Cambridge International Catalogue 2024 - Final - Flipbook - Page 64
CAMBRIDGE IGCSETM AND O LEVEL
GEOGRAPHY (0460/0976/2217)
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Tropical rainforests
Pressure
In low pressure systems air is rising. Low pressure
produces rain as the air may rise high enough,
cool, condense and form clouds and rain. This can
happen in very warm areas, such as in equatorial areas,
at mountain barriers and at weather fronts, when
en warm
air is forced over cold air. In contrast, where there
is high pressure, air is sinking, and rain formation is
prevented. The world’s great hot deserts are located
where there is high pressure caused by sinking air.
Activities
Ac
Ho does latitude affect the amount of heat a place
1 How
receives?
2 Why are equatorial areas not getting any hotter nor
polar areas any colder?
3 What is meant by the term ‘specific heat capacity’?
4 Explain why temperature decreases with height.
5 Why is there a large temperature difference between day
and night in hot deserts, but not in equatorial areas?
Tropical rainforests
View sample material
from our Student’s Books
Tropical evergreen rainforests are located in equatorial
areas, largely between 10°N and 10°S (Figure 3). There
are, nevertheless, some areas of rainforest that are
found outside these areas but these tend to be more
seasonal in nature. The main areas of rainforest include
the Amazon rainforest in Brazil, the Congo rainforest in
central Africa, and the Indonesian-Malaysian rainforests
of South-East Asia. There are many small fragments of
A
Wide-spaced
umbrella-shaped
crowns, straight
trunks and high
branches
Interesting note
Tropical rainforests cover 6 per cent of the world’s
land surface but hold 50 per cent of the world’s
species. The Amazon rainforest alone is home to
10 per cent of the world’s known species.
Tropical rainforest
Tropic of Cancer
Equator
Tropic of Capricorn
▲ Figure 3 World distribution of tropical rainforests
Vegetation
The vegetation is evergreen, enabling photosynthesis
to take place all year round. This is possible due to
30
25
20
C
Densely packed
conical-shaped
crowns
(a)
A Emergent (top)
tree canopy
B Large trees of
middle layer
5
F Root layers
C Lower tree
layer
▲ Figure 5 Adaptations of rainforest plants: (a) Drip-tip and
(b) Pitcher plant
D Shrub/small tree
layer
E Ground vegetation
F Root zone
▲ Figure 4 Vegetation structure of the tropical rainforest
5
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(b)
15
10
D
Sparse
vegetation of
shrubs and
saplings
2.5 CLIMATE AND NATURAL VEGETATION
the high temperatures all year, and the presence
of water throughout the year. The vegetation is
layered, and the shape of the crowns varies at each
layer. Species at the top of the canopy receive
most of the sunlight, whereas species that are
located near the forest floor are adapted to darker
conditions and generally have a darker pigment so
as to photosynthesise at low light levels. There is a
great variety in the number of species in a rainforest
– this is known as biodiversity. A rainforest may
contain as many as 300 different species in a single
hectare. Typical rainforest species include figs, teak,
mahogany and yellow woods.
Tropical vegetation has many adaptations. Some
trees have leaves with drip-tips (Figure 5a), which
are designed to get rid of excess moisture. In
contrast, other plants have saucer-shaped leaves in
order to collect water. Pitcher plants have developed
an unusual means of getting their nutrients. Rather
than taking nutrients from the soil, they have
become carnivorous and get their nutrients from
insects and small frogs that are trapped inside
the pitcher (Figure 5b). This is one way of coping
with the very infertile soils of the rainforest. Other
plants are very tall. To prevent being blown over by
the wind, very large trees have developed buttress
roots that project out from the main trunk above
the ground, which gives the plant extra leverage
in the wind.
45
40
35
B
Medium-spaced
mop-shaped
crowns
Height above ground (m)
Look inside
rainforest, such as those on the island of Madagascar
and in the Caribbean. Everywhere tropical rainforests
are under increasing threat from human activities such
as farming and logging. The result is that rainforests
are disappearing and those that remain are not only
smaller,
smaller but broken up into fragments.
6
Rainforest animals
Although rainforests cover less than 6 per cent of
the earth’s surface, they account for over 50 per
cent of all animal species on Earth. Many species,
such as orangutans, are arboreal (live entirely in
the trees) and rarely come to the ground where
they would be easy prey for large carnivores. Top
carnivores, such as jaguars, tigers and leopards,
are highly camouflaged. Many species are highly
camouflaged to avoid becoming prey, such as
stick insects and the Indian leaf butterfly. Sloths
are covered with a layer of green algae which
camouflages their fur in their arboreal environment.
Some animals have evolved to look larger or scarier
than they really are: the larvae of the lobster moth
look like scorpions, but are defenceless. Many
butterflies have designs that look like large eyes on
their wings, in order to confuse potential predators.
Some species, such as the monarch butterfly, are
poisonous, which helps deter predators. It is more
than likely that many rainforest species have not yet
been discovered – between 2010 and 2013 over 440
new species of plants and animals were discovered
in the Amazon, alone.
Soils
Rainforests are the most productive land-based
ecosystems. Ironically, the soils of tropical
rainforests are quite infertile. This is because most
of the nutrients in the rainforest are contained
in the biomass (living matter). Rainforest soils
are typically deep due to the large amount of
weathering that has taken place, and they are often
red in colour, due to the large amounts of iron
present in the soil. Nevertheless, there are some
areas in which tropical soils may be more fertile:
in floodplains and in volcanic areas the soils may
be enriched by flooding or the weathering of fertile
lava flows.
The nutrient cycle is easily disrupted (Figure 6).
Tropical rainforests have been described as ‘deserts
covered by trees’. Once the vegetation is removed,
nutrients are quickly removed from the system,
creating infertile conditions and even deserts.
