June 3, 2015
Why is Australia so weird?
The mythology of Australian Aborigines, known as the Dreamtime, is filled with imagery of Australia’s wondrous fauna. Later, European naturalists stood open-mouthed at the unique animal life before them. All of these people had to be careful when going for a walk, a swim, or even to the toilet.We come across animal stories from Australia almost weekly (raining spiders, anyone?) that leave us in the same boat as those European naturalists: open-mouthed and in awe that such a thing actually exists. This, in turn, causes us to wonder, "Why is Australia so weird?" And it's a good question. There has to be some evolutionary reason why everything on that island/country/continent can kill you, or just confuse the hell out of you (we're looking at you, platypus).
So, needing absolution, we turned to some experts who could help us--even if many of them later wrote back, "That's a good question. I have no idea."
Some, though, had answers, and this is what they had to say:
A very large life-raft
Here’s Professor Rick Shine, from the School of Biological Sciences at the University of Sydney:
“Australia has a unique fauna because it was isolated from the rest of the world for very long periods. The Australian continent was surrounded by ocean for many millions of years, and so the plants and animals on that very large life-raft were able to evolve in distinctive ways.
“Many types of animals that are common in other continents, such as dogs, cats and monkeys, never found their way to Australia (until people brought them over).”
A dubious distinction
On the high density of poisonous snakes, Professor Shine says: “The high proportion of venomous snakes compared to harmless ones is an evolutionary accident. Snakes of the cobra family found their way here, probably because they are adept at swimming long distances in the ocean and so were early colonisers of the island continent.
“Their descendants spread out to occupy a wide range of ecological niches, but they retained their ancestor’s venomous capacity. So Australia has the dubious distinction of being the only continent in which most snake species are venomous.”
The isolation of Australia was again a factor, but where snakes were concerned, it was a case of being able to swim far enough, rather than that of evolution.
Larisa DeSantis of Vanderbilt’s Department of Earth and Environmental Sciences also offered some fascinating insights into the evolution of Australia’s fauna.
She told us, “Australia has so many bizarre creatures largely due to Earth's history. When Gondwana began to break apart, South America, Antarctica and Australia all remained connected. This connection allowed animals to pass freely between these now distinct continents and is how marsupials made their way to Australia. Recent genetic evidence demonstrated that marsupials actually evolved in South America, but due to the connection between South America, Antarctica, and Australia, marsupials easily migrated to Australia.
“However, roughly about 85 million years ago Australia began to separate from Antarctica (these continents were likely not fully separated until around 40 million years ago). Because of Australia's early isolation from Antarctica and South America, but after the evolution of marsupial mammals and their migration into Australia, those marsupials then evolved in isolation for millions of years - resulting in many of the bizarre forms we know of today - including tree kangaroos, wombats, bilbies, spotted quolls, Tasmanian devils, and many others.”
The difference with South America
So, why don't we see as many "unique" species in South America - the home to marsupial evolution?
“Well, we did,” Professor DeSantis explained. “And there were lots of odd animals including a marsupial saber-toothed 'cat-like' animal called Thylacosmilus. In fact, South America became separated from Antarctica likely a few million years after Australia and much of their flora and fauna evolved in isolation up until roughly 3-5 million years ago, when South and North America then became connected in what is referred to as the Great Biotic Interchange (GBI).”
North American animals entered South America, and South America also lost many of its unique species. Australia, meanwhile, was still evolving alone.
One weird and scary beast
“When plants and animals evolve in isolation, unique forms can evolve," she continued. "However, these plants and animals are often more vulnerable to novel predators. A classic example is when man arrived on islands with flightless birds (e.g., New Zealand), or snakes arrived in Guam and decimated the bird populations, or dingos and foxes brought to Australia and nearly eliminated quokkas (the ones that remain are largely on islands where there are no foxes or dingos).
“Also, it is not clear if dingos (Canis lupus dingo) and/or humans are the cause for the reduction of Tasmania devils to the island of Tasmania (they used to be widespread throughout Australia). Equally, Thyalcines (the marsupial wolves or marsupial tigers - not closely related to either dogs or tigers, but they have canine-like bone morphology, as well as skulls and stripes like tigers) became restricted to Tasmania before going extinct in the 1930.
“Other more bizarre animals like the giant short faced kangaroo and the killer wombat (it evolved from that group, but is also referred to as the marsupial lion) known as Thylacoleo went extinct during the late Pleistocene, potentially due to either climate change, humans, or both. Thylacoleo was one weird and scary beast - with the bite force around 70 percent of a lion despite only being 30 percent of its size.”
Scary but vulnerable
In answering why island animals are more vulnerable, Professor DeSantis said: “An analogy I like to draw is elections. The tougher the primary the more likely the remaining candidate will be able to compete during the final election. When animals and plants evolve in isolation they can become more vulnerable to novel predators or pathogens - this is one of the reasons Australia is so vigilant about quarantines and making sure they keep certain pathogens and invasive species out of the country. Cane toads and rabbits are an example of invasive species gone amuck in Australia.
“Invasive species while a problem today (and more so because humans travel so much, move species in ships, on our shoes, within us, etc.), have also been a problem in the past with the connection of land masses (not just North and South America, but also Europe/Asia with North America via the Bering Land Bridge).
“Some Australian marsupials also have unique adaptations for living in more arid conditions, including environments where resource availability is not always predictable. In fact many kangaroos can have three stages of young at one time, one out of the pouch (but still nursing), one in the pouch, and an embryo developing. In harsh times, milk production stops as does reproduction, but this 'strategy' can be beneficial in harsh and less predictable environments.”
Harsh and unpredictable Australia certainly was and is, but the environment provided one of the world’s great playgrounds for nature lovers.