Australia: The Land Where Time Began
Australia's rainforests occur as islands among the drier, mostly arid, habitats, of 99+% that cover the Australian continent. Since Europeans colonisation 75 % of the small amount still surviving have been destroyed. It has now been realised that Australian rainforests have a number of features that are unique in the world. It is the only country to have rainforests ranging from tropical to cool temperate. Australian rainforests are the only ones having plants and animals to have evolved in isolation for up to about 50 million years.
A number of different types of rainforests have developed according to various combinations of rainfall, soil type, seasonality, altitude, soil nutrients and plant-animal associations. The resulting diversity is described as rainforest types, subtropical, subtropical warm temperate, cool temperate, monsoonal, and dry.
In Australia tropical rainforest is the most common type, accounting for about 33 % of the total rainforest area. It occurs in a coastal strip from Cooktown to Townsville. Between Ingham and Cooktown, the wet tropics, is where most of this type of rainforest occurs. In this stretch are found the most luxuriant of Australia's rainforests. Rainfall averages about 2800 mm/yr, but has been up to 10,000 mm/yr. The temperature averages 24o C, and never drops below 14o C.
Many primitive Gondwanan plants are found in the wet tropics. These rainforests have the largest concentration of primitive angiosperm families - 13 of the world's 19. Included in them are what are believed to be closely related species to the ancestral sclerophyllous plants, now more characteristic of the arid regions of the continent. There is also the highest diversity of ferns and epiphytes. Also found here is the world's largest cycads - 20 m tall.
The wet tropical rainforests have the highest diversity of animals in present day Australia. There are 230 species of vertebrate, 35 of which are mammals. This is only about half the diversity in the Riversleigh deposits, an ancient wet tropics. There are also 140 birds, 25 frogs, and 30 reptiles. Of these 160 species are totally dependent on the rainforest. 54 of the 230 vertebrates found here are found only in this area of rainforest.
Some of the creatures unique to this rainforest are Musky Rat-kangaroos, Tree Kangaroos, 4 species of ring-tailed possum, the Atherton Antechinus, Thornton Peak Melomys, Southern Cassowary, Golden Bowerbird, and relict skinks. The rainforests of the wet tropics have the highest concentration of plant and animal relicts of Gondwana. Also found here are some ancient groups of Gondwanan invertebrates. The Stag Beetle, Sphaenognathus queenslandiae, with its closest living relatives in the Andes of South America.
The wet tropics have the highest concentration of flora and fauna surviving from Gondwanan forests.
The remaining rainforests of Australia are now scattered islands of what was once, in Gondwana, a vast forest covering the present-day southern continents. The rainforest remnants in Australia are refugia for those plants and animals stranded when the pancontinental rainforests were broken up into small patches. The angiosperms possibly arose in these forests about 120 million years ago. In this, the rainforests are different from all other vegetation formations in Australia. Among the primitive plant families that survived to the present are Winteraceae, Eupomatiaceae, Monimiaceae, Lauraceae, and Cunoniaceae.
By 45 million years ago the break from East Antarctica was well under way, though that rift took millions of years to be completed, by about 38 million years ago. At the time of its break from Antarctica Australia was mostly covered by tropical rainforests. Fossil evidence from southern Australia, Maslin Bay in South Australia, show tropical rainforest covered the area at the time the deposits were laid down, 45-50 Ma (million years ago).
As Australia drifted north the cooling of the earth that took place after this time was avoided in the northern parts of Australia for longer than the other southern continents because as the climate cooled the continent slowly moved towards the equator. Like other parts of the southern continents, the rainforests in southern Australia gradually transformed into cool temperate forests. Present-day Tasmania has one of the worlds last remaining rainforests of this type.
As Australia drifted north, it moved through differing climatic zones, encountering different ocean and atmospheric currents that influence the climate, and hence the biota (flora & fauna) that evolves as the climate changes.
A unique feature of rainforests is the ability of tree seedlings to become established in deep shade. Seeds continually fall from the canopy, often high above, germinating soon after, then after putting down roots and a shoot with a small number of leaves, they enter an arrested state of development that can last for years, until a gap opens in the canopy, than all the seedlings begin to grow as fast as they can to fill the gap, racing the other seedlings from their own species as well as other species, to be first to fill the canopy gap. This process is known as 'gap-phase dynamics', and is a continuous process in rainforests. This process, together with their shade tolerance, makes rainforests different from all other Australian plant communities, where regeneration is driven by infrequent events such as fire, flood, or the infrequent and erratic rain, that often leads to flooding. It never rains but it pours.
Ficus species and possumwood (Quintinia sieberi) cheat, avoiding the competition for space of 'gap-phase dynamics' by germinating high in the canopy, starting life as epiphytes. They send roots to the forest floor, often many metres below, and once the roots reach the soil the plants change from epiphytes to trees and gradually encase their host tree with thick roots until they crush it to death, the host eventually rotting to leave the mass of roots where its trunk once stood.
When environmental factors are right, the rainforest can invade other plant communities, such as eucalypt forests, the shade tolerance of rainforest species allowing pioneer species to establish beneath the trees of the neighbouring community. Once established, these pioneer species provide the shade and moisture conditions that allow the other rainforest species to become established. Many rainforest trees are longer lived than eucalypts, eventually taking over from the eucalypts as the eucalypts cannot establish in shaded conditions. It is often thought that in places where there are tall eucalypts with a rainforest canopy beneath is a place where eucalypts are invading rainforest, but it is often the other way around, the rainforest invading the eucalypt forest. The eucalypt forests only flourish when there is open space for the seeds to germinate and establish, as after fire. The seeds and seedlings of rainforest species have low or no tolerance to fire, so it is the fire-related dynamic balance that determines the boundaries between the rainforest and the eucalypt forest.
Unlike other vegetation types, many rainforest species produce fleshy seeds designed for dispersal by birds and mammals. The seeds of these fruit usually pass through the guts of the dispersers unharmed and are deposited in their droppings. It has been found that pigeons and flying foxes are long-distance dispersers of seeds. making them important in the ecology of the rainforest, particularly in isolated patches of forest. Another method of dispersal among rainforest species is by seeds with wings or hairy coverings that are carried some distance by the wind.
Many vertebrates live on the fleshy fruits, but the leaves support a vast number of invertebrates, more being discovered all the time. Some, such as the beetle Novacastria nothofagi, which is specialised to feed on the Antarctic Beech Nothofagus moorei. The tree species vary in the amount of their foliage consumed by the various herbivorous invertebrates. The giant stinging tree Dendrocnide excels loses about a third of their foliage, while red cedar Toona ciliata, loses only about 5 % of its foliage. The young and shade leaves tend to be more heavily grazed than the older and sun leaves which are small and harder and accumulate chemicals toxic to the grazers such as tannins and phenolics. The amount of defoliation suffered by the rainforest's various species differs between rainforest communities at different altitudes - cool temperate forests lose more of their foliage than do those of subtropical communities that lose the least. This difference in foliage loss to insect grazers also applies to the same species in different types of community. An example is sassafras, Doryphora sassafras, which suffers 3-4 times the foliage loss in cool temperate forests than it does in subtropical forests.
Because of the great diversity of plants comprising rainforest, and a large number of nutritious fruit and foliage, there is a correspondingly wide diversity of animals such as birds, frogs, insects and other invertebrates, that live in the rainforest. There are, however, relatively few vertebrates that are restricted to the rainforests of New South Wales, most that visit them also live in other vegetation types, some visiting the rainforest only when the food is in season. This distinguishes the New South Wales forests from the tropical rainforests with the highly diverse fauna.
There are many fruit-eating birds in rainforests. Some of these are fruit pigeons (Ptilinopus sepcies), the double-eyed fig parrot (Cyclopsitta diophthalma coxeni), bowerbirds and riflebirds (Ptilinorhynchidae). The calls are often heard but the birds are rarely seen. Together with flying foxes, these are important seed dispersers in rainforests. Many of the birds inhabiting rainforests are to some degree nomadic, moving between altitudes and from north to south and back, as fruits and flowers come into season.
Ground-feeding birds, such as the noisy pitta (Pitta versicolor), lyrebirds (Menura species), Australian brush turkey (Alectura lathami), rufous scrub-bird (Atrichornis rufenscens), logrunner (Orthonyx temuminckii), and Lord Howe Island woodhead (Gallirallus sylvestris),
Compared with other vegetation types such as eucalypt forests, woodlands and heaths, nectar-feeding birds are not common.
Red-necked pademelons (Thylogale thetis) and Red-legged pademelons (Thylogale stigmatica) can be seen at dusk and dawn in rainforests along tracks and open spaces where there is a combination of grass and shelter. There are also fawn-footed melomys (Melomys servinipes), dusky antechinus (Antechinus swainsonii). These species also inhabit wet sclerophyll forests with a mesic (moist) understorey.
Many frogs inhabit rainforests, as well as the areas around them. There are several rare species of frogs in the rainforsts of New South Wales. Most species of Philoria, frogs of the leaf litter, the barred or stuttering frogs (Myxophyes species) and the pouched frogs (Assa darlingtoni), are all found mostly in rainforests. Male pouched frogs carry their tadpoles in pouches on the front of their hind legs, the young eventually emerge as fully developed frogs after about 2 months. Reptiles found in rainforests include the angle-headed dragon (Hypsilurus spinipes), the beech skink (Cautila zia), the leaf-tailed gecko (Saltuarius swaini) and the skink (Saproscincus rosei).
A distinctive suite of invertebrates are found in the forests of New South Wales, many of which originated in Gondwana. Some of these are velvet worms (phylum Onychophora) and many molluscs in the rotting logs and leaf litter.
|Author: M.H.Monroe Email: email@example.com Sources & Further reading|