Wetlands 3: An Environment Subject to Change

WETLANDS 3: An Environment Subject to Change

South Alligator River, Kakadu National Park (Ian Morris)

In the previous section (Types of Wetlands in the Top End), you will have gained an overall impression of the variety of wetlands that exist in the Top End and of the temporary nature of many of them. The changes that can occur are dramatic. In the flat area of the Top End, floodwaters tend to spread out over large areas. When heavy rains fall in their catchment areas, places like, for example, the Barkly Tableland have wide expanses of water thronged with waterbirds. Several months later in the late dry season they are parched expanses of dry, cracked mud.

In considering wetland life it is necessary to determine the extent of change and the regularity of the change brought about by seasonal weather patterns. Various types of wetlands can be described as:

• having regular changes in water levels on a 24-hour basis, such as mangroves and estuaries
• having irregular rainfall throughout the year, such as tropical wet-dry wetlands which experience six months of no rain and six months of heavy monsoonal rains
• having changes in water levels dictated by human needs and not subject to seasonal influences (dams and irrigation systems)

Many water sources in the Top End, such as paperbark swamps and floodplains, only have surface water in the wet and early dry seasons. In the late dry season all water drys up. Consequantially, many wetland plants and animals will not only be adapted to an aquatic life (e.g. moving in water, gaining oxygen) but will have also developed strategies for dealing with periods when their habitat is reduced in area or is completely dried up.

In the Top End wetlands some animals are escapers. These animals move with the receding waters. Magpie Geese (Anseranas semipalmata) and other waterbirds congregate around refuge waterholes. The Arafura File Snake (Acrochordus arafurae) that spreads throughout the channels of the wetlands in the wet season, retreats to the main rivers and permanent streams in the dry. Many aquatic fish die if they become trapped in a watersrouce that drys up, and so are eaten by remaining predators, such as the waterbirds etc, before they 'escape' to the permanent waterholes. Remnants of the wet season schools that do not become trapped also follow the receding waters to permanent waterholes, where they too spend the dry season. When the wet season arrives again there is a two-way movement. Myriads of aquatic plants and small animals appear on the floodplains, brought about by the nutrients washed down by the rains. The fish near the coast move upstream to the feeder creeks and floodplains as soon as they can. Those surviving from the upper reaches of the catchment that became isolated in the dry by receding waters, move downstream where they use the floodplains as a breeding habitat. Many species may spawn more than once in a good wet season.

The avoiders are animals who escape by going into a torpor (aestivating) and slowing down their body processes when the recede and waters dry up. Northern Snake-necked Turtles (Chelodina rugosa) dig underground chambers for themselves in the mud, which they occupy for the latter part of the dry season. Saltwater, and sometimes Freshwater (Crododylus johnstoni), Crocodiles (Crocodylus porosus)  also aestivate in the mud, where they spend the dry season, if they cannot follow the receding waters to a permanent water source. At the end of the wet Top End frogs mostly disappear from view, each species selecting its preferred retreat which may be in hollow logs, under rocks, or in underground chambers. There are also some fish species that can aestivate in the mud when the water drys up. 

Microinvertebrates, pond snails, freshwater shrimps and some fish species use the 'interrupted life cycle' strategy. These animals produce eggs that remain unhatched until conditions are suitable. Similarly, the larval or pupal stages of some microinvertebrates can remain in a suspended state for considerable lengths of time. An example of this can be seen in aquatic insects, which carry out the majority of their life cycles in the wet and so are less dependent on these water sources as adults, as they have usually emerged from the water by the time the water drys up and can move to permanent waterholes. However many species leave dormant eggs in the drying mud for the next wet season.

Many aquatic plants species in the Top End, such as waterlillies and spike brush etc, have root storage systems that keep them alive in areas where surface waters are likely to evaporate in the late dry season. These plants die off above ground but reshoot again when the rains return the following wet season.