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Surviving the dry

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.

As the Dry progresses, life contracts with the water and becomes concentrated around the swamps and lagoons which  retain moisture. Floodplains dry out and grasses become yellow and desiccated, inviting fire. Rock-hard black soils, once slushy mud, crack into geometrical patterns. Life is hard for many creatures. Some birds take the option of leaving the savanna lands for the wetter coastal areas. Other animals are not so mobile, but have developed strategies to deal with this time of hardship. Indeed, alternating periods of wet and dry may be vital to their lifecycle. For yet others, the Dry is a time of plenty when fish and other aquatic creatures, concentrated in diminishing pools of water, become easy prey.

Many of the reptiles which inhabit seasonally dry wetlands are adapted to withstanding periods of low food supplies. Crocodiles eat less when the temperature is lower, a time which coincides with the lean dry season. Arafura file snakes grow extremely slowly and have a metabolic rate which is lower than most reptiles.

Many frogs of the savanna lands survive dry periods by burrowing — usually backwards — into moist mud, using hard shovel-shaped structures on their feet. They then shed the outer layers of dead skin to form a cocoon around themselves. This hardens, functioning like a plastic bag to reduce evaporation, allowing the frog to remain underground until rain softens the earth around it. Since these frogs may be entombed half a metre or more beneath mud which has hardened to the consistency of a brick, it stands to reason that only very heavy rainfall — producing suitable conditions for breeding — will rouse the frog.

Breeding is an urgent affair which must be completed before the water dries up again so it more rapid than in other frog types. Males immediately start calling for mates, eggs hatch into tadpoles within hours and fully-formed frogs may appear within a week.

The northern bullfrog (left), which lives across the Top End, the Kimberley and north-west Queensland, is normally seen only after rain induces it to go above ground. It is one of the largest frogs in Australia, growing to 10 cm in body length.

Pig-nosed turtles live in the river systems of the Northern Territory. They are unusual turtles in that they have soft shells and paddle-shaped flippers rather like those of marine turtles. The females lay their eggs in sand banks during the dry season. The embryos inside develop within about 70 days but stay put, biding their time in a dormant state, until the first rains or rising floodwaters penetrate the nest. Then the hatchlings burst out of their eggs, all at once.

Some adult turtles, notably long-necked turtles (right), survive the dry season by burying themselves in the soft mud of a drying lagoon. Usually they burrow to a depth of 12 cm, some even lying head-down, with a 3–5 mm diameter hole acting as a breathing hole. The mud eventually becomes baked hard but the turtle, by going into a state of torpor (aestivation) similar to hibernation, is able to remain alive. Although invisible from above, the breathing hole and a slight bulging of the surface may indicate the turtle’s position — clues used by indigenous people for whom these animals are an important food source. They use long sticks to locate the turtles. Where mud is not available, some turtles aestivate among the roots of overhanging trees.

Crustaceans of various types are important wetland inhabitants. Although they breathe through gills and, unlike insects, have no waterproof ‘skins’, a large number of species are able to survive dry periods. Many of the small shrimps produce eggs which are drought-resistant. Some yabbies, on the other hand, are able to burrow into mud and encase themselves in a drought resistant egg-like case until wet conditions eventually soften the walls and allow the inhabitant to emerge.

Freshwater ‘sidewalker’ crabs dig burrows about a metre long, plugging the entrance with mud. Inside they go into a state of suspended animation, taking up water from the trapped humid air. Unlike marine crabs, the young have no free-swimming larval stage. They hatch as juveniles — but can be carried around under the mother’s body in an arrested state of development for months waiting for good rains. There is only one common species, Holthuisana transversa, which is widely distributed in central and northern Australia.

planingale

Deep cracks in parched clay and mud provide shelter for a number of animals, notably planigales (left) and rodents. Planigales are ferocious dasyurids — carnivorous marsupials — some of which are small enough to fit in a standard matchbox. Their flattened heads and sinuous movements allow them to fit into the tiniest cracks until flushed out by the first rains.

Dusky rats, found in the Top End, inhabit these cracks in huge numbers. They have been found at Fogg Dam, a wetland east of Darwin, in densities of 150 per hectare, which has been calculated as one tonne per square kilometre. They are preyed on heavily by water pythons which occur at levels of 700 per square kilometre. These two animals together create one of the greatest mass of animals (biomass) per square kilometre ever recorded anywhere in Australia, and perhaps the world.

Crocodiles can also dig themselves into mud to survive times of drought. The only alternative for a croc whose billabong dries up may be to attempt a hazardous overland journey in search of a wetter place — but then may find it already full of other crocs ready to fight to defend their patch. Many plants, such as water lilies, survive the dry by going into a ‘resting’ phase, allowing their leaves to die but staying alive as roots, tubers or rhizomes. Many also set seed, some of which are extremely long-lasting — one lotus seed is recorded as having germinated after 237 years.