Chapter 48. Biomes and Global Ecology

Terrestrial biomes reflect the distribution of climate.

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In Chapter 29, we saw that plants perform a physiological balancing act, opening their stomata to allow carbon dioxide into leaves, but losing water vapor through transpiration (evaporation from leaves) as a result. Evapotranspiration is the sum of evaporation directly from soils and water bodies plus the amount transpired by plants. In many communities, transpiration returns a large amount of water vapor to the atmosphere. For example, a single acre of corn can transpire 3000–4000 gallons (10,000–15,000 L) of water per day.

As we’ve seen, temperature varies with latitude and altitude, as do humidity and wind. Potential evapotranspiration is the amount of evapotranspiration that temperature, humidity, and wind would cause if water supply weren’t limiting. Potential evapotranspiration, then, reflects the demand on the water resources of an ecosystem, just as precipitation represents the supply. The ratio of water demand to supply is known as the potential evapotranspiration ratio, and it effectively tells us what type of vegetation can be supported in a given region.

Deserts, with low annual precipitation, have a high potential evapotranspiration ratio (up to 20 or more): The limited moisture in soils evaporates rapidly, and plants must keep transpiration (and therefore photosynthetic rate) to a minimum. The plants’ ability to store water in their tissues provides a hedge defense against drought, and so succulents—plants like cactus, which store water in their stems—are common. At the other extreme, rain forests have potential evapotranspiration ratios as low as 0.2: High precipitation permits nearly unlimited transpiration (and therefore high photosynthetic rates), enabling the development of wet forests with high biomass. Intermediate potential evapotranspiration ratios characterize the temperate zones that encompass much of the United States, Europe, and parts of eastern Asia, as well as mid-altitudes, below the highest mountain peaks but well above the coastline. Both latitude and altitude influence the potential evapotranspiration ratio through their effects on temperature and precipitation, and the combined effects of these factors are reflected in the distribution of biomes around the world (Fig. 48.9).

FIG. 48.9 Evapotranspiration and precipitation. The amounts of both potential evapotranspiration and precipitation change with latitude and altitude, and the ratio between the two is a primary determinant of vegetation type, primary production, and species diversity. Data from: International Water Management Institute Global Monthly Dataset.

Data from: International Water Management Institute Global Monthly Dataset.

Quick Check 2 Why can wet soils occur both near the equator, where rainfall is high, and at high latitudes, where rainfall is low?

Quick Check 2 Answer

Soil moisture reflects both rainfall and evaporation. Therefore, wet soils occur both near the equator where rainfall is high, and at high latitudes where rainfall is low but where evaporation is also low owing to cold climate, which limits the rate of evaporation of water from the soil.

In similar biomes on different continents, dominant plants generally look much the same, but commonly they are not closely related. Plants have independently evolved adaptations to particular climatic regimes by convergence. For example, the cacti that grow in deserts in western North America look very similar to plants in arid regions of Africa, but the plants in Africa belong to a different family entirely (Fig. 48.10). Convergent evolution is also seen in trees found in rain forests in different regions of the world; distantly related trees have evolved leaves with long tips that allow excess water to drip away, a reddish color in new leaves that protects them from intense sunlight, and roots with high buttresses that support tall trees atop thin soil.

FIG. 48.10 Convergence of plant form. (a) Cacti in the deserts of North America and (b) euphorb plants in Africa look similar because they are both adapted to conserve and store water, but they are not closely related.

In terrestrial biomes, vascular plants dominate primary production, although mosses, lichens, algae, and cyanobacteria also contribute. Plants also provide a physical structure for the biome, with high canopies in wet temperate forests and tropical forests, but shorter vegetation where water or nutrients are scarce, or where the growing season (the part of the year when temperature and precipitation permit plant growth) is short.

Primary consumers include a variety of animals, but in many environments, mammals and insects are the principal herbivores, grazing on leaves, fruits, seeds, and stems. Insects and vertebrates also play major roles as secondary consumers. Predatory insects in various terrestrial biomes include hornets, the praying mantis, and larval ant lions that trap their prey in small sandy pits. Terrestrial biomes are often characterized by the carnivorous vertebrates that live there, which are among the most charismatic of all animals—for example, lions, tigers, and wolves. In all terrestrial biomes, despite the activities of consumers, detritus from both plants and animals ends up on the ground and may accumulate in soil.

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Decomposers, especially fungi and bacteria, feed on this detritus, returning CO₂ to the atmosphere and nutrients to the soil. Still more protists and animals feed on these decomposers, populating a detritus-based wood web within the soil. Soil animals include minute nematodes (Chapter 44) as well as larger earthworms that eat both detritus and decomposers, ventilating the soil as they tunnel in search of food.

The principal biomes on land are summarized in Fig. 48.11.

FIG. 48.11 Some common land biomes

Tundra

Tundra is the coldest biome, and short days in winter limit the growing season. Tundra occurs close to the North Pole, above 65° N. The South Pole is largely surrounded by the ice and seas of Antarctica, and so there is very little area with plants. Precipitation is low, but because rates of evaporation are also low and because drainage is commonly poor, the ground is usually waterlogged and permanent ice occurs below a few centimeters of soil. Primary producers are mostly mosses, lichens, herbs, and low shrubs. Grasses and sedges occur in drier places, as do other flowering plants. Plant diversity is low, and most plants are small. Caribou are conspicuous grazers, but other primary consumers, including rabbits, birds, and insects, occur as well. Wolves and foxes are key predators. Fungi and bacteria decompose organic detritus, but low temperatures and water-logged soils limit rates of respiration.

Alpine

The Alpine biome is similar to tundra but lacks permanent ice below the soil, and the temperatures vary more widely. Alpine areas occur throughout the world, often at about 10,000 feet (3000 m) at lower latitudes, but always just below the snow line. Because of their altitude, these are windy, cold places. The thin atmosphere provides only limited protection from UV radiation. Many alpine plants are therefore low and slow growing. Grasses are abundant, as are herbs that can provide spectacular wildflower displays during the short growing season. Alpine grazers include mountain goats, llamas, yaks, and marmots, as well as seasonally active insects. Predators include wolves and cats such as the Himalayan snow leopard.

Taiga

These cool, moist forests occur from 50° N to 65° N. The short summer brings rain, and most of the plants are low-growing conifers like spruce, fir, larch, and pine, with an understory dominated by shrubs in the blueberry and rose families. Mosses commonly form an extensive and soggy groundcover. The soils are deep with accumulated organic matter because the low temperatures result in slow decomposition, but they are also acidic and poor in nutrients, helping to explain the limited stature of most plants. Birds, insects, and other invertebrate animals are more abundant and diverse than those of tundra biomes, but mammals are the most conspicuous consumers. Grazers include elk, moose, caribou, porcupines, hares, and a diversity of rodents. Bears, lynx, wolverines, weasels, mink, wolves, and foxes are well-known predators.

Temperate coniferous forest

Two broad areas of temperate coniferous forest occur below 50° N in North America, northern Japan, and parts of Europe and continental Asia. Along the Pacific coast of North America, warm summers and mild winters, along with abundant precipitation, permit growth of enormous conifers such as Douglas Fir, Red Cedar, Sitka Spruce, and redwoods. Much of the undergrowth consists of grasses, ferns, and members of the blueberry family. Understory shrubs are uncommon. In the interior of North America, much less precipitation and colder winter temperatures support drought-resistant conifers such as Ponderosa and Lodgepole Pines and Englemann Spruce. Diverse invertebrate species live in the soils, supported by active microbial decomposers. Insect and vertebrate diversity are all higher than in taiga forests.

Deciduous forest

A moderate climate and dominance of hardwood deciduous trees occur across much of eastern North America, Europe, and Asia. Much of this biome has been subjected to human disturbance for agriculture and urban development. There are usually 15 to 25 tree species, including maples, oaks, poplars, and birches. Springtime sun passes through seasonally leafless trees to reach a diverse understory flora. Soils are rich in nutrients from annual leaf fall, and the moderate temperatures and precipitation promote decomposition, while the cool winters promote accumulation of organic materials. Soils contain a diversity of microbes and fungi, along with the protists and invertebrate animals that feed on them. Insects, birds, and mammals are relatively diverse, joined by snakes, lizards, and amphibians.

Temperate grassland

Before human settlement, this biome occupied most of the midwestern United States and south central Canada, dominated by blue-stem and buffalo grasses. Disturbance by fire helps to maintain grass populations in this biome. Lack of precipitation also prevents many species of tree from growing, and those trees that grow are usually in low areas with more available moisture. Because of active decomposers, soils accumulate nutrients, providing some of the most productive agricultural lands in the world. Before colonization by humans, temperate grasslands of North America supported abundant and diverse grazers, including bison, pronghorns, horses, and mammoths. Horses and mammoths became extinct 11,000 years ago, and herds of bison and pronghorn dwindled in the nineteenth century. Burrowing grazers such as prairie dogs remain relatively common, although population sizes have fallen; these species support predators such as ferrets, badgers, foxes, and birds of prey.

Desert

Desert occurs in continental interiors around the world north and south of the equator from 15° to 35° latitude. Wind patterns prevent this biome from receiving more than a few centimeters of precipitation annually. Deep-rooted plants, like cactus and euphorb plants, are adapted to store water. Primary production is low, and soils are poor in nutrients but may have high surface salt due to evaporation. Given the low rates of primary production, primary consumers tend to be small, and include diverse lizards as well as rodents. Predators include snakes, cat species, coyotes, and birds such as owls, hawks, and eagles.

Chaparral

Like that of deserts, the distribution of chaparral reflects a narrow range of climate conditions and occurs on the western edge of continents from 32° to 40° latitude north and south of the equator. Precipitation ranges from 30 to 75 cm per year, usually falling in a period of 2 to 4 months. Typical plants are herbs that die and reseed themselves every year, evergreen shrubs, and small trees. Olives, eucalyptus, acacia, and oaks are typical woody species, always drought resistant and often adapted to withstand fire. Limited precipitation means soils are not rich in organic materials.

Savanna

Tall, perennial grasses dominate this biome, which occurs in warm, relatively dry regions of eastern Africa, South America, and Australia. Rain is seasonal and ranges from 75 to 150 cm per year. Scattered trees and shrubs usually drop their leaves in the dry season to conserve moisture. As in temperate grasslands, fire plays a key role in maintaining this biome. Large mammalian grazers are abundant and diverse; these include the migrating antelopes, zebras, and giraffes well known from Africa, but also kangaroos and other marsupials in Australia and large rodents in South America. Predator diversity can also be high, as exemplified by lions and other cat species, hyenas, and wild dogs. Dingos are important predators in Australian savannas, but were introduced only a few thousand years ago from southeastern Asia. Before that, the top carnivores included large, now-extinct lizards, as well as snakes and smaller lizards still present in the biome.

Tropical rain forest

The most diverse of all terrestrial biomes, tropical rain forests extend north and south of the equator from 10° N to 10° S. Annual precipitation is commonly more than 250 cm, and tree diversity alone often exceeds 300 species per hectare. Trees grow tall, and many have buttressed roots for support. Lianas and other epiphytic plants are common. Most leaves are evergreen and leathery and many have long pointed tips that facilitate drainage of excess moisture. Because of the high temperatures and heavy rains, decomposition by fungi and bacteria is rapid, preventing the accumulation of organic materials in clay-rich or sandy soils. There are few large grazers, but smaller mammals such as primates, bats, and rodents are highly diverse, as are birds, snakes, and lizards. Insects are especially abundant and diverse. Ants alone make up as much as 30% of animal biomass in rain forests, and they are the principal grazers on rain forest trees.