The Great Western Woodlands is an ancient landscape. It today stands largely as it has for millennia—a relatively flat landscape punctuated by breathtaking natural features. It is sitting on part of the Yilgarn Craton, one of the oldest landforms in the world.
For more than 250 million years, the Great Western Woodlands has been landscape that has seen no mountain building or glacial events, or been covered in oceans.
This is an exceptionally long period of time for life to evolve, and has given the landscape a continuous biological heritage that has seen the development of the first flowering plants, the evolution of a complex mosaic of soil types, dinosaurs coming and going, and the appearance of humans.
There have also been large climatic changes.
The landscape has slowly dried out in the past 20 million years, and in the past two million years there have been major fluctuations in rainfall. It is the interplay between the age and complexity of the soils, climate, isolation from eastern Australia and many other factors, which have provided the opportunity for a huge amount of speciation to occur. The complex array of vegetation, landforms and soil types results in many different types of habitat for wildlife.
In this picture – Lake Johnston
Lake Johnston is one of many massive natural Salt Lakes found in the Great Western Woodlands.The salt lakes are remnants of ancient drainage systems or rivers (Palaeochannels or Palaeodrainage systems) that originated in the late Cretaceous period about 65 million years ago.
These drainage systems generally flowed north to south and eventually discharged into the ocean at the Great Australian Bight. As the land surrounding them eroded away, the drainage channels filled with sediments, resulting in the saline playas that we see today. The Palaeodrainage systems now terminate beneath the Nullarbor; none actually reach the ocean. The systems that underlay the salt lakes in the Great Western Woodlands are called the Cowan and Lefroy systems.
These salt lake systems appear to be large expanses of bare salty mud with not much chance of supporting any sort of life. For a large part of the year the saline playas of the salt lakes have no surface water and often have a salt crust.
Even when dry, however, the lake playas support life, most of which is nocturnal.
Iridescent Tiger Beetles and spiders roam the playa at night hunting insects, and Salt Lake Dragon lizards make hunting forays from their burrows on the shore.
Large numbers of aquatic fauna appear after heavy rains fill the lakes and wetlands. While the lake playas are dry, aquatic fauna is ‘resting’ in the form of cysts in the sediments. These cysts are activated by fresh water and hatch out into clam shrimp, fairy shrimp or brine shrimp larvae as well as the much smaller planktonic fauna such as Daphnia and copepods.
Within a week or so of the lake filling, predatory insect larvae such as dragonflies, damselflies and various beetles hatch out—their parents having flown in from elsewhere—and hunt the crustaceans. The lake crustaceans complete their life cycles rapidly, since the lakes and wetlands often dry out or become too saline to support life within three weeks of filling. The fauna must therefore hatch, mature and produce eggs or cysts within that time.
In some years with good rain, the aquatic invertebrates reach such large numbers that wading birds such as avocets and stilts arrive en masse and breed on islands in the larger lakes.
They have been known to produce two successive clutches of eggs if conditions are favourable and the (mainly) crustacean food source continues.
