بیابان و صحراها-Desert
Desert
No precise definition of a desert exists.
From an ecological viewpoint the scarcity of rainfall is all important, as it
directly affects plant productivity which in turn affects the abundance,
diversity, and activity of animals. It has become customary to describe deserts
as extremely arid where the mean precipitation is less than 2.5–4 in. (60–100
mm), arid where it is 2.5–4 to 6–10 in. (60–100 to 150–250 mm), and semiarid
where it is 6–10 to 10–20 in. (150–250 to 250–500 mm). However, mean figures
tend to distort the true state of affairs because precipitation in deserts is
unreliable and variable. In some areas, such as the Atacama in
The deserts of the world; those within the
tropics are hot as well as arid.
The main desert regions of the world are
shown in the illustration. Most lie within the tropics and hence are hot as well
as arid. The Namib and Atacama coastal deserts are kept cool by the Benguela and
Humboldt ocean currents, and many desert areas of central
Plant
production
As a consequence of unreliable rainfall,
plant productivity is much more variable than in most ecosystems. It may vary
from none to 880 dry lb/acre · yr (1000 dry kg/ha · yr) although in most places
there is usually some productivity even when there is no rainfall. The average
biomass is generally low at 0.004–0.144 dry lb/ft2 (0.02–0.7 dry kg/m2) compared
to 9.25 dry lb/ft2 (45 dry kg/m2) in tropical forest and 6.17 dry lb/ft2 (30 dry
kg/m2) in temperate forest. Another feature of desert vegetation is the low
percentage of green, photosynthetic plant biomass. There is also about three
times as much dead as living plant material, a high figure compared to most
other ecosystems. See also: Biological productivity; Biomass
Between 10 and 20% of the living plant
material is eaten by consumers, a figure typical of terrestrial ecosystems,
where the level of consumption may be as low as 5% (compared to over 90% in
ocean ecosystems). Much desert vegetation tends to be hard and prickly and hence
unpalatable to many consumers. Dead plant material does not accumulate and is
utilized by the decomposers, particularly wood-eating termites. Seeds are
abundant and can survive a long time without germinating. Many animals,
including rodents, ants, and birds, depend heavily upon them for food. There is
evidence of both intra- and interspecific competition for seeds, the production
of which varies markedly with variations in rainfall.
A common viewpoint is that desert plants and
animals live in a harsh environment and as a consequence have evolved many
morphological and behavioral adaptations that enable them to “escape” the rigors
of their surroundings. Certainly extreme aridity coupled with extreme heat seems
a stressful environment, but whether desert organisms really possess more escape
adaptations than, say, rainforest organisms is a matter for conjecture.
Nevertheless, specific adaptations to drought and heat can be readily
identified.
Adaptations in
plants
In many deserts, annual rather than
perennial plants form the bulk of the climax vegetation, quite unlike the
situation in most terrestrial ecosystems where perennial plants dominate.
Deserts, especially those within the tropics, periodically “bloom” with
flowering annuals soon after a significant rainfall. In general, the more
unreliable the rainfall in a desert, the greater the abundance and diversity of
annual plants. Estimates vary, but these annuals account for up to half the
primary production in a year, although for a given site they may fail altogether
if there is no rain. Many annuals last for only a very short time. They tend to
have the C4 photosynthetic pathway, which means they grow quickly and
opportunistically. It is these plants in particular that produce the vast
quantities of seed which under extreme aridity can last for years, providing a
“seed bank” until conditions are right for germination. Most ephemeral annuals
do not have special drought-resistant or drought-tolerant adaptations: they are
simply opportunists, much like the annuals of wetter communities which, however,
tend to be successional rather than climax species. See also: Photosynthesis
Succulent plants occur in all deserts but
nowhere as conspicuously as in the
Other perennial plants include trees and
shrubs with long tap roots that can reach underground water, making them
independent of scarce and unreliable rainfall. Some perennials burst into leaf
only after rain, and are hence leafless most of the time. Yet others retain
leaves throughout dry periods; the leaves tend to be small and narrow and
require relatively little water to photosynthesize. These plants, mostly small
shrubs and grasses, invariably have an extensive root system which enables them
to maximize their acquisition of water. See also: Physiological ecology (plant);
Plant-water relations; Plants, life forms of
Adaptations in
animals
A majority of terrestrial invertebrates are
cryptic in coloration, matching almost exactly components of the background on
which they normally live. Many invertebrates and small vertebrates, such as
rodents and snakes, spend the day in holes and burrows, and become active at
night, when it is cooler. In all environments there are many small animals that
escape by hiding, in most places to escape from predators; but in deserts escape
from high daytime temperatures and radiation appears to be the chief adaptive
response. The burrow constructed by the
The largest of all birds, the ostrich,
Struthio camelus, is an inhabitant of arid regions of
Soon after heavy rainfall, holes and
depressions in sand or rock fill with water and within a few hours teem with
microscopic life, chiefly algae, bacteria, and protozoa. After a few days
countless small crustaceans appear, such as fairy and tadpole shrimps. These
grow and reproduce with great rapidity. They can tolerate high water
temperatures and also the high salinity which often builds up in temporary
desert pools. The pools soon disappear and the land may then remain dry for
months, even years, before the next downpour. The shrimps and other organisms
diapause in the soil, usually as eggs which are remarkably resistant to high
temperatures and extreme desiccation. See also: Physiological ecology (animal)
Aestivation
The term aestivation is used to describe a
lengthy period of dormancy during which metabolism is much reduced. Many desert
animals are capable of prolonged aestivation which enables them to survive
during periods of food and water scarcity. In insects such as butterflies, which
have a complete metamorphosis (clear-cut egg, larval, pupal, and adult stages),
aestivation can occur at any stage of the life cycle but is almost invariably
confined to a particular stage for a given species. Before entering aestivation,
most animals seek out a secluded place that provides the best protection from
temperature fluctuations and solar radiation, as well as from predators. Many
species, ranging from lungfish to insect larvae, construct some form of
protective cocoon in which to aestivate. See also: Hibernation and estivation
Migration and
movement
An alternative to aestivation is migration
or other movement away in search of better feeding and breeding conditions. Some
of the African antelopes, such as the oryx, Oryx gazella, undertake
long-distance movements which are correlated with changes in the quality and
quantity of grass upon which they feed. These grass characteristics in turn are
determined by rainfall, so there may be regular movements to and from areas
where the rainfall is predictable and regular, or much more erratic movements in
areas of unpredictable rainfall.
The semiregular migrations of several
species of African weaverbirds (Ploceidae) are strongly associated with the
seeding of wild grasses. The birds may form flocks of thousands or even tens of
thousands of individuals and descend on and devour ripening seeds before moving
on. Some of these birds, including the black-faced dioch, Quelea quelea, have
become serious pests of millet and other cereals grown on land irrigated from
underground water. See also: Migratory behavior
The blooming of desert annuals and the
leafing out of trees and shrubs soon after rainfall often result in a dramatic
increase in abundance of leaf-eating insects. Eggs that have remained dormant
for months produce larvae which can be so numerous as to defoliate vegetation.
In
One of the most successful groups of desert
animals is the grasshoppers. In many deserts there is a considerable variety of
species, some of which are abundant. A few species are able to build up suddenly
in numbers, often in response to unseasonal rain which has promoted a rapid
growth of vegetation. They then undertake long-distance irruptive movements,
followed by further breeding until they reach such numbers that they invade
higher-rainfall areas and devastate crops. One of these grasshoppers, the desert
locust, Schistocerca gregaria, can invade and seriously affect the vegetation of
an area extending through 110° of longitude from West Africa to
Species
diversity
The diversity of species of animals in a
desert is generally correlated with the diversity of plant species, which to a
considerable degree is correlated with the predictability and amount of
rainfall. There is a rather weak latitudinal gradient of diversity with
relatively more species nearer the Equator than at higher latitudes. This
gradient is much more conspicuous in wetter ecosystems, such as forests, and in
deserts appears to be overridden by the manifold effects of rainfall. Animals,
too, may affect plant diversity: the burrowing activities of rodents create
niches for plants which could not otherwise survive, and mound-building termites
tend to concentrate decomposition and hence nutrients, which provide
opportunities for plants to colonize.
Convergent
evolution
Each desert has its own community of
species, and these communities are repeated in different parts of the world.
Very often the organisms that occupy similar niches in different deserts belong
to unrelated taxa. The overall structural similarity between American cactus
species and African euphorbias is an example of convergent evolution, in which
separate and unrelated groups have evolved almost identical adaptations under
similar environmental conditions in widely separated parts of the world.
Convergent structural modification occurs in many organisms in all environments,
but is especially noticeable in deserts where possibly the small number of
ecological niches has necessitated greater specialization and restriction of way
of life. The face and especially the large ears of desert foxes of the Sahara
and of
Desert
community
Ecological change in deserts seems to occur
slowly. Plant and animal succession is much less obvious than in other
communities, although there is some evidence of cyclical change in which species
replace one another. In the
Denis F. Owen
Bibliography
-
G. N. Louw and M. K. Seely, Ecology of Desert Organisms, 1982
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G. M. O. Maloiy (ed.), Comparative Physiology of Desert Animals, 1972
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K. Schmidt-Nielsen, Desert Animals: Physiological Problems of Heat and Water, 1964
-
F. H. Wagner, Wildlife of the Deserts, 1980
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alifazeli=egeology.blogfa.com
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