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Adaptations: case studies
When looking at organisms in your local environment you will try to
identify features which can be interpreted as adaptations. The two
examples below may help you in your understanding of how organisms
are suited to their environments.
Case study 1: mangroves
Mangroves form woody plant communities in warm, shallow tidal water
over more than half of the Australian coastline. Twenty-nine species
occur along the tropical north coast of Australia, but the number of
species decreases further south. Around Sydney only two species are
found: the grey mangrove, Avicennia marina, and the river mangrove,
Aegiceras corniculatum. In South Australia and southern Victoria only
the grey mangrove is found. There are none in Tasmania.
Mangroves are found on sheltered muddy shores and along estuaries.
Depending on the conditions and the species, they range from 2 m high
shrubs to 30 m high trees.
Support and movement
Mangroves are upright woody plants. They are anchored by complex
root systems in the shifting environment of tidal mud flats. They have
vertical anchor roots to which are attached spreading cable roots.
Together these form a dense mass which helps stabilise the mud. Some mangroves support
themselves on ‘stilt’ roots that lift the plant out of
the salty water
Gaseous exchange
There is a lack of oxygen in the water-logged soil, which is exposed at
low tide but covered with water at high tide. Mangroves of the genus
Avicennia have aerial roots or pneumatophores which push upwards
through the mud and salt water. Their tips have pores or lenticels
through which gaseous exchange can occur
Control of water balance
Mangroves are halophytes (salt-tolerant plants). The water available to
them is salt water with a high ion concentration. They control their salt
level in three ways:
• accumulation: their cells maintain higher than normal concentrations
of cell solutes
• secretion: some mangroves have salt glands on their leaves which
actively secrete salt
• exclusion: some mangroves use energy to excrete salt.
Control of internal temperature
Mangrove leaves have a thick cuticle and are hard and leathery. These
features help to control water loss and prevent wilting in hot weather.
In dense mangrove swamps the thick canopy of leaves helps maintain a
lower temperature in the lower layers.
Obtaining light
Mangroves are the dominant plant in their community. The shape of the
plant and arrangement of leaves ensure abundant light is available for
photosynthesis.
The leaves are dark green because they are rich in chlorophyll. The
leaves high on the plant are angled and those lower down are horizontal
to best capture sunlight.
Reproduction
Mangroves have flowers and, following pollination and fertilisation,
fruits containing one seed develop. These seeds begin germination
before they drop from the parent plant (Figure 1.34). The seeds are
buoyant and dispersed by the tide. Their initial development,
particularly of the root system, ensures the new plant can rapidly
anchor itself and grow rapidly once it is deposited in the mud.
FIGURE 1.34
In some mangroves the
seeds germinate while still
attached to the plant.
When fully formed these
propagules drop from the
plant and spear into the
mud, where they begin to
grow.
(a) Ceriops,
(b) Bruguiera.
Case study 2: kangaroos
Members of the genus Macropus are all physically very similar. Larger
specimens (over 20 kg) are called kangaroos, and smaller species are
known as wallabies and wallaroos. Species inhabiting steep, hilly areas
rather than flat plains are usually called wallaroos. Few species are
solitary—most congregate in groups or mobs.
Kangaroos are widespread across Australia. They are all grazing herbivores
that feed on grasses and herbs.
The red kangaroo inhabits inland plains. The western and eastern grey
kangaroos are found in grasslands, eucalypt woodlands and open forests
from Tasmania to central Queensland and across to Western Australia. The
antelopine wallaroo is found in the tropical north of Australia from
Queensland across to Western Australia. The euro, or common wallaroo, is
found in central and southern Australia.
Support and movement
Kangaroos have an internal bony skeleton. Their well-muscled hind legs
are far larger than their forelegs (Figure 1.35). Only the hind legs are
used when the animal is travelling at high speed. This method of
bounding along is more efficient in terms of energy use when compared
with animals which run on all four legs.
When moving slowly, kangaroos ‘hop’. Their weight is pressed down
on their forelimbs and tail and their large hind legs are swung forwards.
FIGURE 1.35
The long tail of kangaroos and
their relatives is an adaptation that
helps the animals to balance when
hopping.
The long tail of the kangaroo is a useful structure. It is used as a
balancing counterweight when bounding, as an extra limb when hopping,
and it helps the kangaroo to remain upright when standing still.
Gaseous exchange
Kangaroos have lungs as internal respiratory surfaces.
Control of water balance
Kangaroos in arid areas, such as the red kangaroo and euro, can survive
for long periods without drinking water, provided there is sufficient
green plant material available. Kangaroos reduce water loss by sweating
only during exercise. When they stop moving, sweating stops.
Control of body temperature
Kangaroos that live in hot, dry regions seek the shade of rock crevices
and caves during the hottest part of the day. Kangaroos in other areas
will bask in the sunshine, but when conditions are hot they seek the
shade of trees and bushes.
When the weather is hot, euros may lick their forelimbs where the
blood vessels run close to the surface and heat is lost from the body. It
is thought that the evaporation of the saliva has a cooling effect.
Obtaining light
Kangaroos have binocular vision. They are mainly nocturnal animals but
may also be active in early morning and early evening.
Reproduction
Kangaroos are marsupial mammals. They have internal fertilisation and a
very short gestation period in the uterus. At birth the young climb into the
mother’s forward-opening pouch. They attach to a teat and continue
development while suckling (Figure 1.36). When they leave the pouch
there is a weaning period before parental care ends.
In the red kangaroo the young are born after 33 days in the uterus and
weigh less than a gram. They remain suckling in the pouch for 235 days.
They leave the pouch weighing 4–5 kg and have a weaning period of up to
4 months, during which they suckle and eat grass.
Kangaroo population numbers are controlled through reproduction.
Under good environmental conditions numbers can increase
rapidly, because female kangaroos can be almost continuously pregnant
when adult. They mate again directly after giving birth. If the
mother is still suckling her newborn young, the fertilised egg does
not develop until the young leaves the pouch. This is known as
delayed implantation. At any one time a female kangaroo may have
a joey (young kangaroo) being weaned, a young one being suckled in
the pouch and an embryo in the uterus awaiting development.
Kangaroos have the amazing ability to produce two kinds of milk at
the same time. The milk produced by the teat for the developing young
in the pouch contains much less fat than the milk produced by the teat
being used by the joey outside the pouch.
When environmental conditions are not good, such as in times of
drought, young joeys do not survive and any fertilised egg does not
implant. Females do not begin reproducing again until conditions improve.