Geoscience Reference
In-Depth Information
concluding that the pulse-reserve-response concept needed some modification to take
into account differences in nutrient availability, variations in antecedent soil moisture,
and differences in the composition and cover of different plant functional types.
The ability of certain desert grass seeds to remain viable for many years between
rains in extreme deserts, such as the eastern Sahara, has given rise to the phenomenon
of
gizu
grazing, in which the nomads of northern Sudan and southern Libya will take
their flocks deep into the desert during times when unusually heavy rain has led to
widespread plant germination and growth. In an original use of the dead remains of
such grasses, Haynes (
1989
) was able to calculate the rate of advance of a barchan
dune in the eastern Sahara from the known historic records of the rare rainstorms that
gave rise to the
gizu
grasses during the last century.
Fires lit by lightning during convectional storms at the start of the rainy season are
common in both tropical savanna andMediterranean environments. As a consequence,
many plants in those regions have adopted a variety of ingenious survival strategies,
including thick insulating bark in the case of Mediterranean oaks to rapid shedding of
burnt bark by eucalypts. Certain tropical grasses, such as 'spear grass' (
Heteropogon
contortus
) in northern Australia, seem to require the passage of grass-fires to ensure
more efficient seed dispersal. However, other types of vegetation are highly sensitive
to fire, including the remnant patches of monsoon rainforest preserved in isolated
localities in the monsoonal tropics of northern Australia. The arrival of humans into
these regions altered the fire regime and often caused considerable damage to trees
during the flowering season. Over time, humans also adapted and modified their
burning practices accordingly (Haynes,
1991
). But the prairies of North America
(and their dependent buffaloes) never survived the burning practices of the European
immigrants.
4.4 Adaptations of animals to life in deserts
The physiological and behavioural adaptations of animals living in deserts are
designed to avoid water loss and minimise heat stress (Evenari et al.,
1971
;Newby,
1984
). Two other stress factors prevalent in high-altitude deserts, such as the arid high
plains or Altiplano of Bolivia and Peru, and the arid uplands of Mongolia and Tibet
are extreme seasonal cold and limited supplies of oxygen at high elevations. Extreme
cold is also a feature of hot deserts during winter nights, when temperatures can drop
well below freezing point. Large mammals minimise heat stress in several ways. The
woollen coats of the llamas, guanacos and vicunas of the Atacama and Patagonian
deserts offer insulation against both hot and cold, as do the woollen coats of sheep,
goats and camels and the hairy coats of yaks. It is no coincidence that the ancestors
of these ruminants were among the earliest animals to be domesticated by the first
desert pastoralists. In addition, all of these animals are able to endure higher ranges
of body temperature than humans can without suffering undue heat stress.
