Download Arid lands in perspective-a biologist`s view

Annals of Arid Zone-21
(2), 98-103, 1982
Arid lands in perspective-a biologist's view
J.L.
CLOUDSLEy-THOMPsoN
Department
of Zoology,
University of London,
INTRODUCTION
Man and desert
have
long been
intimately related.
The desert has seen
the genesis of Christianity
and the birth
of the Moslem religion. It has witnessed
savage warfare throughout
the ages and
has invoked
both the worst and the
best in human nature. In his speech
before the Battle of the Pyramids, in
1798, Napoleon
Bonap-Jrte said: 'Soldiers, consider that frol11 the summit of
these pyramids,
forty
centuries
look
down upon youl• He was not without
a due sense of history.
The ecologist
must also be aware of history and evolution, as well as of animals and plants,
soil and climate, day and night, summer
and winter.
My own introduction
to the desert,
in 1941, was perhaps scarcely less dramatic than that of Napoleon's
privileged
sol diers.
A fleeting vision-from
the
window of the troop train that bore us
westward to the war-of
a gaunt camel
beside a clump of palms silhouetted
against a sea of silvery sand; the landscape bathed in a flood of brilliant light
from a crescent moon hanging in the
velvet star-spangled
sky. A cool breeze
giving little hint of the blazing heat of
the days to follow,
when the Stuka
Birkbeck College,
London (U.K.)
dive-bombers
would
scream towards
us out of the rising sun, their bombs
hurtling earthwards
like vicious black
eggs to explode with deafening crashes
and clouds of yellow and black smoke.
The rallle of Bren machine-guns,
their
barrels almost red-hot
as we replied
with magazine after magazine-burning
vehicles, crashed planes, the squat grey
Panzers of the Deutsch Afrika Korps
firing high-velocity
shells-and
the
sinister,
square, black screens of the
deadly 88 mms that
wrought
such
havoc among our Iightly-armoured
Crusader tanks.
All these made an unforgettable,
but by no means inappropriate, introduction
to a pitiless environment
in which
human
warfare,
throughout
the ages, has matched
the
severity of the incessant
struggle
for
survival of every animal and plant.
Adaptation
of plants and animals to
the harsh desert environment
is a matter
of trtmendous
interest to biologiits.
Another
is the problem
of desertification, a major
world-wide
phenomenon.
The rise of technology
has
changed the face of the earth: it has led
to enormous increases in the possession
of material goods, and to the squandering of the
world's
resources
at. an
ARID
LANDS
alarming
rate.
Even the peoples
of
developing
countries-many
of whom
have never enjoyed, and probably never
will enjoy, a full meal-are
deeply
affccted by the current
population
explosion. Nowhere is this more apparent
than
in the arid lands, where the
deserts are expanding at an ever-increasing speed, largely as a result of Man's
misuse of the environment.
Such is
the vast problem
to which Dr. H.S.
Mann and h is Co \leagues of the Central
Arid Zone Research Institute, Jodhpur,
are devoting so much or their time and
energy.
On the other hand, it is the
topic to wh 'ch are directed many of the
contributions
to this
special 'Felicitation Number', so this article will be
restricted to environmental
adaptations.
Informations,
based
on research,
is
essential to the rationale
of planned
development.
Such information
is also
interesting in its own right. and gathering it can be a source of pleasure and
excitement.
To the biologist, as already
mentioned,
the desert
biome is especially
intriguing
on account
of the
marked
specialization
of its flora and
fauna resulting from the intensity of the
struggle for survival.
WATER
CONSERVATlON
The desert environment
is inhospitable to life for many reasons, but chief
among these is shortage of water.
Not
only is lack of rain the primary cause of
desert conditions,
and the absence of
clouds
responsible
for
extremes
of temperature,
but low humidity
itself
has an adverse
effect on plants and
animals because the rate of evaporation
is so great,
especially
at high tempe-
IN PERSPECTIVE
- A BIOLOGIST'S
VIEW
:
99
ratures.
Desert plants
are able
to
survive by virtue of complicated combinations
of physiology,
anatomy
and
life history.
Annuals
evade more extreme conditions
by completing
their
life-cycles during the short rainy season, and passing the remainder
of the
years as fruits or ..seeds lying dormant
in the soil.
Many desert animals likewise, withstand the dry season in diapause-a
state of suspended
development or
growth,
accompanied
by
greatly decreased metabolism.
Thus, a
vernal rain fauna appears at the time of
inflorescence,
when the desert is transformed by an abundance
of plant and
animal life.
Flowers
are visited by
butterflies
and moths,
bees,
wasps,
hover-flies, bee-flies and other Diptera.
The droppings of camels and goats are
rolled away by dung beetles, and grass
seeds are harvested by industrious
ants.
Termites
extend
their
subterranean
galleries to the soil surface and indulge
in nuptial flights while predators,
such
as scorpions, camel-spiders
(Solifugae),
spiders, ant-lions, bugs, wasps, robberflies and predatory
beetles, glut themselves on an abundance of food.
The importance of a limited season
of plant growth
in arid environments
is reflected in the rutting and birth of
various species of gazelles which calve
about one month after the onset of the
rains when plenty of grazing is avaIlable.
In contrast to most domesticated animals
the camel, too,
has a
pronounced
rutting
season at the time
of rainfall and its pregnancy
lasts for
12 months.
The fertility
of jerboas,
voles and other rodents is interrupted
100 :
J.L. CLOUDSLEY-THOMPSON
during dry weather and, at this time,
the population
level drops considerably.
Drought-evading ephemerals of the
desert are not true xerophytes-they
are
really mesophytic because their activity
takes place only wben moisture is
available. To some extent, tbe same
may be true of animals, but usually
even those tbat aestivate show some
morphulogical and physiological adaptations to aridity. Truly xerophytic
plants, and the animals of arid environments, show many striking similarities
to one another in their responses to tbe
rigours of drought and heat.
Setae,
hairs, scales and other structures that
create a boundary layer red ucing transpiration and the flow of heat from tbe
environment are common to both.
Plant stomata and insect spiracles are
similar in the possession of a complex
passage which resists the diffusion of
water vapour. Cuticular transpiration
in both plants and arthropods is reduced by the presence of waxes having
high melting points, and both plants and
animals may possess mechanisms for
the excretion of surplus salt, the uptake
of moisture from unsaturated air, tbe
retention of metabolic water, and so
on.
Surface-to-volume relationships are
an important
factor in determining
transpiration and heat flux relationships
between
plants,
as well as
animals, and their environments. Large
succulents-euphorbias
and cacti-heat
up more slowly than small-leaved
desert shrubs but the latter never reach
such high temperatures because transpi-
ration is less even from their relatively
small, Larrow leaves. Similarly, desert
woodlice, such as Hemilepistus spp., are
not only unusually large but are particularly resistant to water loss by
transpiration through tbe cuticle when
compared with species from more mesic
environments.
Cuticular permeability
is exceptionally low in unfed desc:rt
ticks
and
the
critical
transition
temperatures of tbeir epicuticular waxes
extremely high. Similarly, the permeability of insect cuticle is particularly
low among dt:sert species.
Plants and animals adapted to hot,
dry, environments
often ~how an
enhanced ability to survive drought.
Leaves of the creosote bush Larrea
divaricata may tolerate desiccation so
extreme that they become brown and
brittle, and yet are capable of recovery.
The Saharan solifugld Ga/eodes granti
can withstand a loss of two-thirds of
its body weight, the gecko Tarentola
annularis one of 35 per cent, while the
camel is unique among mammals in
also being able to tolerate a loss of
about one-third of its body weight.
THERMAL
ADAPTATIONS
Membranes permeable to oxygen and
carbon dioxide are also permeable to
water vapOUr. The air leaving any photosynthetic or respiratory surface is normally saturated with water vapour and
some loss of moisture is therefore inevitable. While the function of transpiration is to limit heat stress, the attainment of this objective conflicts with the
need for water conservation.
In desert
regions, where water shortage is acute,
ARID LANDS IN PERSPECTIVE-A
BIOLOGIST'S VIEW : 101
the balance between these two incompossess this useful facility, but beetle
patible requirements almost invariably
larvae may do so. Even so, none of
swings in favour of water conservation.
these animals can afford to utilize
The logical consequence of this is that
evaporative cooling for purposes of
all the smaller animals avoid excessive
thermoregulation.
Indeed, of those
heat by their behavioural responses,
animals
that
are day-active,
the
while larger mammals such as camels,
majority seek shade when the sun
gaze lIes, eland, oryx and ad dax ante- is hottest
or orient their bodies
lope, as well as the ostrich, tolerate hy- so that the least possible area is
perthermia during the day, storing exce- exposed to insolation. In this, locusts,
ssive heat which is dissipated at night.
lizards and camels show a thermal
In contrast, xerophytic plants are unable
response in common with that of
to avoid excessive insolation,
but sur- wilting plants whose leaves droop so
vive by the development of excessive
that their flat surfaces are no longe'r at
rooting systems which increase the up- right angles to the rays of the sun.
take of water. In succulents such roots
Just as many desert plants and
spread laterally but are seldom more
animals can to lerate extreme desiccathan 3-4 cm below the surface of the
tion, so can they survive unusually high
soil so that they exploit to the maximum
temperatures that would be lethal to
every shower of rain even if water does
their relatives
from
more humid
not penetrate far into the ground. Nonregions. Not only do the larger homesucculent desert perennials, on the
otherms tolerate voluntary hypertherother hand, often have extraordinarily
mia, as already mentioned, but many
deep tap roots-those
of the mesquite
arthropods
survive
extremely high
Prosopis velulina
aod of Acacia spp.
temperatures for 24 hours or more at
extending over IS m to reach a water
very low humidities. Examples include
table far beneath the soil surface.
Galeodes granti 50°C, the scorpion LeuiJust as plant roots are able to
rus quinquesfriafus
47°C, and various
absorb water from damp soil, so many tenebrionid beetles 43-46°C.
desert arachnids
are able to absorb
Smaller animals cannot afford to
capillary
moisture from damp sand.
expand water for evaporative cooling
Some desert arthropods
also demonsbut, in general, they do not need to do
trate their natural superiority by taking
so because they escape from the midday
up moisture
from unsaturated
air.
heat by retiring into shady places or
These include mites and ticks (Acari),
cool burrows. Desert rodents do not
bristletails
(Thysanura),
book-lice
sweat, but they possess an emergency
(Psocoptera), fleas (Siphonaptera), the thermoregulatory
mechanism and proAmerican desert cockroach (Arenivaga
duce a copious flow of saliva in resinvestigata) and a few other wingless
ponse to heat stress. This soaks the
insects. Neither
termites (Isoptera)
fur under the chin and throat, provinor adult desert beetles appear to
ding temporary
relief when body
•
102 :
J.L.
ClOUDSLEY-THOMPSON
temperatures
Some reptiles,
approach
especially
lethal limits.
tortoises, also
employ
thermoregulatory
salivations.
In addition,
tortoises
discharge urine
over the back legs when their temperatures are dangerously
high.
The function of the large bladder
of desert
tortoises
has long puzzled naturalists.
We now know the answer:
urine is
stored not only as a defence against
predatory
enemies but also for emergency cooling.
Other
morphological
adaptations
of animals to desert conditions
include
counter-current
heat
exchanges.
Although
the body
temperatures
of
gazelles, for instance,
may reach 46°C.
the blood that supplies
the brain· is
cooled by means of heat exchange
in
the carotid
rete, a network
of small
blood vessels in the cavernous
sinus.
This sinus is fitled with venous blood
that drains from the nasal
passage,
where it has been cooled by evaporation
from the moist mucous membranes.
In
a defert gazelle the brain temperature
may thus be nearly 3°C lower than that
of the blood
in the central
arteries.
Counter-current
heat shunts are important in reptilian thermoregulation.
They
help to maintain
core temperatures
at
an optimum,
both during
hot periods
of the day, and when cooling
takes
place at dusk so that activity can be
prolonged
into the night.
:Most so-called
'desert
of animals-such
as webbed
adaptations'
feet and toes
fringe~ with elongated scales, the pointed rostrum and modified ears and eyes
of desert
lizards-are,
in fact, specific
modifications
for
living
in
sand.
Adaptations
of plants to life in deserts
include sunken
stomata, small, waxy
leaves, and the water-storage
tissues of
euphorbias
and cacti.
DEFENCES
At the beginning
of this article,
I
emphasised the severity of struggle
for
survival imposed
on desert plants and
animals.
Climatic
extremes
of arid
environments
are countered
by behavioural
means,
by physiological
adaptations
that reduce water loss or
conserve
moisture,
and enhance
the
toleration
of
high
temperature.
Predation has also played a vital evolutionary
role.
Many desert plants are
thorny
as an adaptive
response
to
intensive browsing, while desert animals
are almost always either sand-coloured
and cryptic, or black.
The function
of
cry psis in deserts is nearly
al ways
one
of
defence:
among
spiders
and insects it is invariably a response to
predation.
Black
coloration,
on the
other hand, has an aposematic functionunless it occurs on black sand and lava
flows where it may be cryptic.
It may
also be an evolutionary
legacy : if
so, it presumably had an adaptive function at one time. White may be cryptic
on white sand or aposematic on darker
substrates.
The thermal significance of
colour
is slight.
CONCLUSION
In this brief essay it has been possible to review only the major adaptations
of pllnts
and animals
to the
rigours of desert life.
Nevertheless,
I
have tried to show how the inter-related problems of heat and drought have
ARID LANDS IN PERSPECTIVE-A
resulted in comparable modifications of
the physiology and morphology
of
both plants and animals. The desert is
a relatively simple biome and the problems it poses for the organisms that
inhibit it are relatively few. At the
same time, however, climatic extremes,
BIOLOGIST'S VIEW:
103
intra-specific competition
and
food
shortage have engendered such intensity of selection that extremes of environmental adaptation have been evolved. Herein lies a subject of major
interest to biologists.