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Tropical Rain Forest
Tropical rain forests are hot and wet year-round and
are home to more species than all other biomes combined.
The leafy tops of tall trees – extending from 50 to 80
meters above the forest floor – form a dense covering
called a canopy. In the shade below the canopy, a second
layer of shorter trees and vines forms a dense understory.
Organic matter that falls to the forest floor quickly
decomposes and the nutrients are recycled. The soil in a
tropical rain forest is thin and nutrient-poor.
Canopy
dense covering
formed by tall
trees
Understory
shorter trees and
vines
Tropical Savanna
Tropical savannas are characterized by warm
temperatures and seasonal rainfall. Receiving more
seasonal rainfall than deserts, but less than tropical forests,
tropical savannas are characterized by a cover of grasses.
Savannas are spotted with isolated trees and small groves
of trees and shrubs. Compact soils, fairly frequent fires,
and the action of large animals such as rhinoceroses
prevent some savanna areas from turning into forests.
Tall, perennial grasses
and drought-resistant
trees dominate the
savanna.
Animal Adaptations in the
African Savanna
Migration
During the rainy season,
birds, insects, and both
large and small mammals
thrive in the savannah, but
the rainy season only lasts
6 to 8 months. During the
dry season, surface water
from the rain is quickly
absorbed into the ground
because the soil is extremely porous. Competition for water
during the dry season is intense. Consequently, most birds
and many of the large mammals migrate during the dry
season in search of water. Because drought conditions are
sometimes localized, the migration may be just to another
area within the savannah. When drought conditions exist
for a long time and over a wide area, the animals may
migrate to another biome until the rainy season begins
again.
Although elephants do migrate, they have a
physical adaptation that allows them to access water that is
not available to other animals. Baobab trees store water in
their large trunks. The elephant's physical strength and
anatomy allow it to tear open the trunk of the baobab tree
and to suck the water from it. An adaptation used by small
burrowing animals is to remain dormant during times of
drought--much like bears do during the winter in other
biomes
Lightning
During the dry season,
lightning frequently ignites the brown,
dry grasses that cover the savannah.
Many of the animals have adapted to
living with the fires. The ability to fly or
to run fast enables most birds and
large mammals to escape the flames.
Some birds, such as the Fork-tailed
Drongos, actually are attracted to the
active fires. These birds feast on
fleeing or flame-roasted insects.
Although small burrowing animals may
not be able to outrun the flames, they
frequently survive the fire by digging
beneath the soil and remaining there
until the flames pass by them.
Why do giraffes have such long
necks?
Competition for Food
The “competing browsers hypothesis” was originally suggested
by Charles Darwin and only challenged recently. It suggests that
competitive pressure from smaller browsers, such as kudu, steenbok,
and impala, drove the elongation of the neck so giraffes could reach
nutrients competitors could not. This advantage is real – giraffes can and
do feed up to 5 m, while most of their competitors, kudu, can only feed
up to about 2 m (7 ft).[ There is also research suggesting that browsing
competition below 2 m is intense, and giraffes feed more efficiently
(gaining more leaf biomass per bite) higher in the canopy.
However, scientists disagree about just how much time giraffes
spend feeding at levels unreachable to other browsers. Although giraffes
can feed as low as 0.5 m and as high as 6 m off the ground, it appears
that they most often feed between 2 and 4 m (7–14 ft). However,
elephants also routinely feed at heights up to 5 m (they knock down only
a minority of the trees they feed on), and are likely competitors at these
heights. Competition for food with other giraffes could also favor the
evolution of tall necks.
Competition for females
The other main theory, the sexual selection hypothesis,
proposes that the long necks evolved as a secondary sexual
characteristic, giving males an advantage in "necking" contests to
establish dominance and obtain access to sexually receptive females.
In support of this theory, males have proportionally larger necks than
females, and males with longer, bigger necks are more successful in
dominance displays and courtship behavior. However, a major criticism
of this theory is that it fails to adequately explain why female giraffes
also have long necks.
Temperate Grasslands
Characterized by a rich mix of grasses and underlain by
some of the world’s most fertile soils, temperate
grasslands – such as plains and prairies – once
covered vast areas of the Midwest and central United
States. Since the development of the steel plow, however,
most have been converted to agricultural fields. Periodic
fires and heavy grazing by large herbivores maintain the
characteristic plant community. Temperature Grasslands
generally have warm to hot summers and cold winters with
moderate, seasonal precipitation.
Grassland animals
Herbivores:
•Mule deer
•Pronghorn antelopes,
•Rabbits
•Prairie dogs
•Bison
Predators:
•Coyotes
•Badgers
•Wolves
•Owls
•Hawks
•Prairie chickens
Insects:
•Ants
•Grasshoppers
Reptiles:
•snakes
Desert
All deserts are dry – a desert biome is defined as having
annual precipitation of less than 25 cm. Beyond that,
deserts vary greatly, depending on elevation and latitude.
Many undergo extreme temperature changes during the
course of a day, alternating between hot and cold. The
organisms in this biome can tolerate the extreme
conditions. Soils in the desert are rich in minerals, but poor
in organic material.
Tundra
The Tundra is characterized by permafrost, a layer of
permanently frozen subsoil. During the short, cool
summer, the ground thaws to a depth of a few centimeters
and becomes soggy and wet. In winter, the topsoil freezes
again. This cycling of thawing and freezing, which rips and
crushes plant roots, is one reason that tundra plants are
small and stunted. Cold temperatures, low precipitation,
high winds, the short growing season, and humus-poor
soils also limit plant height.
Permafrost
A layer of soil that stays frozen all year
The Calliergon giganteum has adapted
well to its cold climate. When it is not
growing, it stores nutrients so new
leaves can be made quickly next
spring. The more leaves the more they
can photosynthesize. It is adapted to
the incredibly strong winds found in
the tundra by growing near to the
ground. Because it can grow under
water it is protected from the drying
winds and cold, dry air of the frozen
tundra. Its long life and slow growth
are probably adaptations to the short
growing season and the cold.
Cushion plants - grow in a
low, tight clump and look
like a little cushion.
Cushion plants are more
common in the tundra
where their growth habit
helps protect them from the
cold.
Temperate Forest
Temperate forests contain a mixture of deciduous
and coniferous tress. These forests have cold winters that
halt plant growth for several months. In autumn, the
deciduous trees shed their leaves. In the spring, small
plants burst out of the ground and flower. Summers are
often warm. Soils of temperate forests are often rich in
humus, a material formed from decaying leaves and other
organic matter that makes soil fertile. This biome is also
characterized by year-round precipitation.
Deciduous trees
trees that shed their leaves
in the autumn
Food web of the temperate
forest
Boreal Forest (Taiga)
Along the northern edge of the temperate zone are dense
evergreen forests of conifers. (Conifers are trees that
produce seed-bearing cones which usually have leaves
shaped like needles.) These biomes are called boreal
forests, or taiga. Winters are bitterly cold, but summers
are mild and short – but long enough to allow the ground to
thaw. The word boreal comes from the Greek word for
“north,” reflecting the fact that boreal forests occur mostly in
the Northern hemisphere. This area is characterized by
acidic, nutrient poor soils.
Conifers
trees that produce
seed-bearing cones
and usually have
leaves shaped like
needles.
Partial food web showing consumers of the Taiga.