Download Biomes Text Final

BIOMES
{ecological communities dictated
by local conditions of
sunlight, temperature, humidity
and precipitation}
A biome consists of all plants, animals and other organisms that make up a distinct natural
community in a climatic region. Local conditions of sunlight, temperature, humidity, and precipitation determine what plants and animals exist in a given biome. Geologic features and
human activity also influence the composition of biomes. In some biomes conditions are
favorable for growth, and large numbers of plant and animal species thrive. In other biomes
the environment is more challenging, and the number of species and individual organisms is
smaller.
Biomes may be terrestrial or aquatic. Terrestrial biomes reflect the amazing variety of living
communities on the earth’s surface: from the extreme conditions of the ice cap or the desert, to
the temperate environments found in grasslands and deciduous forests, to the lush habitat provided by tropical rain forests. Sometimes the borders between biomes are quite distinct; in
other cases, where the division is more gradual, the plants and animals of two biomes mingle
in a transition community called an ecotone.
Aquatic biomes may be marine or freshwater; together, they make up the largest ecosystem on
earth. All aquatic biomes are linked together through the global water cycle, in which water
molecules are constantly exchanged among oceans, clouds, glaciers, rivers, lakes and the many
other places where water is found. Organisms living in aquatic biomes are influenced by
regional climate, waves and tides, as well as the amount of salt, availability of nutrients and the
depth of the water. Some biomes, such as seashore and wetland, are semiaquatic: these living
communities exist in a constant give-and-take between terrestrial and aquatic influences.
Like the earth itself, biomes can and do change. All biomes are living communities, and thus
they respond to both internal and external conditions. Climate change and human activity are
two factors which can powerfully alter the composition of biomes. The process may be gradual, as in the expansion of deserts, or the change can be drastic and abrupt, such as when grassland is plowed under for agricultural or industrial development.
Clustered like great subsurface cities, coral reefs are living organisms whose complex constructions provide a home for numerous species of fish, eels, crabs, mollusks, squid, sea urchins
and many other marine species. Coral reefs are ecosystems as diverse as tropical rain forests.
To survive, corals require both warmth—seas no colder than 70° F, on average for the coldest
month—and clear water. Corals can’t live above the low tide line—unlike barnacles, they can’t
survive being exposed to the air between low and high tides. And they don’t grow in waters
deeper than 65 feet or so, since at that depth there is not enough light to sustain a reef. These
requirements mean that reef-building corals will only thrive in tropical and subtropical
waters. Even in the most favorable conditions, most corals grow slowly, often no more than
half an inch per year.
CORAL REEF
Corals can only grow well in clear water. This is because they depend on a special relationship
with tiny single-celled algae for survival. The algae, called zooxanthellae, live both inside the
cells of the coral and on its surface. And to get enough light for photosynthesis, the process of
using sunlight to make food, the algae need clear water.
The greatest concentration of coral species is found in the waters of Southeast Asia, in countries like Indonesia and the Philippines, where there are more than 700 coral species. Coral
diversity is not as great in other ocean areas; there are only 200 coral species in the Red Sea, 57
in the Arabian Gulf and only 35 in the Caribbean.
Coral reefs provide a home for a wide variety of plants and animals. Multicellular filamentous algae and macroalgae (seaweed) often grow in patches to be eaten by grazing fish.
Sponges, which look like plants but are actually animals, will occupy the lower reaches of a
reef and those areas with less clear water. Sea Urchins hide in the reef during the day, like
many crabs, mollusks and small invertebrates. They emerge nightly to feed on the coral algae
growth. Moray eels lodge in sections of the reef, darting out to eat small fish. Parrotfish feed
on the corals, biting off pieces, grinding them up and digesting the soft parts. Meanwhile, large
predators like tiger sharks patrol the reef looking for a meal.
CULTIVATED LAND
Most ecological zones are created by the prevailing natural conditions: days of available sunlight, temperature, water supply, soil quality, weather patterns and the forms of mountain and
valley. However, there is one habitat most greatly influenced by man: cultivated land or farmland, where the natural array of plants and animals has been changed by human planting, fertilizing, weeding and harvesting.
Natural meadows and grasslands provide homes for a wide range of plant and animal species.
One estimate puts the number of different types of plants originally in the temperate grassland
of North America in the hundreds, the number of mammals at more than 80 and the number
of bird types at more than 300. This astonishing variety is greatly reduced when land is farmed
to produce a single crop. Where many plants once flourished, only one comes to dominate.
When grasslands are plowed, many species of animals lose natural cover and must move on to
make new homes. The removal of seed-bearing plants that humans classify as “weeds” can
reduce the food supply for small birds. And when cultivated land is heavily sprayed with
insecticide, the poison kills or weakens many helpful as well as harmful insects. This destruction of insect populations further reduces food supplies for birds. Other types of animals, like
birds and small mammals, also suffer from heavy insecticide use.
When land is cultivated, wallows and grassland potholes are drained and filled in for use in
planting crops, eliminating habitat for duck and geese. The loss of wetlands to farming also
can affect small mammal and fish populations. And runoff of fertilizer from fields can alter
the chemical balance of streams and rivers.
Some of the effects of cultivation are good for animals. Plowing provides birds with some benefits. When the earth is turned over earthworms are brought to the surface, providing easy
meals for robins and starlings. And the nooks and crannies of farm buildings can provide nesting places for owls and swallows.
Erosion is another problem that can be caused by cultivation. High winds and strong storms
can erode even well-managed farmland. Plowed fields that do not yet have the cover provided by crops are particularly susceptible to wind erosion. In an effort to reverse the effects of the
famous “Dust Bowl” years of the 1930s, when tons of topsoil were blown away by storms, the
Great Plains Shelterbed Project planted more than 223 million trees between 1935 and 1943.
The trees were put in to form wind breaks, but were not completely successful. They sometimes had a negative effect by taking vital ground water from already struggling crops.
DESERT
More than a third of the Earth’s land area is parched and barren, with wide waterless expanses of swirling dust and sun-baked sand. These desert lands are zones of great heat and cold
where only the hardiest and best-adapted life survives.
Around the world, deserts often form in the subtropics beneath areas of atmospheric high
pressure. The air in these high-pressure zones is stable and clear, allowing for maximum sunlight to fall on the earth’s surface. This allows the surface temperature to build, evaporating
the small amount of rain that does fall. With no cloud cover to retain the heat at night, desert
air can quickly get chilled as heat is radiated out into the emptiness of space. Deserts sometimes form near mountains that cause moisture-laden clouds to drop their rain, leaving the
land downwind parched. Another cause can be cold water upwelling along a coast. The air
above is then chilled, causing its moisture-carrying abilty to be drastically reduced.
Many deserts receive less than five inches of rain per year. For the plants and animals that
make the desert their home, this lack of water not only determines their behavior but also
plays a big role in their size and shape.
Desert plants must minimize the amount of water they release to the environment. One way
they do this is with small leaves. The smaller the surface area of a leaf, the less water vapor it
can give up. One class of plant, the cactus, eliminates leaves entirely. It concentrates the chlorophyll it needs for photosynthesis in its outer skin. The tall Saguaro cactus uses its skin this way
and saves water in its pulpy, spongelike inner core.
Animals in the arid lands need some method of escaping from the scorching midday sun. A
common approach is to live in underground burrows, like the North African jerboa, or the
kangaroo rat of North America. Insects will burrow into the sand or hide in rock crevices. In
the deserts of North America, one species of squirrel uses its tail like a bushy parasol.
Desert lands are growing. Some studies have revealed a growth of more than 40 square miles
of desert every day.
ICE CAP
The habitat of an ice cap is deceptive. Viewed from the surface, the expanse of polar ice can
appear desolate and uninhabited. However, below this world lies another, far more vibrant
realm of cold-water organisms. Seals, fish, crustaceans, diving birds, whales and many other
creatures live in what is for them a welcoming habitat.
While the ice caps of the northern Arctic and southern Antarctic polar regions are similar,
there are definite differences. The Arctic ice cap is predominantly frozen sea water and is usually no thicker than seven feet. The Antarctic ice cap, however, is made up of fresh-water ice
thousands of feet thick sitting atop the Antarctic landmass. At opposite ends of the earth, these
ice world habitats are sufficiently different that no species is common to both poles (save for
the migratory Arctic tern that visits both poles).
In the north, the ice cap consists primarily of sea ice floating in a great ice island in the Arctic
Ocean. (There is also a permanent ice cap that covers the large island of Greenland.) This vast
jumble of fractured ice floes, some as large in area as several kilometers square, rotates counterclockwise, following the Arctic Ocean current. The ice cover shrinks in summer and grows
again in the northern winter.
The master of the Arctic ice, and one of the few animals equipped to venture forth on it, is the
polar bear. These formidable bears (an average adult male will be 8.5 feet high and weigh 900
pounds) are gifted swimmers and have been seen 50 miles from land or any significant pack
ice. Polar bears exist primarily on a diet of seals.
While bears are the animals seen on the surface of the northern ice cap, seals and penguins
populate the southern ice cap. Ungainly, flightless birds on land, penguins are fast, agile swimmers that are more at home in the water.
The vast Antarctic is home to many species of seals including: Antarctic fur seals, crabeater
seals, leopard seals, Weddell seals and Ross seals. Since there are no polar bears in Antarctica,
the seals have no enemies other than human and thus have developed huge populations—
some estimates put the crabeater seal population in excess of 30 million.
Anchoring the base of the Antarctic food chain are small, shrimp-like crustaceans called krill.
Because they are a staple food source for seals, penguins, whales, fish and birds like the albatross, changes in their populations can have widespread effects on many Antarctic creatures.
TROPICAL RAIN FOREST
Along the equator, in a zone thick with heat and humidity, lies the earth’s richest, most diverse
habitat—the tropical rain forest. The abundance of plant life is due to the ready availability of
water, light and favorable temperatures. These conditions also allow plentiful food supplies for
a dazzling number of animal species. This riot of biological activity means that up to 300
species of tree can be found within a square mile of Brazilian rain forest and up to 600 species
of beetle can be found in the canopy of a single tree.
With such favorable conditions for year-round growth, the trees of the forest canopy reach
impressive heights. The tallest trees, called emergents, can stand 130 to 160 feet above the
ground. For all their great size, however, these trees have relatively short life spans—only a
few hundred years compared to 1,000 years or more for long-lived temperate forest trees like
those found in the Pacific Northwest of the U.S. Most rain forest trees grow to a height of 80
to 120 feet, forming a dense green umbrella that blocks much of the light from reaching the
forest floor. To grab their share of the life-giving light, an unusual class of plants, called epiphytes, entwine themselves around the branches of canopy trees. They are not parasites, but
only use the trees for support and gather their water from the abundant rainfall.
Those plants and trees, like the nutmeg, that do grow beneath the main canopy must use light
efficiently to survive. The forest floor-dwelling begonia, for example, has leaves with red
undersides to take advantage of the red wavelength light that bounces off the surrounding
vegetation.
Even with all the rapid growth and profusion of species, the soil of the rain forest is not particularly rich. The leaves and other organic material that drop to the forest floor don’t break
down into nutrients and fortify the soil but, instead, are quickly recycled into new plants.
Thus, when the rain forest is cleared for farming, the nutrient-poor soil is quickly exhausted.
Much of the animal activity in the rain forest takes place in the forest canopy. Birds, bats, monkeys, frogs, and insects will spend the majority of their lives in the canopy, high above the
ground. Lower down in the trees of the undercanopy, leopards, jaguars and predatory snakes
wait to drop on their unfortunate victims. The forest floor is the home of larger mammals like
elephants, deer and gorillas.
SAVANNA
Endless expanses of grass that are home to the earth’s fastest and largest land animals, savannas are found in broad strips of land on either side of the equator in the tropics and subtropics. With a climate balanced between the heavy rains of the tropical forest and the arid waste
of the desert, the savanna produces a scattering of trees, but the dominant plants are the profuse grasses. These great seas of grass, along with all other life on the savanna, have adapted
to varied conditions ranging from sweltering drought to dousing rains that rule the life cycle
of the savanna.
The grasses of the savanna generally grow much higher than those found in temperate grassland like the North American prairie. While grasses are the dominant plants, the savanna also
produces numerous trees like the sharp-thorned acacia and sturdy, thick-trunked baobab.
Ranging over the flat savanna grasslands is an amazing variety of herd animals and powerful
predators. The grazing herds of gazelles, wildebeests and zebras demonstrate the productive
nature of the savanna, which can support huge numbers of animals. By migrating with the
seasons, the herds don’t overuse the grass in any one area. Also, each type of animal crops the
grass to a different length. Thus the zebras, which have upper and lower teeth, eat the tallest,
coarsest grass. Next come the wildebeests, which, like the other species of antelopes, have teeth
only on their lower jaws and a toothless upper palate. Once they are finished, a species like the
Thomson’s gazelle will move in and eat the lowest and smallest plants.
Meanwhile, the grazers are followed, chased, and sometimes killed by the predators: lions,
leopards, cheetahs, hyenas, jackals and wild dogs. Lions, the largest of the carnivores, hunt in
groups and bring down the biggest prey—sometimes including even a hippopotamus or a
young elephant. Both the stealthy leopard and the blazingly fast cheetah are solitary hunters.
Hyenas, jackals and wild dogs will hunt in large packs, using their advantage in numbers to
run their prey to the ground. These three species are also opportunistic scavengers, often cleaning up a carcass left by a pride of satisfied lions.
TAIGA
Blanketing the northern reaches of North America, Europe and Asia just south of the tundra
belt, the great coniferous forest is a thick swath of dense pine woodlands, kettlehole ponds and
soggy marshes. This 6,800-mile expanse is often referred to as taiga, a Russian name for
marshy pine forest. Along its northern edge, taiga begins at the latitude where there is enough
light (30 days per growing season) and the temperature rises high enough (50° F) for trees to
grow. Extending southward to an average width of 1,500 miles, the pine forest gives way to
the broadleaf deciduous forests of the Temperate Zone.
Evergreen trees like pine, hemlock, spruce, cedar and fir are well adapted to the harsh conditions of the taiga, which can have the coldest temperatures on earth outside of Antarctica. The
thin leaves (needles) of these conifer species lose the minimum amount of water vapor. This is
advantageous in an environment where most of the water is locked up in ice and snow for
more than half the year, bringing plants to the edge of drought conditions. When the thaw
comes in the spring, the cool climate doesn’t promote evaporation, so snow and ice melt accumulates in ponds, marshes and bogs and the soil becomes waterlogged. Due to the cold, there
is less bacterial activity and the waxy needles of the conifers break down slowly, leading to
acidic soil with few nutrients.
Very few animal species, and individual animals, live in the taiga because of its long, snowy
winters. Among the seasonal inhabitants of the taiga are the caribou, which migrate south
from the tundra and spend the winter in the forest where they can gain some shelter from the
vicious polar winds. To help conserve heat, the animals of the taiga are among the largest of
their species. For example, the heftiest member of the deer family is the moose, which can
weigh up to 1,400 pounds and stand seven feet tall. The wolverine is the biggest and most
aggressive of the weasel family, while the capercaillie is the largest example of a grouse.
TUNDRA
Circling the top of the globe between the shifting ice of the Arctic and the northernmost limit
of tree growth lies the fragile expanse of lichen, mosses and shrubs called the tundra. Locked
in an uneven cycle that ranges from -70° F in the long, dark winters to 70° F in brief but sunfilled northern summers, the tundra, which covers a tenth of the earth’s surface, is home to a
hardy collection of plants and animals.
A defining element of tundra is the thick layer of permanently frozen ground called permafrost that underlies the thin vegetation layer. Even the long sunny days of the northern
summer merely thaw the top few inches of the permafrost, which can reach depths of up to a
mile. Like an impervious bedrock slab, the permafrost prevents meltwater from ice and snow
from draining down into the soil, so this water collects in numerous ponds and bogs.
While the tundra is generally flat and featureless, the combination of meltwater and permafrost produces ice-cored hillocks called pingos. These soil and vegetation-covered mounds,
which can reach 150 feet high and 1,800 feet across at the base, form when meltwater is forced
to the surface and freezes. Sometimes the central ice lens of a pingo will melt, leaving a kettlehole pond surrounded by a ring of slumped earth.
Tundra plants are challenged not only by the long winters but also by the steady wind that can
dehydrate them and the permafrost that limits their root growth. Many tundra plants, like
dwarf willow and dwarf birch, grow horizontally rather than growing upward into the icy
wind. Other tundra species that have adapted to the short summer are bearberry, cranberry,
various grasses and sedges, mosses and lichens.
A signature animal species of the tundra is the caribou (or reindeer). Large herds of caribou
migrate from the northern forests out onto the tundra to feed on the short-lived bloom of tundra vegetation. The wet, boggy conditions of the tundra are perfect for the mass breeding of
the caribou’s nemesis: the mosquito. Great swarms of mosquitoes and black flies can descend
on caribou herds, driving some of the animals insane with discomfort.
Smaller animals like foxes, hares and lemmings must adapt to the long, bitter winters and
make maximum use of the all too brief summers.
WETLAND
Marshes, swamps and peat bogs are found the world over. Because of the wide diversity of
wetland types, they are among the most difficult biomes to classify. However, all wetlands are
transitional zones where land and water habitats merge. This mixing of environments allows
wetlands to support both land-based and water-based organisms. Many wetland areas undergo yearly changes, shrinking during the dry season and expanding their borders in times of
heavy precipitation.
The water in wetlands fills the soil and blocks oxygen from reaching soil microbes. Because of
the lack of oxygen, bacteria and fungi can’t break down dead organic matter. Thus, plant matter accumulates, giving wetland areas a distinctive layer of thick muck. Plants that have adapted to wetlands are those that can live in these low oxygen conditions, like water lilies, red
maples, mangroves and cypress.
In the tropics, mangrove trees thrive in wetland areas. Red mangroves colonize the edges of
tropical swamps; their arching, pedestal roots allow the trees to exchange gases with the air
and also work to trap soil and extend new land into the swamp.
Over the course of centuries, wetlands in northern areas are often transformed into bogs.
These are lakes and marshes that have accumulated layers of sediment that promote the
growth of sphagnum moss along the margins. Floating mats of grasses and moss grow into the
pond and eventually cover it with a thick layer of growth. Dead plant matter accumulates and
is packed together into peat. Blocks of peat can be cut, dried and then burned for fuel.
Wetland areas in the warm weather zone of the tropics are the home of the alligator and crocodile, large reptiles that have been around since the age of dinosaurs. Like all large predators,
these animals have suffered from human encroachment and many populations are in decline.
Probably the best known natural engineer, the beaver, is a wetland dweller. Second only to
man in changing the environment to suit its needs, a group of beavers can turn a flowing
stream into a large pond or marsh. By felling many hundreds of trees a year, a beaver family
creates a habitat that provides a home and food for fish, amphibians, waterfowl and large
mammals like moose.