|
Tropical savannas can feature woodlands like these above ...
|
...or open plains like this
|
General questions |
Climate | People of the
tropical savannas | Land use in the tropical
savannas |
What are tropical savannas?
Tropical savannas are landscapes of grass and scattered trees
that occur throughout the world’s tropics. Tropical savannas
can be almost treeless grasslands or denser woodlands – as
long as the canopy cover of the trees is not so dense that it
shades out the grass.
Note that some people use a narrower definition of tropical
savannas, restricted to landscapes that are largely grassland with
scattered trees or scrub. Landscapes with a continuous grass layer
below and regular tree cover above, as seen in much of far northern
Australia, would be called tropical woodland rather than
savanna under this defintion.
We use the broader definition of tropical savanna that includes
both woodlands and grasslands because the ecosystem processes and
management issues are similar across both landscapes in north
Australia.
Where do you find them?
Tropical savannas are found in Africa, Australia, South America,
India and South-East Asia – (see map). They cover a little
less than a third of the world's land surface. In Australia,
tropical savannas encompass around one quarter of Australia,
stretching from the Indian Ocean in the west to the Pacific in the
east. They border desert country to the south, rainforest on the
east coast and are fringed by floodplains and peppered with monsoon
forest patches in the north.
In Africa, tropical savannas form a broad semi-circle from the
western Ivory Coast across to a southern border with northern
Namibia. Approximately 45% of South America is savanna and exists
as two large patches north and south of the equator. Approximately
10% of India and South-east Asia is considered savanna.
 |
|
Encyclopaedia Britannica's map of
the world's tropical savannas
|
How do Australian savannas differ from African ones?
Australia, unlike Africa, does not have large animals such as
giraffe, zebra and wildebeest. In Australia, much of the plant
material is eaten and re-cycled by insects such as termites! More
than 40 thousand years ago, however, there were large grazing
animals in Australian tropical savannas, such as giant wombats and
kangaroos. Debate continues as to why these animals became extinct.
Some argue that it was mainly because of climate change, others say
that the ancestors of Australian Aborigines who arrived on the
continent at least 40 thousand years ago, could have hunted them
into extinction, and many say it was a combination of climate
change and human action that removed many of the large animals from
the Australian savannas.
Why are the tropical savannas important?
People consider tropical savannas to be important for a number
of reasons. We list two of the most prominent reasons below, taking
Australia’s tropical savannas as an example.
High in Biodiversity Tropical savannas are a
major reservoir of biodiversity – some areas of tropical
savanna may have a similar biodiversity level to tropical
rainforest. They are home to hundreds of species of native plants,
mammals, birds, reptiles and amphibians and tens of thousands of
different species of invertebrates. Many species in all these
groups are found nowhere else in the world.
There are a few reasons for this. The tropics are generally
found to have a higher biodiversity for any given area than regions
in higher latitudes. This may relate to several factors, such as:
(i) less frost; (ii) higher energy levels from the more intense
sunshine reaching tropical ecosystems; (iii) the large areas of
rainforest that are found in the tropics; (iv) tropical
temperatures and humidity don't vary as much over time as they do
in higher latitudes. Additionally, the tropics tend to be less
intensively developed than the temperate areas of the world. This
is particularly the case in Australia, and much of the natural
habitat in northern Australia is relatively intact compared to that
in southern Australia.
Rich in Culture The tropical savannas are also
important in cultural terms. The Australian tropical savannas for
example, are home to a diverse and distinctive Aboriginal culture.
Aboriginal communities manages a large fraction of the land area of
the region. The didgeridu and the band Yothu Yindi come from the
tropical savannas.
There is also a long history of pastoralism in northern
Australia which has some of the largest cattle stations in the
world.
Tourists and locals also value the tropical savannas for their
wild and spectacular natural features. This part of Australia has
world heritage areas like Kakadu National Park, Purnululu National
Park.
What sort of climate do Australia’s tropical savannas
have?
The tropical savannas of northern Australia have two starkly
different seasons: the 'wet' and the 'dry'. The intensity and
length of these seasons will vary depending on the latitude,
topography and distance from the coast. These are European
classifications – Aboriginal people will divide the year more
finely with often six or more distinct seasons recognised.
The wet months, December to March, are hot and humid
interspersed with torrential downpours and contrast with the dry
months of May to August which have low humidity, little to no rain
and cooler, wider-ranging temperatures. These two major seasons are
separated by brief periods of variable conditions.
The Wet Season (December to March) At this time most of
the tropical savannas, but particularly the northern sectors, are
influenced by warm, moist, monsoonal north-westerlies. These winds,
originating in equatorial regions, bring prolific rains to northern
coastal areas. The annual average rainfall gradually diminishes,
from more than 2000 mm to less than 600 mm, with distance inland
and to the south. Rainfall also decreases from east to west, with
areas of coastal Queensland (not savanna country but rainforest
areas) receiving over 3000 mm of rainfall in some years!
The southern savannas, situated below the range of regular
monsoon winds, are influenced more directly by rain-bearing
'pseudo' or 'quasi-monsoons'. The pseudo-monsoon winds bring a
moist, westerly flow in from the Indian Ocean while the trade winds
of the quasi-monsoon originate in the Pacific Ocean to the
east. As their names imply, these winds do not bring the
reliable rainfall of the true monsoons, but are more variable.
Throughout the wet, temperatures near the coast hover around the
low 30s (oC) with warmer conditions inland where occasional days in
excess of 50oC have been recorded. Temperatures at this time of
year may be moderated by the passage of storms or persistent cloud
cover. However, 'climate discomfort' days are numerous as
it’s often humid as well as hot.
The change from wet to dry (late March to April) It is noted for
its hot periods with increased hours of sunshine, calm or variable
winds, fewer and less intense storms and reduced humidity. Towards
the end of this period temperatures drop as the north-west monsoon
winds are replaced by the south-east trade winds. The dry season
arrives earlier in the southern regions.
The Dry Season (May to August) With dry south-east winds
(moister on the east coast), cooler temperatures, greater
temperature variation during the day, clear skies and low humidity.
Light rains may fall, particularly in the south-east portions,
during the early months but can occur in any month. Occasionally
cold air from the south penetrates well into the tropics and
night-time temperatures may drop enough for frosts to form in some
inland regions in the east and west. However, the savannas are
mostly frost free. As the dry progresses through August and
September the temperatures begin to rise as the sun moves more
directly overhead and the south-easterly winds begin to
subside.
The change from dry to wet (September to
November) At the transitional pre-monsoon period, the monsoon
trough moves south over the savannas again. It is a time of calm
and variable winds. The intermittent westerly winds bring increased
humidity and scattered thunderstorms develop. Temperatures increase
and so hot and humid days become more intense and numerous –
hence this time of year is called the “build-up”. When
the monsoons fully develop again it comes as a relief to many
people, and the new wet season has arrived.
How does this climate compare to those of other tropical
savannas?
When compared with the climate of the largest tropical savannas
– those of western and eastern Africa –
Australia’s climate is distinguished by a couple of
features.
Firstly, in the wetter northern region of Australia’s
savannas the climate is remarkable for the stark division between
the wet and dry seasons: in the wet up to 2400 mm of rain can fall
and there is always a wet season; yet in the dry there is regularly
none or almost no rain for two or three months. In the wetter parts
of Africa the rainfall is more variable.
Secondly in the drier, southern parts of the Australian
savannas, there is not such a regular division between the seasons.
The climate of this region is influenced by the El Nino/Southern
Oscillation phenomenon with some years having much less rain than
others. In some years there can be rain in the dry season. In other
years there can be very little rain in the wet season.
Who lives in Australia’s tropical savannas?
Not many people. Australia's tropical savannas are relatively
sparsely populated compared to other areas in Africa or Asia. With
15.4% of Australia's area, the northern and eastern tropical
savanna zones contain only 0.8% of Australia's population. There is
also a uniquely high proportion of Aboriginal and Torres Strait
Islander peoples among the inhabitants of the tropical north,
amounting to around a quarter of the population.
Despite the low overall numbers of people, there is still great
variation in the population density within Australia’s
tropical savannas. In the west – in the Kimberley and the
more remote regions of the Northern Territory, the population is
very sparse with a relatively high percentage of Aboriginal people
and cattle station and mining workers. Most of the people in the
western part of the savannas, however, live in major centres like
Darwin, Broome and Katherine where major employers are in
government service, the tourist industry and the defense forces.
Moving east to Queensland the density of people and cattle stations
increases, although they are still pretty sparse in the gulf and
Cape York. The highest densities occur on the east coast where
Townsville is the major centre and the largest town in the tropical
savannas.
How long have people been living in Australia’s tropical
savannas?
There is still some speculation around the date of human
occupation of Australia, however, it is becoming more widely
accepted that Aboriginal occupation goes back 40,000 years and may
go back as far as 60,000 years ago. This would make the Aboriginal
culture of the tropical savannas one of the oldest on Earth.
What are the main ways people make a living in
Australia’s tropical savannas?
The main areas of employment can be summarised as: the Grazing
Industry, management of National Parks, Tourism, the Australian
Defense Force, the Mining industry, Aboriginal land use and the
Horticulture industry. There are also urban centres which require
infrastructure, goods and services and therefore generate
employment in areas such as local and state government, hospitals,
schools, and hospitality.
Until a few decades ago, cattle grazing was the main economic
base of the tropical savannas, accompanied by lesser, but more
intensive uses such as mining, agriculture or urban development.
More recently however, mining and tourism have become the dominant
economic industries, although their use of land area remains
relatively small. In terms of sheer numbers most people in the
tropical savannas live in the major towns where provision of
government and other services, tourism and defense are the major
occupations.
Across the vast landscapes that make up the rangelands of the
tropical savannas, there has been a rise in resource values and
land-use options other than traditional cattle grazing. These
include managing for biodiversity, and an increase in land
ownership by Aboriginal people. Many Aboriginal groups, however,
still want to carry on cattle grazing on land they have acquired.
The areas of recreation and tourism have been on the rise over the
past few decades, and tourism has now reached the point of yielding
a higher regional income than the cattle industry.
The map below shows the major land uses in the tropical
savannas.
