From Tropical Topics newsletter, No. 68
June, July 2001, produced by Stella Martin at the Queensland
Environmental Protection Agency. Download the PDF to read the whole
issue.
Many introduced plants and animals find savanna conditions ideal
and are doing considerable damage to the wetlands. Flowering water
lilies (Nymphaea spp) make a spectacular sight during the
dry season. There are a number of species, with flowers ranging
from white to pale blue and pink. Although they appear to float on
the water surface, these plants are actually anchored to the
bottom, up to 3 metres below. Seeds are contained within spongy
berries. As they develop, the stalk forms a coil which pulls the
fruit below the water, a strategy which may protect them from being
eaten. Aboriginal people eat the fruits and stems raw and bake the
tubers, which are considered a good medicine for diarrhoea. They
also bake the oily seeds, eating them whole or grinding them to
make damper.
Lotus lilies (Nelumbo nucifera), although they similar to
water lilies, belong to a different family. Their immaculate leaves
can grow from 50cm to nearly one metre in floating on the surface
or held above the water. The short lived flowers, which are also
held high above, open at first light but drop their petals by
midday. They are a spectacular dark pink with dense yellow stamens.
Curiously, it seems that the plants can regulate temperature of
their flowers, keeping it within the 30–35oC even
when surrounding temperatures drop as 10oC. This may
serve to attract pollinating insects. are embedded in distinctive
conical woody receptacles. These plants are an important source of
food for Aboriginal people, who use them in a similar way to water
lilies. They are also grown and eaten widely across Asia.
Revered as a symbol of purity in Hinduism Buddhism, the lotus
has leaves which never get dirty. The trick to this has recently
been discovered by German scientists and adapted to create self
cleaning paint for building exteriors and roof seems that the
secret is a combination of chemical repulsion and the bumpy, waxy
surface of This provides no flat surface for water to cling each
drop rolls off. Dirt is more strongly attracted the surface tension
of the water than to the is 'captured' by the water and carried
away.
Water snowflake (Nymphoides indica) is one of the most
commonly seen aquatic plants, easily recognised by the delicately
fringed petals of its white flowers. The related wavy marshwort,
(Nymphoides crenata) has similar flowers which are
yellow.
Pandanus of various species line wetlands, many of them fruiting
during the dry season. These plants are important for Aboriginal
people. They serve as seasonal indicators — the
appearance of the fruit signals the dugong-hunting season in some
areas — and are used for food, medicine and materials.
Leaves, with thorns removed, are useful for making baskets, mats,
ropes, dilly bags and so on while the seed kernels within the fruit
can be eaten, baked or raw.
Magpie goose and goslings Photo: Peter
Whitehead
|
Magpie geese rely on two main habitat types and two main food
plants. During the breeding season, from March to June, the adults
frequent patchy grass and sedgelands where they, and their young,
feast on the seeds of wild rice (Oryza meridionalis). As
these areas begin to dry out, the geese move to swamps where
bulkuru (Eleocharis dulcis) grows. Also known as water
chestnuts, these plants produce starchy tubers in large numbers;
one site studied averaged five million bulbs per hectare. From July
to September the geese shuffle through the sticky mud, digging up
the tubers with their hooked bills and gorging themselves on this
high energy food. As the wetlands continue to dry out the geese
congregate in ever-increasing numbers on the remaining swamps. The
late dry season is a lean period, when food supplies run low, but
when the rains come the birds are able to feed on young shoots of
grass until the next crop of wild rice seeds is ready. Magpie geese
were once widespread across Australia but alterations to wetlands,
poisoning and shooting have led to their disappearance in southern,
temperate areas and they are now found only in the tropics.
However, their wetlands are under threat in the north, with
introduced plants such as para grass and olive hymenachne rapidly
filling up many magpie geese feeding sites, rendering them useless
to the birds. Although the geese do sometimes eat grass, it does
not provide the necessary nutrients in the long term. They need
seeds and tubers.
 |
File snakes are remarkably loose skinned snakes with a flabby
appearance. Their common name comes from the pointed scales which
give their skin a very rough texture. These snakes live in water,
where they are very agile, but they are really quite helpless on
land. They can remain submerged for long periods with their
nostrils closed by a flap of skin on the roof of the mouth.
Nonvenomous, file snakes are an important food for Aboriginal
people. There are two species. The little file snake lives mainly
in estuaries and mangroves, while the larger Arafura file
snake (left) ventures further inland and is most commonly
found in freshwater streams and lagoons. These snakes have a habit
of wrapping their tails around underwater logs or roots to use them
as an anchor while striking at passing fish, the chief item on
their menu.
Merten's water monitor is at home in the water, feeding mainly
on fish, freshwater crabs, frogs and carrion. Growing to about one
metre in length it is usually seen basking on rocks, logs and
branches overhanging creeks or swamps, but quickly drops into the
water when disturbed. It is a good swimmer, its long vertically
flattened tail providing propulsion. Nostrils placed high on its
snout mean that, like a crocodile, it can remain submerged with
just the tip of its nose above the water. Underwater, it uses its
long body and tail to herd fish into the shallows where they can be
easily snapped up.
Saratoga are found in clear, slow-flowing or still waters around
Cape York, the Gulf of Carpentaria and as far west as the Adelaide
River, east of Darwin. They are able to tolerate low oxygen levels
and are suspected of using their swim bladders as
‘lungs’ for an extra supply. They can grow to almost a
metre in length and to over 10 kg in weight.
Saratoga have an interesting and ancient history, belonging to a
once numerous group of fish with a fossil history dating back 55
million years. There are now only eight species of saratoga left in
the world, two of them in Australia. Since the others are found in
Africa, South America and Southeast Asia, saratoga are seen as
evidence that these continents were once joined in the
supercontinent, Gondwana. While most freshwater fishes evolved from
marine ancestors, saratoga and lungfish are among the very few
species, still in existence, which evolved in freshwater.
Saratoga have bony mouths and are the only fish known to have
bony tongues. It is surprising, therefore, that they are mouth
brooders. The female lays between 30 and 130 eggs, each the size of
a marble, which she carries in her mouth for five to six weeks
until they hatch. (She doesn't feed during this period).
Afterwards, the young fry stay close to her head for a similar
period, darting back into her mouth if in danger.
Long-necked turtles, also known as snake-necked turtles, have
elongated necks which can be tucked in under the front edge of the
shell in a sideways folding motion. Together, the head and neck can
be as long as the shell. Sometimes concealing themselves in
sediment at the bottom of a waterhole, these turtles ambush prey,
using their long necks to strike out, with speed and accuracy, and
sucking fish, crustaceans and molluscs into their large mouths.
Normally, inundation would be expected to destroy any turtle or
crocodile eggs but some northern long-necked turtle mothers
deliberately deposit theirs under the water. Development does not
begin until the water evaporates in the dry season, leaving them
high and dry (but buried under sediments). Experiments with the
eggs have shown that they can develop quite normally even after
they have been immersed in water for up to 12 weeks. This strategy,
which may have developed in low lying situations where there is
little land above water outside the dry season, may help to protect
the eggs from terrestrial predators. Researchers, keen to
investigate the rumour, finally proved it by catching a pregnant
female, inserting an egg-shaped radio transmitter into her ovaries
and, a few days later, found the nest which was buried in sediments
in 15 cm of water. When the water evaporated, the eggs
developed.
Shorter-necked turtles include aquatic plants and fruits, such
as pandanus in their diet. Indeed, adults of the northern snapping
turtle, which is found in rivers throughout northern tropical
Australia and the east coast of Queensland, are almost entirely
vegetarian although they prefer meat in captivity.
The Gulf snapping turtle was known only from a fossil 25,000
years old, found at the Riversleigh fossil site—until a live
one was discovered in 1995 in one of the rivers draining into the
Gulf of Carpentaria. This 'living fossil', which is one of the
largest freshwater turtles in Australia, is very similar to the
northern snapping turtle.
Darters can be seen in and around most wetlands. In common with
closely related cormorants, but unlike most other waterbirds, they
have plumage which absorbs water allowing them to sink below the
surface. Often only the bird's long S-shaped neck can be seen, the
reason for its other common name, snake bird. Although large webbed
feet allow a darter to swim strongly, it does not chase its prey
under the water, but instead stalks it or waits until it comes
within reach. It then shoots out its long neck, specially equipped
with hinged vertebrae for speedy movement, and spears it with its
long dagger-like bill. Fish are its main food, but it also eats
small turtles, aquatic insects and plants.
 |
Darters have a habit of standing on branches with their wings
outstretched. It has always been assumed that this is to dry out
their waterlogged plumage. However, studies of cormorants, which in
a similar way, suggest that it also aids digestion. Experiments, in
which cormorants were given cold fish and some were given warm fish
showed that the spread and flapped their wings more. The theory is
that the flapping wing produce heat which in turn, warms the fish
in the bird’s stomach.
In other parts world female darters are dark, like the males.
Australian female darters, however, a pale front with a grey-brown
back.
Danger
Although wetlands in the dry season, with their tranquil waters,
delicate water lilies and dabbling waterbirds are the epitome of
peace, this can be deceptive. Wherever there is high productivity,
there are large appetites and most animals need to be wary. Both
freshwater and estuarine crocodiles are liable to turn up wherever
there is water in northern tropical Australia. Young ones feed on
insects, crustaceans and frogs, the type of prey changing as they
grow. Adult freshwater crocs eat large amounts of fish but food
from terrestrial origins, such as invertebrates, birds, and small
reptiles and mammals make up as much as 40 per cent of their
diet.
