mountain—the leeward side—is quite different. As the air passes the peak and descends on the far side of the mountains, it is warmed, so its moisture-holding capacity increases. Sucking up all available moisture, the air dries the surrounding landscape, often producing a desert. This effect, called a rain shadow, is responsible for deserts such as Death Valley, which is in the rain shadow of Mount Whitney, the tallest mountain in the Sierra Nevada.
Similar effects can occur on a larger scale. Regional climates are areas that are located on different parts of the globe but share similar climates because of similar geography. A so-called Mediterranean climate results when winds blow from a cool ocean onto warm land during the summer. As a result, the air’s moisture-holding capacity is increased and precipitation is blocked, similar to what occurs on the leeward side of mountains. This effect accounts for dry, hot summers and cool, moist winters in areas with a Mediterranean climate such as portions of southern California or Oregon, central Chile, southwestern Australia, and the Cape region of South Africa. Such a climate is unusual on a world scale. In the regions where it occurs, many unusual kinds of endemic (local in distribution) plants and animals have evolved.
Key Learning Outcome 36.7. Temperatures fall with increasing latitude and also with increasing elevation. Rainfall is higher on the windward side of mountains, with air losing its moisture as it rises up the mountain; descending on the far side, the dry air warms and sucks up moisture, creating deserts.