Wetland Wildlife 3: Animals of the Wetlands

WETLANDS WILDLIFE 3: Animals of the Wetlands

Saltwater crocodile (Crocodylus porosus), Yellow Waters (Ian Morris)

Insects of the wetlands

Insects rarely attract the attention they deserve in conservation management programs. They have a poor public image, and indeed are often considered merely as pests. However, if the aim of conservation is to maintain biological diversity, then attention should focus on insects because they constitute more than 90% of all animal species. Similarly, if the aim of conservation is to maintain the ecological integrity of ecosystems, then attention should also focus on insects because of their fundamental roles in ecosystem structure and function. (Press and others, 1995).

There are an estimated 10,000 species of insects in the Top End wetlands. They are close to the bottom of many food webs and support either, directly or indirectly, most of the larger and more well-known animals of the region. Insects are one of the major components of the wetlands National Estate.

In northern Australia there are no big herbivores of the grasslands as in other continents -  the wildebeest of Africa, the bison of America. Here, the grazing role is taken over by insects, mainly grasshoppers. Termites supplement the grasshoppers' activities by playing a major part in recycling nutrients, and about 50 species of termites have been recorded from the Top End wetlands and bordering woodlands. The Cathedral Termite (Amytermes) builds the largest mounds, and termite mounds are important nesting sites for many mammals, birds and reptiles of the region.

Figure 1. Aquatic insects of the wetlands

Representatives of the major groups of aquatic insects are remarkably similar the world over. The dragonflies flying over a pond in Australia are very similar in appearance to the ones seen in America, Asia or Europe and the same applies to other groups of aquatic insects. A number of aquatic insects of the wetlands are unknown outside the Top End. Fifteen of the dragonfly species alone are endemic (confined to the region).

The Top End insects have, in many cases, adapted to the short season of plenty by developing rapid life histories. They quickly pass through the egg, larval and pupal stages, to become adults before the wet season is finished, an example of how nonliving environmental factors, such asrainfall, temperature changes, soil texture, can affect the organisms within an ecosystem.

The list (Figure 1) is not a complete inventory of Top End aquatic insects. There are a number of different beetles that are closely related to the diving beetles: some are much smaller and others, not truly aquatic, may be found in the mud at the water's edge. There are four families or different types, of water striders or pond measurers. Some aquatic insects differ not so much in appearance as in habitat preferences, with some inhabiting the saline waters of the estuaries, some freshwater billabongs,  and others the rivers.

Other invertebrates of the wetlands

Besides insects there are a number of other invertebrates that live in freshwater habitats. They include hydra (12 mm long), freshwater relatives of sea anemones and corals, who capture their prey with stinging cells. Freshwater snails graze stone surfaces and the larger molluscs, such as Freshwater Mussels (Velesunio angas), feed on the organic matter in the detritus (fine sediments) on stream beds.

Crustaceans in the Top End include microcrustaceans, such as Daphnia spp. and copepods, small freshwater shrimps, Long-armed Freshwater Prawns (Macrobrachium spp.), Freshwater Crabs and Redclaw Yabbies (Cherax quadricarinus). There are also a variety of worms: small black flatworms 15 mm long (planarians), true worms and free-living roundworms.

Like the insects, these invertebrates are important links between the energy producers - plants ang green algae - and the larger animals of the wetlands. They are also important indicators of the health of a wetland and are used by Waterwatch to determine the quality of the water.

Frogs

Frogs are associated in people's minds as being part of wetlands life, and in the Top End they are present in considerable numbers during the wet season only to disappear from sight during the dry season. Although equipped with lungs, frogs also take in oxygen through their skins that have to be moist for the process to take place. This method of obtaining a supplementary supply of oxygen is a useful adaptation for aquatic environments, but in the wet/dry tropics it can be a handicap and frogs have had to evolve strategies for seeking refuge in the dry season. The frog's physiology is such that when the humidity reaches a low point, it automatically seeks shelter. This explains the lack of frog numbers in the dry season, even around permanent waterholes. Each species of frog has its own preference for a place of refuge, and this spatial separation avoids unnecessary competition.

Green tree frog (Litoria caerulea)

Several frogs, such as the Ornate Burrowing Frog (Platyplectrum ornatus), spend the dry season deep in the soil aestivating, emerging in the wet season to breed. Others, including the Roth's Tree Frog (Litoria rothii) and Marbled Frog (Limnodynastes convexiusculus), find refuge in muddy sites. The large Green Tree Frog (Litoria caerulea) favours hollow branches, and breeds in pools of static water when the wet arrives. 

All frogs are equipped with powerful hind legs that act like z-shaped springs for hopping and swimming, but their feet differ according to their habits. Tree frogs have slightly flattened fingers and toes that are tipped with flat adhesive discs to assist in climbing, and their feet have more webbing for swimming. Burrowing frogs have cylindrical fingers and toes and the foot has a cutting edged tubercule (lumpy growth) on the sole to help in digging.

Frogs catch insects by uncoiling a long tongue that is folded and attached by strong muscles behind the tip of the lower jaw. This adaptation to capture fast moving prey with a whip-like tongue action is efficient, but frogs may also seize larger prey in their mouths. Food taken by frogs varies from species to species, but when the opportunity arises to make use of a plentiful food resource, many species will make the best use of that opportunity. The swarming of mayflies in the late wet and the increased numbers of grasshoppers in the late dry are two examples of how changes in insect populations can affect the food intake of frogs. Recorded food sources are termites, bugs, spiders, beetles, ants, springtails, flies, caterpillars, grasshoppers, moths, butterflies, mayflies, centipedes, cockroaches, small bats and other frogs.

Frogs are prey to a variety of birds such as Glossy, Sacred and Straw-necked Ibis (Plegadis falcinellus, Threskiornis aethiopica (molucca) and T.spinicollis), Black-necked Storks, or Jabiru (Ephippiorhynchus asiaticus), Cattle, Great, Intermediate and Little Egrets (Ardea ibis, A.alba, A.intermedia and A.garzetta), Rufous Night Herons (Nycticorax caledonicus), and Blue-winged Kookaburras (Dacelo leachii). The Keelback (Tropidonophis (Amphiesma) mairii), Macleay's Water Snake (Enhydris polylepis), Slaty-Grey snake (Stegonotus cucullatus), Northern Snake-necked Turtle (Chelodina rugosa) and juvenile Water Pythons (Liasis fuscus (Bothrochilus)) are reptile predators. Many fish species are attracted to the swarms of tadpoles that gather on the floodplains in the wet. Some aquatic insects, such as the water tiger (larva of the diving beetle) and water scorpions also take tadpoles. Other predators of adult frogs include the Northern Quoll (Dasyurus hallucatus), Ghost Bats (Macroderma gigas) and Freshwater Crocodiles (Crocodylus johnstoni).

The onset of the wet season triggers the start of the breeding season, as most species are dependent upon water for breeding. Almost all breeding takes place in temporary water, because permanent waterholes and billabongs have resident populations of fishes that will eat eggs, tadpoles and adults. The onset of the wet provides a variety of temporary breeding sites: small ponds, puddles, potholes and the margins of the floodplains themselves. Tadpoles have gills and as they grow develop the lungs of an adult. The higher the temperature, the shorter the period for eggs to hatch, and for tadpoles to metamorph into adults. This is beacuse the high temperatures increase the chance of temporary waterbodies drying up - killing the resident eggs and tadpoles.

The Frogwatch NT website (http://www.frogwatch.org.au) can be used to help identify frogs from all around the Northern Territory.

Larger Animals: Fish, Birds, Reptiles and Mammals

The following information gives an overview of the main groups of animals of the wetlands and the range of biodiversity that exists within each group. Individual animals will be treated in more detail when we discuss the different habitats of the wetlands in the next section.

The illustration of a typical Top End waterhole or billabong (Figure 2) shows how a number of reptiles, fishes, bird and mammals of the wetlands share the same habitat and environmental conditions. The activities of each is related to the presence and activities of other animals within the ecosystem, and a natural balance is maintained so that no one organism can totally crowd out or eliminate others. The exception to this rule is when an introduced animal or plant, like Salvinia molesta, with no natural enemies, is brought into the ecosystem.

Figure 2. Life in a Top End billabong

The wetlands birds are divided into those which enter the water, such as ducks, cormorants, pygmy geese, and those which wait on the margins for food, like kingfishers. There are 289 bird species in the wetlands and fringing woodlands of the Top End, including twelve species of ducks and thirty-five other waterbirds.

Fish of the wetlands include a number of small but colourful species, which maintain an energy flow between small aquatic insects and larger animals of the wetlands, such as Barramundi (Lates Calcarifer), Saratoga (Scleropages leichardti) and Egrets. There are approximately 55 freshwater fish species in the Top End waterways.

Merten's Water Monitor (Varanus mertensi), (Ian Morris)

Some reptiles of the Top End wetlands have been mentions already, and their diversity of species, with thirty-nine snake and seventy-six lizard species, allows for a number of specialisations in habitat. An example of this is the Merten's Water Monitor (Varanus mertensi), which is able to stay underwater for several minutes searching for shrimps and small fishes. 

The Water Rat (Hydromys chrysogaster) is the only Australian mammal, besides the Platypus (Ornithorhynchus anatinus), that is fully adapted to an aquatic environment. It lives in a burrow in the river bank and has partially webbed hind feet. There are also twenty-eight species of bats that use a number of habitats for roosting sites, from paperbark swamps to rocky gorges. Other mammals, such as the Dusky Rat (Rattus colletti) and Pale Field Rat (Rattus tunneyi) move onto the floodplains and banks of billabongs during the dry season when the waters have receded. Wallabies, wallaroos and dingoes.