ALERT member John Terborgh, who has studied ecosystems across the world, has just spend six months in Australia. A truly deep thinker -- and winner of a rare MacArthur 'Genius' Award -- John ponders here the nature of Australia. Biologically speaking, he asks, is it a continent or just a big island?
At first glance, the answer is trivial. Australia is, in fact, an island. But with an area of nearly 7.7 million square kilometers, it is an island of continental proportions. As a continent, it is the smallest of the seven, only three-quarters as large as Europe.
But Nature does not adhere to the same notions of geography as we humans do. So does the biology of Australia better support the island or continent model?
Islands versus Continents
To address this question we need to know how islands and continents differ biologically.
Islands typically have impoverished ecosystems, meaning that their faunas are relatively lacking in diversity and deficient in certain groups of species, particularly top predators.
In terms of species diversity, Australia does moderately well. With roughly 800 bird species, over 200 mammals, and nearly 1,000 reptiles, it compares favorably to North America (north of Mexico) and greatly outshines Europe. However, it falls short of Mexico, which occurs at similar latitudes but is only a quarter the size of Australia (just under 2 million square kilometers).
Deficient in Predators
In terms of top predators, however, Australia is severely deficient. The few big predators it once had -- such as the marsupial 'lion', giant monitor lizard, and Thylacine or marsupial 'wolf' -- are all now extinct.
The marsupial lion and giant monitor disappeared soon after humans first arrived on the continent, roughly 50,000 years ago, whereas the last Thylacine died in the Hobart Zoo in Tasmania in 1936 -- widely persecuted for its habit of preying on sheep.
The largest undisputed islands, such as New Guinea, support a few owls and raptors plus large constricting snakes and the odd carnivorous lizard. But they don't have any native canids (dog family) or felids (cat family), the two dominant groups of predators found on all the other continents (except Antarctica, which of course is cloaked in ice).
Outside of Australia, the continents not only support owls, raptors, and big snakes, but also a multitude of canids, felids, bears, and mustelids (badgers, wolverines, weasels, and their kin), plus other assorted predators of lesser renown.
Implications of a Weak Predation Regime
This contrast suggests that Australia possesses a relatively weak predation regime -- and one that entirely lacks the more recently evolved carnivore groups found on the other ice-free continents. In this trait, Australia is decidedly island-like.
A rather wimpy predation regime helps to explain some other distinctive features of the Australian fauna. Apart from the introduced dingo, which has been extirpated over much of the continent, Australia’s larger mammals (kangaroos, wallaroos, wombats, koala) have no natural enemies and tend to increase to spectacular numbers in the absence of management.
Herbivores introduced to predator-free islands typically undergo similar population explosions, increasing until checked by a storm, drought, severe winter, or simple starvation. In Australia’s semi-arid interior, drought plays the role of grim reaper.
Under a weak predation regime, animal numbers tend to increase, intensifying competition between individuals and species. The manifestations of heavy competition in Australian birds is obvious even to a casual observer. Birds of many kinds, but especially the many species of honeyeaters, engage incessantly in disputes and chases, both within and between species.
Under the pressure of intense competition for resources, natural selection confers an advantage not only to aggressiveness, but to certain life-history adjustments.
Juveniles are at a particular disadvantage in the presence of aggressive adults, so juvenile survival can be enhanced by extending the period of parental care. In Australia, one sees this in the highly territorial, extended family groups of fairy wrens, scrub wrens, thornbills, babblers, and many others. Such cooperative, group-living species are especially common in Australia.
Another way natural selection can enhance juvenile survival is through reduced fecundity, which ensures that each offspring receives more parental care. Kangaroos, for instance, raise just one joey at a time, and Australia’s remarkable scrub birds have very small clutch sizes.
Another outcome of weak predation is that animals can live longer. Some of Australia’s birds are notably long-lived, such as the sulfur-crested cockatoo, which can live for more than a century. And in New Zealand -- equally deficient in predators -- the situation is even more extreme.
A Different Creator?
Many of Australia’s endemic birds and mammals are of ancient origin, dating to the breakup of Gondwanaland some 80 million years ago.
As the Australian continent slowly drifted northward toward Asia, more recently evolved groups such as rodents, falcons, and swallows arrived by dispersing over the sea. A water barrier is a strong filter that allows some groups to cross, but not others.
Thus, Australia, anomalously, has no woodpeckers, vultures, hornbills, trogons, primates, felids, canids, or ungulates, among other common groups. In other words, animal groups that are widespread across the other continents are lacking here.
In this sense Australia is strikingly unbalanced, an island characteristic. So unbalanced, in fact, that when a young Charles Darwin visited Australia in the 1830s, he mused that it seemed to have been made by a "distinct Creator".
All the other ice-free continents, in contrast, have had relatively recent physical contact with one another. This has allowed the free interchange of their respective faunas and the spread of some groups, like canids and felids, around the world.
Although Australia is counted by geographers as one of the world’s seven continents, many attributes of its fauna more closely resemble those of an island, which, in fact, it is.
Like other large islands, Australia is home to many endemic groups of plants and animals, and possesses only moderate faunal diversity. Some widespread groups have been able to reach Australia from Asia despite the water barrier, whereas others have not, leaving Australia with a strikingly imbalanced assemblage of major animal groups.
Most conspicuously and importantly lacking are modern carnivores, especially canids and felids. Without these, and without the Thylacine or its extinct megafaunal carnivores, much of the Australian fauna experiences weak predation pressure, a condition that results in further island-like behavioral and life-history features in its fauna.
These include intense competition for resources, manifested by high levels of aggression between species and individuals, lower fecundity, long periods of parental care, and greater longevity.
Hence, I conclude that, in biological terms, Australia is an island, not a continent. Geographers (and doubtless, many Australians) might disagree.