THE LIVING WORLD
Unit Eight. The Living Environment
Living on land ourselves, we humans tend to focus much of our attention on terrestrial ecosystems. A biome is a terrestrial ecosystem that occurs over a broad area. Each biome is characterized by a particular climate and a defined group of organisms.
While biomes can be classified in a number of ways, the seven most widely occurring biomes (color-coded in figure 36.21) are (1) tropical rain forest (dark green), (2) savanna (pink), (3) desert (pale yellow), (4) temperate grassland (tan), (5) temperate deciduous forest (brown), (6) taiga (purple), and (7) tundra (light blue). The reason that there are seven primary biomes, and not one or 80, is that they have evolved to suit the climate of the region, and the earth has seven principal climates. The seven biomes differ remarkably from one another but show many consistencies within; a particular biome often looks similar, with many of the same types of creatures living there, wherever it occurs on earth.
There are seven other less widespread biomes also shown in figure 36.21: chaparral; polar ice; mountain zone; temperate evergreen forest; warm, moist evergreen forest; tropical monsoon forest; and semidesert.
Figure 36.21. Distribution of the earth's biomes.
The seven primary types of biomes are tropical rain forest, savanna, desert, temperate grassland, temperate deciduous forest, taiga, and tundra. In addition, seven less widespread biomes are shown.
If there were no mountains and no climatic effects caused by the irregular outlines of the continents and by different sea temperatures, each biome would form an even belt around the globe. In fact, their distribution is greatly affected by these factors, especially by elevation. Thus, the summits of the Rocky Mountains are covered with a vegetation type that resembles tundra, whereas other forest types that resemble taiga occur farther down. It is for reasons such as these that the distributions of the biomes are so irregular. One trend that is apparent is that those biomes that normally occur at high latitudes also follow an altitudinal gradient along mountains. That is, biomes found far north and far south of the equator at sea level also occur in the tropics but at high mountain elevations (see figure 36.12).
Distinctive features of the seven major biomes—tropical rain forest, savanna, desert, temperate grassland, temperate deciduous forest, taiga, and tundra—along with several of the less widespread biomes are now discussed in more detail.
Rain forests, which experience over 250 centimeters of rain a year, are the richest ecosystems on earth. They contain at least half of the earth’s species of terrestrial plants and animals— more than 2 million species! In a single square mile of tropical forest in Rondonia, Brazil, there are 1,200 species of butterflies—twice the total number found in the United States and Canada combined. The communities that make up tropical rain forests are diverse in that each kind of animal, plant, or microorganism is often represented in a given area by very few individuals. There are extensive tropical rain forests in South America, Africa, and Southeast Asia. But the world’s tropical rain forests are being destroyed, and with them, countless species, many of them never seen by humans. Perhaps a quarter of the world’s species will disappear with the rain forests during the lifetime of many of us.
Tropical rain forest
In the dry climates that border the tropics are found the world’s great grasslands, called savannas. Landscapes are open, often with widely spaced trees, and rainfall (75 to 125 cm annually) is seasonal. Many of the animals and plants are active only during the rainy season. The huge herds of grazing animals that inhabit the African savanna are familiar to all of us. Such animal communities occurred in the temperate grasslands of North America during the Pleistocene epoch but have persisted mainly in Africa. On a global scale, the savanna biome is transitional between tropical rain forest and desert. As these savannas are increasingly converted to agricultural use to feed rapidly expanding human populations in subtropical areas, their inhabitants are finding it difficult to survive. The elephant, rhino, and cheetah are now endangered species; the lion and giraffe will soon follow them.
In the interior of continents are found the world’s great deserts, especially in Africa (the Sahara), Asia (the Gobi), and Australia (the Great Sandy Desert). Deserts are dry places where less than 25 centimeters of rain falls in a year—an amount so low that vegetation is sparse and survival depends on water conservation. One quarter of the world’s land surface is desert. The plants and animals that live in deserts may restrict their activity to favorable times of the year, when water is present. To avoid high temperatures, most desert vertebrates live in deep, cool, and sometimes even somewhat moist burrows. Those that are active over a greater portion of the year emerge only at night, when temperatures are relatively cool. Some, such as camels, can drink large quantities of water when it is available and then survive long, dry periods. Many animals simply migrate to or through the desert, where they exploit food that may be abundant seasonally.
Halfway between the equator and the poles are temperate regions where rich grasslands grow. These grasslands once covered much of the interior of North America, and they were widespread in Eurasia and South America as well. Such grasslands are often highly productive when converted to agriculture. Many of the rich agricultural lands in the United States and southern Canada were originally occupied by prairies, another name for temperate grasslands. The roots of perennial grasses characteristically penetrate far into the soil, and grassland soils tend to be deep and fertile. Temperate grasslands are often populated by herds of grazing mammals. In North America, the prairies were once inhabited by huge herds of bison and pronghorns. The herds are almost all gone now, with most of the prairies having been converted to the richest agricultural region on earth.
Mild climates (warm summers and cool winters) and plentiful rains promote the growth of deciduous (“hardwood”) forests in Eurasia, the northeastern United States, and eastern Canada. A deciduous tree is one that drops its leaves in the winter. Deer, bears, beavers, and raccoons are the familiar animals of the temperate regions. Because the temperate deciduous forests represent the remnants of more extensive forests that stretched across North America and Eurasia several million years ago, these remaining areas—especially those in eastern Asia and eastern North America—share animals and plants that were once more widespread. Alligators, for example, are found only in China and in the southeastern United States. The deciduous forest in eastern Asia is rich in species because climatic con ditions have remained constant.
Temperate deciduous forest
A great ring of northern forests of coniferous trees (spruce, hemlock, larch, and fir) extends across vast areas of Asia and North America. Coniferous trees are ones with leaves like needles that are kept all year long. This ecosystem, called taiga, is one of the largest on earth. Here, the winters are long and cold. Rain, often as little as in hot deserts, falls in the summer. Because it has too short a growing season for farming, few people live there. Many large mammals, including elk, moose, deer, and such carnivores as wolves, bears, lynx, and wolverines, live in the taiga. Traditionally, fur trapping has been extensive in this region. Lumber production is also important. Marshes, lakes, and ponds are common and are often fringed by willows or birches. Most of the trees occur in dense stands of one or a few species.
Tundra: Cold Boggy Plains
In the far north, above the great coniferous forests and below the polar ice, there are few trees. There the grassland, called tundra, is open, windswept, and often boggy. Enormous in extent, this ecosystem covers one-fifth of the earth’s land surface. Very little rain or snow falls. When rain does fall during the brief arctic summer, it sits on frozen ground, creating a sea of boggy ground. Permafrost, or permanent ice, usually exists within a meter of the surface. Trees are small and are mostly confined to the margins of streams and lakes. Large grazing mammals, including musk-oxen, caribou, reindeer, and carnivores such as wolves, foxes, and lynx, live in the tundra. Lemming populations rise and fall on a long-term cycle, with important effects on the animals that prey on them.
Chaparral consists of evergreen, often spiny shrubs trees that form communities in regions with what is “Mediterranean,” dry summer climate. These regions clude California, central Chile, the Cape region of South Africa, southwestern Australia, and the Mediterranean area itself. Many plant species found in chaparral can germinate only when they have been exposed to the hot temperatures generated during a fire. The chaparral of California and adjacent regions is historically derived from deciduous forests.
Polar ice caps lie over the Arctic Ocean in the north and Antarctica in the south. The poles receive almost no precipitation, so although ice is abundant, freshwater is scarce. The sun barely rises in the winter months. Life in Antarctica is largely limited to the coasts. Because the Antarctic ice cap lies over a landmass, it is not warmed by the latent heat of circulating ocean water and becomes very cold. As a result, only prokaryotes, algae, and some small insects inhabit the vast Antarctic interior.
Tropical upland forests occur in the tropics and semitropics at slightly higher latitudes than rain forests or where local climates are drier. Most trees in these forests are deciduous, losing many of their leaves during the dry season. This loss of leaves allows sunlight to penetrate to the understory and ground levels of the forest, where a dense layer of shrubs and small trees grow rapidly. Rainfall is typically very seasonal, measuring several inches daily in the monsoon season and approaching drought conditions in the dry season, particularly in locations far from oceans, such as in central India.
Tropical monsoon forest
Key Learning Outcome 36.11. Biomes are major terrestrial communities defined largely by temperature and rainfall patterns.
Inquiry & Analysis
The lumber industry practice called "clear-cutting” has been common in many states. Loggers find it more efficient to simply remove all trees from a watershed, and sort the logs out later, than to selectively cut only the most desirable mature trees. While the open cuts seem a desolation to the casual observer, the loggers claim that new forests can become established more readily in the open cut as sunlight now more easily reaches seedlings at ground level. Ecologists counter that clear-cutting permanently changes the forest in ways that cannot be reversed.
Who is right? The most direct way to find out is to clear-cut an area and watch it very carefully. Just this sort of massive field test was carried out in a now-classic experiment at the Hubbard Brook Experimental Forest in New Hampshire. Hubbard Brook is the central stream of a large watershed that drains a region of temperate deciduous forest in northern New Hampshire. The research team, led by then-Dartmouth College professors Herbert Bormann and Gene Likens, first gathered a great deal of information about the forest watershed. Starting in 1963, they censused the trees, measured the flow of water through the watershed, and carefully documented the levels of minerals and other nutrients in the water leaving the ecosystem via Hubbard Brook. To keep track, they constructed concrete dams across each of the six streams that drain the forest and monitored the runoff, chemically analyzing samples. The undisturbed forest proved very efficient at retaining nitrogen and other nutrients. The small amounts of nutrients that entered the ecosystem in rain and snow were approximately equal to the amounts of nutrients that ran out of the valleys into Hubbard Brook.
Now came the test. In the winter of 1965 the investigators felled all the trees and shrubs in 48 acres drained by one stream (as shown in the photo), and examined the water running off. The immediate effect was dramatic: The amount of water running out of the valley increased by 40%. Water that otherwise would have been taken up by vegetation and released into the atmosphere through evaporation was now simply running off.
It was clear that the forest was not retaining water as well, but what about the soil nutrients, the key to future forest fertility?
The red line in the graph above shows nitrogen minerals leaving the ecosystem in the runoff water of the stream draining the clear-cut area; the blue line shows the nitrogen runoff in a neighboring stream draining an adjacent uncut portion of the forest.
1. Applying Concepts
a. Variable. In the graph, what is the dependent variable?
b. Scale. What is the significance of the break in the vertical axis between 4 and 40?
2. Interpreting Data
a. What is the approximate concentration of nitrogen in the runoff of the uncut valley before cutting? of the cut valley before cutting?
b. What is the approximate concentration of nitrogen in the runoff of the uncut valley one year after cutting? of the clear-cut valley one year after cutting?
3. Making Inferences
a. Is there any yearly pattern to the nitrogen runoff in the uncut forest? Can you explain it?
b. How does the loss of nitrogen from the ecosystem in the clear-cut forest compare with nitrogen loss from the uncut forest?
4. Drawing Conclusions
a. What is the impact of this forest's trees upon its ability to retain nitrogen?
b. Has clear-cutting harmed this ecosystem? Explain.
1. Energy from the sun is captured and converted into chemical energy by
2. As energy is transferred from one trophic level to the next, substantial amounts of energy are lost to/as
a. undigestible biomass.
d. All answers are correct.
3. The number of carnivores found at the top of an ecological pyramid is limited by the
a. number of organisms below the top carnivores.
b. number of trophic levels below the top carnivores.
c. amount of biomass below the top carnivores.
d. amount of energy transferred to the top carnivores.
4. Hydrologists, scientists who study the movements and cycles of water, refer to the return of water from the ground to the air as evapotranspiration. The first part of the word refers to evaporation. The second part of the word refers to transpiration, which is evaporation of water
a. from plants.
b. through animal perspiration.
c. off the ground shaded by plants.
d. from the surface of rivers.
5. The carbon cycle includes a store of carbon as fossil fuels that is released through
d. All answers are correct.
6. The element phosphorus is needed in organisms to build
7. A rain shadow results in
a. extremely wet conditions due to the lack of wind over a mountain range.
b. dry air moving toward the poles that cools and sinks in regions 15 to 30 degrees north/south latitude.
c. global polar regions that rarely receive moisture from the warmer, tropical regions, and are therefore dryer.
d. desert conditions on the downwind side of a mountain due to increased moisture-holding capacity of the winds as the air heats up.
8. As one travels from northern Canada south to the United States, the timberline increases in elevation. This is because as latitude
a. increases, temperature increases.
b. decreases, temperature increases.
c. increases, humidity decreases.
d. decreases, humidity increases.
9. In freshwater lakes during the summer, layers of sudden temperature change called the _____ form.
b. profundal zone
10. Which of the following biomes is not found south of the equator?
a. polar ice cap
d. tropical monsoon forest