Biomes
biome is a large, distinctive complex of plant communities created and maintained by climate.
How many biomes are there?
A study published in 1999 concluded that there are 150 different "ecoregions" in North America
alone. But I shall cast my lot with the "lumpers" rather than the "splitters" and lump these into 8
biomes:
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tundra
taiga
temperate deciduous forest
scrub forest (called chaparral in California)
grassland
desert
tropical rain forest
temperate rain forest
The figure shows the distribution of these 8 biomes around the world.
A number of climatic factors interact in the creation and maintenance of a biome. Where
precipitation is moderately abundant — 40 inches (about 1 m) or more per year — and
distributed fairly evenly throughout the year, the major determinant is temperature. It is not
simply a matter of average temperature, but includes such limiting factors as:
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whether it ever freezes;
length of the growing season
If there is ample rainfall, we find 4 characteristic biomes as we proceed from the tropics (high
temperatures) to the extreme latitudes (low temperatures). In order, they are:
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tropical rain forest or jungle
temperate deciduous forest
taiga
tundra
Tropical Rain Forest
In the Western Hemisphere, the tropical rain forest reaches its fullest development in the jungles of
Central and South America.
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The trees are very tall and of a great variety of species.
One rarely finds two trees of the same species growing close to one another.
The vegetation is so dense that little light reaches the forest floor.
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Most of the plants are evergreen, not deciduous.
The lushness of the tropical rain forest suggests a high net productivity, but this is illusory. Many of the
frequent attempts to use the tropical rain forest for conventional crops have been disappointing. Two
problems:
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The high rainfall leaches soil minerals below the reach of plant roots.
The warmth and moisture cause rapid decay so little humus is added to the soil.
The tropical rain forest exceeds all the other biomes in the diversity of its animals as well as
plants. Most of the animals — mammals and reptiles, as well as birds and insects — live in the
trees.
The closest thing to a tropical rain forest in the continental United States are the little wooded
"islands" found scattered through the Everglades in the southern tip of Florida. Their existence
depends on the fact that it never freezes, and they often escape the fires that periodically sweep
the Everglades.
Temperate Deciduous Forest
This biome occupies the eastern half of the United States and a large portion of Europe. It is
characterized by:
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hardwood trees (e.g., beech, maple, oak, hickory) which
are deciduous; that is, shed their leaves in the autumn.
The number of different species is far more limited than in the jungle.
Large stands dominated by a single species are common.
Deer, raccoons, and salamanders are characteristic inhabitants.
During the growing season, this biome can be quite productive in both natural and agricultural
ecosystems.
Taiga
The taiga is named after the biome in Russia.
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It is a land dominated by conifers, especially spruces and firs.
It is dotted with lakes, bogs, and marshes.
It is populated by an even more limited variety of plants and animals than is the temperate
deciduous forest.
In North America, the moose is such a typical member that it has led to the name: "sprucemoose" biome.
Before the long, snowy winter sets in, many of the mammals hibernate, and many of the birds
migrate south.
Although the long days of summer permit plants to grow luxuriantly, net productivity is low.
Tundra
At extreme latitudes, the trees of the taiga become stunted by the harshness of the subarctic climate.
Finally, they disappear leaving a land of bogs and lakes.
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The climate is so cold in winter that even the long days of summer are unable to thaw the
permafrost beneath the surface layers of soil.
Sphagnum moss, a wide variety of lichens, and some grasses and fast-growing annuals dominate
the landscape during the short growing season.
Caribou feed on this growth as do vast numbers of insects.
Swarms of migrating birds, especially waterfowl, invade the tundra in the summer to raise their
young, feeding them on a large variety of aquatic invertebrates and vertebrates.
As the brief arctic summer draws to a close, the birds fly south, and
all but a few of the permanent residents, in one way or another, prepare themselves to spend
the winter in a dormant state.
Biomes established by altitude
Temperature is the major influence on the biomes discussed above. Because temperatures
decline with altitude as well as latitude, similar biomes exist on mountains even when they are at
low latitudes. As a rule of thumb, a climb of 1000 feet (about 300 m) is equivalent in changed
flora and fauna to a trip northward of some 600 miles (966 km).
Field studies in various parts of the Northern Hemisphere have shown that in recent decades
many species of animals and plants have
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shifted their ranges farther north (averaging 16.9 kilometers per decade);
shifted their ranges higher in the mountains (averaging 11.0 meters per decade).
These observations add to the growing body of evidence that global warming is affecting a broad
assortment of living things.
Biomes established by rainfall
The other major biomes are controlled not so much by temperature but by the amount and
seasonal distribution of rainfall.
The prevailing winds in the western half of North America blow in from the Pacific laden with moisture.
Each time this air rises up from the western slopes of, successively, the Coast Ranges, the Sierras and
Cascades, and finally the Rockies, it expands and cools. Its moisture condenses to rain or snow, which
drenches the mountain slopes beneath. When the air reaches the eastern slopes, it is relatively dry, and
much less precipitation falls. How much falls and when determine whether the biome will be
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temperate rain forest
grassland
desert or
chaparral
Temperate Rain Forest
The temperate rain forest combines high annual rainfall with a temperate climate. The Olympic
Peninsular in North America is a good example. An annual rainfall of as much as 150 inches (381 cm)
produces a lush forest of conifers.
Grasslands
Grasslands are also known as prairie or plains. The annual precipitation in the grasslands
averages 20 inches (~51 cm) per year. A large proportion of this falls as rain early in the growing
season. This promotes a vigorous growth of perennial grasses and herbs, but — except along
river valleys — is barely adequate for the growth of forests.
Fire is probably the factor that tips the balance from forest to grasslands. Fires — set by lightning
and by humans — regularly swept the plains in earlier times. Thanks to their underground stems
and buds, perennial grasses and herbs are not harmed by fires that destroy most shrubs and trees.
The abundance of grass for food, coupled with the lack of shelter from predators, produces
similar animal populations in grasslands throughout the world. The dominant vertebrates are
swiftly-moving, herbivorous ungulates. In North America, bison and antelope were conspicuous
members of the grassland fauna before the coming of white settlers.
Now the level grasslands supply corn, wheat, and other grains, and the hillier areas support
domesticated ungulates: cattle and sheep.
When cultivated carefully, the grassland biome is capable of high net productivity. A major
reason: rainfall in this biome never leaches soil minerals below the reach of the roots of crop
plants.
Desert
Annual rainfall in the desert is less than 10 inches (25 cm) and, in some years, may be zero. Because of
the extreme dryness of the desert, its colonization is limited to
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plants such as cacti, sagebrush, and mesquite that have a number of adaptations that conserve
water over long periods;
fast-growing annuals whose seeds can germinate, develop to maturity, flower, and produce a
new crop of seeds all within a few weeks following a rare, soaking rain.
Many of the animals in the desert (mammals, lizards and snakes, insects, and even some birds)
are adapted for burrowing to escape the scorching heat of the desert sun. Many of them limit
their forays for food to the night.
The net productivity of the desert is low. High productivity can sometimes be achieved with
irrigation, but these gains are often only temporary. The high rates of evaporation cause
minerals to accumulate near the surface and soon their concentration may reach levels toxic to
plants.
Chaparral
The annual rainfall in the chaparral biome may reach 20–30 inches (64–76 cm), but in contrast to
the grasslands, almost all of this falls in winter. Summers are very dry and all the plants — trees,
shrubs, and grasses — are more or less dormant then.
The chaparral is found in California. (The photo shows the chaparral-clad foothills of the Sierra
Nevada in California.) Similar biomes (with other names, such as scrub forest), are found around
much of the Mediterranean Sea and along the southern coast of Australia.
The trees in the chaparral are mostly oaks, both deciduous and evergreen. Scrub oaks and shrubs
like manzanita and the California lilac (not a relative of the eastern lilac) form dense, evergreen
thickets. All of these plants are adapted to drought by such mechanisms as waxy, waterproof
coatings on their leaves.
The chaparral has many plants brought to it from similar biomes elsewhere. Vineyards, olives,
and figs flourish just as they do in their native Mediterranean biome. So, too, do eucalyptus trees
transplanted from the equivalent biome in Australia.