CONCEPTS IN BIOLOGY

PART IV. EVOLUTION AND ECOLOGY

16. Community Interactions

16.4. Types of Communities

Ecologists recognize that the world can be divided into large, regional, terrestrial communities known as biomes. Biomes are communities of organisms with widespread geographic distribution that are adapted to particular climatic conditions. The primary climatic factors that determine the kinds of organisms that can live in an area are the amount and pattern of precipitation and the temperature ranges typical for the region (figure 16.14). The map in figure 16.15 shows the distribution of the major terrestrial biomes of the world.

FIGURE 16.14. The Influence of Precipitation and Temperature on Vegetation

Temperature and moisture influence the kind of vegetation that can live in an area.

FIGURE 16.15. Biomes of the World

Major climatic differences determine the kinds of plants and animals that can live in a region. These regional communities are called biomes.

Temperate Deciduous Forest

Temperate deciduous forest exists in the parts of the world that have moderate rainfall (75 to 130 centimeters per year) spread over the entire year and a relatively long summer growing season (130 to 260 days without frost). This biome, like other land-based biomes, is named for a major feature of the community, its dominant vegetation. The predominant plants are large trees that lose their leaves during the fall. Thus, they are called deciduous (figure 16.16). The temperate deciduous forest covers a large area from the Mississippi River to the Atlantic Coast and from Florida to southern Canada. This type of biome is also found in much of Europe and parts of eastern Asia.

FIGURE 16.16. Temperate Deciduous Forest

(a) Climagraph for Chicago, Illinois. (b & c) A temperate deciduous forest develops in areas that have significant amounts of moisture throughout the year but where the temperature falls below freezing for parts of the year. During this time, the trees lose their leaves. (d) Raccoons are common animals in the temperate deciduous forest of North America. This kind of forest once dominated the eastern half of the United States and southeastern Canada.

Because the trees are the major producers and new leaves are produced each spring, the important primary consumers in this biome are leaf-eating insects. These insects are food for a variety of birds that raise their young in the forest during the summer and migrate to more moderate climates in the fall. Many other animals, such as squirrels, some birds, and deer, use the trees’ fruits for food. Carnivores, such as foxes, hawks, and owls, eat many of the small mammals and birds typical of the region. Another feature of the temperate deciduous forest is an abundance of spring woodland wildflowers, which emerge early in the spring before the trees have leafed out.

The region where temperate deciduous forests exist encompasses a large geographic area with somewhat different climatic conditions in different locations. Therefore, there are differences in the species of trees (and other organisms) usually found in different areas within this biome. For instance, in Maryland the tulip tree is one of the state’s common large trees, whereas in Michigan it is so unusual that people plant it in lawns and parks as a decorative tree. Aspen, birch, cottonwood, oak, hickory, beech, and maple are typical trees of the temperate decidous forest. Typical animals of this biome are many kinds of leaf-eating insects, wood-boring beetles, migratory birds, skunks, porcupines, deer, frogs, opossums, owls, and mosquitoes.

In much of this region, the natural vegetation has been removed to allow for agriculture, so the original character of the biome is gone, except where farming is not practical or the original forest has been preserved or allowed to regenerate.

Temperate Grassland (Prairie)

Temperate grassland, or prairie, exists where the rainfall (30 to 85 centimeters per year) is not adequate to support the growth of trees; the dominant vegetation consists of various species of grasses (figure 16.17). Typical are long periods during the year when there is no rainfall. Trees are common in this biome only along streams, where they can obtain sufficient water. Interspersed among the grasses are many kinds of prairie wild-flowers. The dominant animals are those that use grasses as food; large, grazing mammals (bison and pronghorn antelope); small insects (e.g., grasshoppers and ants); and rodents (e.g., mice and prairie dogs). A variety of carnivores (e.g., meadowlarks, coyotes, and snakes) feed on the herbivores. Most of the bird species are seasonal visitors to the prairie. At one time, fire was a common feature of the prairie during the dry part of the year.

FIGURE 16.17. Temperate Grassland

(a) Climagraph for Tehran, Iran. (b) Grasses are better able to withstand low rainfall than are trees. Therefore, in areas that have moderate rainfall, grasses are the dominant plants. (c & d) Pronghorns and grasshoppers are common herbivores and (e) meadowlarks are common consumers of insects in North American grasslands.

In North America, temperate grasslands are found throughout much of the region between the Mississippi River and the Rocky Mountains. This biome is also common in parts of Eurasia, Africa, Australia, and South America.

Today, most of the original grasslands, like the temperate deciduous forest, have been converted to agricultural uses. Grasslands with adequate rainfall or where irrigation is possible are used to grow crops like corn and wheat. Grasslands that are too dry to allow for farming typically have been used as grazing land for cattle and sheep. The grazing of these domesticated animals has modified the natural vegetation, as has farming in the moister grassland regions.

Savanna

Savannas are tropical biomes of central Africa, northern Australia, and parts of South America; they have distinct wet and dry seasons (figure 16.18). Although these regions may receive 100 centimeters of rainfall per year, there is an extended dry season of 3 months or more. Because of the extended period of dryness, the dominant vegetation consists of grasses. In addition, a few thorny, widely spaced, drought- resistant trees dot the landscape. Many kinds of grazing mammals are found in this biome—various species of antelope, wildebeest, and zebras in Africa; various kinds of kangaroos in Australia; and a large rodent, the capybara, in South America. Another animal common to the savanna is the termite, a colonial insect that typically builds mounds above the ground.

FIGURE 16.18. Savanna

(a) Climagraph for Nairobi, Kenya. (b) Savannas develop in tropical areas that have seasonal rainfall. (d) Thomson's gazelles They typically have grasses as the dominant vegetation with drought and fire-resistant trees scattered through the area. Grazing animals such as elephants and gazelles (b & d) are common herbivores and lions and secretary birds (c, e) are common carnivores in African savannas.

During the wet part of the season, the trees produce leaves, the grass grows rapidly, and most of the animals raise their young. In the African savanna, seasonal migration of the grazing animals is typical. Many of these tropical grasslands have been converted to grazing for cattle and other domesticated animals.

Mediterranean Shrublands (Chaparral)

The Mediterranean shrublands are located near an ocean and have a climate with wet, cool winters and dry, hot summers. Rainfall is 40 to 100 centimeters per year. As the name implies, this biome is typical of the Mediterranean coast and is also found in coastal southern California, the southern tip of Africa, a portion of the west coast of Chile, and southern Australia.

The vegetation is dominated by woody shrubs that are adapted to withstand the hot, dry summer. Often the plants are dormant during the summer. Fire is a common feature of this biome, and the shrubs are adapted to withstand occasional fires. The kinds of animals vary widely in the different regions of the world with this biome. Many kinds of insects, reptiles, birds, and mammals are found in these areas. In the chaparral of California, rattlesnakes, spiders, coyotes, lizards, and rodents are typical inhabitants (figure 16.19).

FIGURE 16.19. Mediterranean Shrubland (Chaparral)

(a) Climagraph for Rome, Italy. (b) Mediterranean shrublands are characterized by a period of winter rains and a dry, hot summer. The dominant plants are drought-resistant, woody shrubs. (c & d) Common animals in the Mediterranean shrubland (chaparral) of California are the California quail and black-tailed jack rabbit.

Very little undisturbed Mediterranean shrubland still exists. The combination of moderate climate and closeness to the ocean has resulted in all Mediterranean shrublands being heavily altered by human activity. Agriculture is common, often with the aid of irrigation, and many major cities such as Los Angeles and San Diego are located in this biome.

Tropical Dry Forest

The tropical dry forest is another biome that is heavily influenced by seasonal rainfall. Tropical dry forests are found in parts of Central and South America, Australia, Africa, and Asia (particularly India and Myanmar). Many of the tropical dry forests have a monsoon climate in which several months of heavy rainfall are followed by extensive dry periods ranging from a few to as many as eight months. (See figure 16.20.) The rainfall may be as low as 50 centimeters or as high as 200 centimeters.

FIGURE 16.20. Tropical Dry Forest

(a) Climagraph for Acapulco, Mexico. (b) Tropical dry forests typically have a period of several months with no rain. In places where the drought is long, many of the larger trees lose their leaves. The coati (c) is a common animal in the tropical dry forests of the Americas. The endangered one-horned rhinoceros (d) is an inhabitant of the tropical dry forest of Asia.

Since the rainfall is highly seasonal, many of the plants have special adaptations for enduring drought. In regions that have long dry periods, many of the trees drop their leaves during the dry part of the year. Many of the species of animals found here are also found in more moist tropical forests of the region. However, there are fewer kinds in dry forests than in rainforests.

Many of these forests occur in areas of very high human population. Therefore, harvesting wood for fuel and building materials have heavily impacted these forests. In addition, many of these forests have been converted to farming or the grazing of animals.

Desert

Deserts are very dry areas found throughout the world wherever rainfall is low and irregular. Typically, the rainfall is less than 25 centimeters per year. Some deserts are extremely hot; others can be quite cold during much of the year. The distinguishing characteristic of desert biomes is low rainfall, not high temperature. Furthermore, deserts show large daily fluctuations in air temperature. When the Sun goes down at night, the land cools off very rapidly, because there is no insulating blanket of clouds to keep the heat from radiating into space.

A desert biome is characterized by scattered, thorny plants that have few or no leaves. However, often the stems are green and carry on photosynthesis (figure 16.21). Because leaves tend to lose water rapidly, the lack of leaves is an adaptation to dry conditions. Many of the plants, such as cacti, can store water in their fleshy stems. Others store water in their roots. Although this is a very harsh environment, many kinds of flowering plants, insects, reptiles, and mammals live there. The animals usually avoid the hottest part of the day by staying in burrows or other shaded, cool areas. Staying underground or in the shade also allows animals to conserve water. Many annual plants also live in this biome, but the seeds germinate and grow only after the infrequent rainstorms. When it does rain, the desert blooms.

FIGURE 16.21. Desert

(a) Climagraph for Cairo, Egypt. (b) The desert receives less than 25 centimeters of precipitation per year, yet it teems with life. Cacti, sagebrush, lichens, snakes, small mammals, birds, and insects inhabit the desert. (c) Coyotes are common in North American deserts. Because daytime temperatures are often high, most animals are active only at night, when the air temperature drops significantly. Cool deserts also exist in many parts of the world, where rainfall is low but temperatures are not high. (d) Collared lizards are common reptiles in many deserts of the United States.

Boreal Coniferous Forest

Boreal coniferous forest—also known as the northern coniferous forest or the taiga—is dominated by evergreen trees (conifers) that are especially adapted to withstand long, cold winters with abundant snowfall. This biome is found throughout the world in northern regions, including parts of southern Canada, extending southward along the Appalachian and Rocky Mountains of the United States, much of northern Europe, and Asia.

Typically, the growing season is less than 120 days, and rainfall ranges between 40 and 100 centimeters per year. However, because of the low average temperature, evaporation is low and the climate is humid. Most of the trees in the wetter, colder areas are spruces and firs, but some drier, warmer areas have pines. The wetter areas generally have dense stands of small trees intermingled with many other kinds of vegetation and many small lakes and bogs (figure 16.22). The animals characteristic of this biome include mice, snowshoe hare, lynx, bears, wolves, squirrels, moose, midges, and flies. These animals can be divided into four general categories: those that become dormant in winter (e.g., insects and bears); those that are adapted to withstand the severe winters (e.g., snowshoe hare, lynx); those that live in protected areas (e.g., mice under the snow); and those that migrate south in the fall (most birds).

FIGURE 16.22. Boreal Coniferous Forest, or Taiga

(a) Climagraph for Moscow, Russia. (b & d) The boreal coniferous forest, or taiga occurs in areas with long winters and heavy snowfall. The trees have adapted to these conditions and provide food and shelter for the animals that live there. (c) In North America the lynx and snowshoe hare are common animals.

Temperate Rainforest

Temperate rainforest exists in the coastal areas of northern California, Oregon, Washington, British Columbia, and southern Alaska. Temperate rainforest is also found in New Zealand and the southwest coast and tip of South America. In these coastal areas, the prevailing winds from the west blow over the ocean and bring moisture-laden air to the coast. As the air meets the coastal mountains and is forced to rise, it cools and the moisture falls as rain or snow. Most of these areas receive 200 centimeters or more precipitation per year. This abundance of water, along with fertile soil and mild temperatures, results in a lush growth of plants.

Sitka spruce, Douglas fir, and western hemlock are typical evergreen coniferous trees in the temperate rainforest of North America. Undisturbed (old growth) forests of this region have trees as old as 800 years that are nearly 100 meters tall. Deciduous trees of various kinds (e.g., red alder, big leaf maple, black cottonwood) also exist in open areas where they can get enough light. All the trees are covered with mosses, ferns, and other plants that grow on the surface of the trees. The dominant color is green, because most of the surfaces have a photosynthetic organism growing on them (figure 16.23).

FIGURE 16.23. Temperate Rainforest Biome

(a) Climograph for Juneau, Alaska. (b) The temperate rainforest is characterized by high levels of rainfall that support large evergreen trees and the many mosses and ferns that grow on the surface of the trees. (c & d) The blacktail deer is common in this biome, which is also the home of the endangered northern spotted owl.

When a tree dies and falls to the ground, it rots in place and serves as a site for the establishment of new trees. This is such a common feature of the forest that the fallen, rotting trees are called nurse trees. Fallen trees also serve as food for a variety of insects, which are food for a variety of larger animals.

Because of the rich resource of trees, 90% of the original temperate rainforest has already been logged. Many of the remaining areas have been protected, because they are home to the endangered northern spotted owl and marbled murrelet (a seabird).

Tundra

Tundra is the most northerly biome (figure 16.24). It is characterized by extremely long, severe winters and short, cool summers. The growing season is less than 100 days and, even during the short summer, the nighttime temperatures approach 0°C. Rainfall is low (10 to 25 centimeters per year). The deeper layers of the soil remain permanently frozen, forming a layer called the permafrost. Because the deeper layers of the soil are frozen, when the surface thaws the water forms puddles on the surface.

FIGURE 16.24. Tundra

(a) Climagraph for Fairbanks, Alaska. (b & c) In the northern latitudes and on the tops of some mountains, the growing season is short and plants grow very slowly. Trees are unable to live in these extremely cold areas, in part because there is a permanently frozen layer of soil beneath the surface, known as the permafrost. Because growth is so slow, damage to the tundra can still be seen generations later. (d & e) Caribou and snowy owls are common in the tundra.

Under these conditions of low temperature and short growing season, very few kinds of animals and plants can survive. No trees can live in this region. The typical plants and animals of the area are grasses, sedges, dwarf willow, some other shrubs, reindeer moss (actually a lichen), caribou, wolves, musk oxen, fox, snowy owls, mice, and many kinds of insects. Many kinds of birds are summer residents only. The tundra community is relatively simple, so any changes have drastic and long-lasting effects. The tundra is easy to injure and slow to heal; therefore, it must be treated gently—the construction of the Alaskan pipeline has left scars that might still be there 100 years from now.

Tropical Rainforest

Tropical rainforest is at the other end of the climate spectrum from the tundra. Tropical rainforests are found primarily near the equator in Central and South America, Africa, parts of southern Asia, and some Pacific islands (figure 16.25). The temperature is high (averaging about 27°C), rain falls nearly every day (typically, 200 to 1,000 centimeters per year), and there are thousands of species of plants in a small area. Balsa (a very light wood), teak (used in furniture), and ferns the size of trees are plants from the tropical rainforest. Every plant has other plants growing on it. Tree trunks are likely to be covered with orchids, many kinds of vines, and mosses. Tree frogs, bats, lizards, birds, monkeys, and an almost infinite variety of insects inhabit the rainforest. These forests are very dense, and little sunlight reaches the forest floor. When the forest is opened up (by a hurricane or the death of a large tree) and sunlight reaches the forest floor, the opened area is rapidly overgrown with vegetation.

FIGURE 16.25. Tropical Rainforest

(a) Climagraph for Singapore. (b, c, & d) Tropical rainforests develop in areas with high rainfall and warm temperatures. They have an extremely diverse mixture of plants and animals such as birds, butterflies, and monkeys.

Because plants grow so quickly in these forests, people assume the soils are fertile, and many attempts have been made to bring this land under cultivation. In reality, the soils are poor in nutrients. The nutrients are in the organisms; as soon as an organism dies and decomposes, its nutrients are reabsorbed by other organisms. Typical North American agricultural methods, which require the clearing of large areas, cannot be used with the soil and rainfall conditions of the tropical rainforest. The constant rain falling on these fields quickly removes the soil’s nutrients, so that heavy applications of fertilizer are required. Often, these soils become hardened when exposed in this way. Although most of these forests are not suitable for agriculture, large expanses of tropical rainforest are being cleared yearly because of the pressure for more farmland in the highly populated tropical countries and the desire for high-quality lumber from many of the forest trees.

Table 16.2 summarizes the distinguishing features of each of the biomes. Outlooks 16.1 describes a special kind of tropical forest that involves periodic flooding.

TABLE 16.2. Summary of Kinds of Terrestrial Biomes

The Relationship Between Elevation and Climate

The distribution of terrestrial communities is primarily related to temperature and precipitation. Air temperatures are warmest near the equator and become cooler toward the poles. Similarly, air temperature decreases as elevation increases. This means that, even at the equator, cold temperatures are possible on the peaks of tall mountains. Therefore, as one proceeds from sea level to the tops of mountains, it is possible to pass through a series of biomes that are similar to what one would encounter traveling from the equator to the North Pole (figure 16.26).

FIGURE 16.26. The Relationship Among Elevation, Latitude, and Vegetation

As one travels up a mountain, one experiences changes in climate. The higher the elevation, the cooler the climate. Even in the tropics, tall mountains can have snow on the top. Thus, it is possible to experience the same change in vegetation by traveling up a mountain as one would experience traveling from the equator to the North Pole.

OUTLOOKS 16.1

Varzea Forests—Seasonally Flooded Amazon Tropical Forests

The Amazon River and it many tributaries constitute the largest drainage basin (about 40% of South America) and the highest volume of flow of any river system in the world—about 20 percent of worldwide river flow. The water is supplied by abundant rainfall. Many areas receive over 300 cm of rain per year—in the basin and snowmelt from the Andes. Since the snowmelt and, to a certain extent the rainfall, is seasonal, the Amazon and its tributaries are characterized by seasonal flooding. Much of the river basin is flat. The city of Iquitos is about 3,600 kilometers from the ocean but the river at that point is only 100 meters above sea level. Because of the flat terrain, extensive areas along the river are often flooded under several meters of water. The area flooded extends several kilometers from the river. This creates a seasonal wetland forest known as the varzea. The land farther from the river that does not flood is known as the terra firme.

This seasonally flooded area accounts for about 4% of the total area of the Amazon rainforest. The vegetation of the varzea is different from that of the terra firme because the trees and other vegetation must be able to withstand extensive periods of flooding.

The animals of the river and the varzea are greatly affected by the flooding. Animals of the river move into the forest with the flood and use forest resources as food. Varzea forest areas are critical to the freshwater fisheries of the Amazon Basin. Many fish actually change their diet and become fruit eaters when they are able to enter the flooded forest. In the drier portions of the year when the river recedes, they return to the main river channel and are carnivores. In addition to using the forest for food, the fish also distribute the seeds of fruits in their feces. Other river animals such as the caimans and the giant river otter also move into the forest with the flood.

The river nearly reaches to the top of this bank during floods that occur every year

The terrestrial animals of the forest face a different problem. As the river rises they are forced to retreat to higher ground and often become trapped on islands. This results in intense competition for food. Monkeys and birds are less troubled by the flooding. Many of them rely on fruits of trees, which is available even during the flood, as their primary food source. The monkeys can simply travel from tree to tree and the birds can fly over the water.

The periodic flooding of the area deposits silt, which provides a fertile soil. Therefore, the varzea is affected by human activity as people use the land along the river during the dry season to raise crops. Often the crops are a mixture of normal forest plants along with crops like bananas, rice, and root crops. Because of the flooding, people who live along the river build their houses on high ground and often on stilts. The rivers are also the primary highways of the region and small boats are the most common form of transportation.

Varzea forest

16.4. CONCEPT REVIEW

9. List a predominant abiotic factor in each of the following biomes: temperate deciduous forest, boreal coniferous forest, temperate grassland, Mediterranean shrubland, tropical dry forest, desert, tundra, temperate rainforest, tropical rainforest, and savanna.

10. List a dominant producer organism typical of each of the following biomes: temperate deciduous forest, boreal coniferous forest, temperate grassland, Mediterranean shrubland, tropical dry forest, desert, tundra, temperate rainforest, and savanna.