Was the Bass Plain a bridge or a filter?

by Chris Johnson | Jun 30, 2024 | Once was a grassy plain

Between about 40,000 and 10,000 years ago, when the sea was lower, a land bridge connected the places we now call Lutruwita/Tasmania and Victoria. But this connection was more than just a bridge: the Bass Plain was a vast country that at its maximum extent was larger than Tasmania is now.

What was that country like? What species of plants and animals lived there? We can answer these questions for the places that still lie above sea level, especially the islands of Tayaritja/the Furneaux Group, because we know what species live there now and, from studies of pollen and fossils, we can reconstruct what was there before the sea rose to its present level.

But those islands were once the mountainous highlands of a mostly low-lying country. They would have been unusual, different from the much larger areas which are now under the Bass Strait.

We can get a view of that lost country from a surprising angle: by looking at differences between the species that live in Victoria and in similar environments in Lutruwita/Tasmania.

The Bass Plain was a vast landscape as large as a country. It was an open, low lying and mostly treeless environment. But there were also wetlands, patches of woodland, and thickets of woody heath and shrubland. Photograph of a scorpionfly on a sheoak by John Tann. CC BY 2.0.

About the Author

Chris Johnson

Professor Chris Johnson is an ecologist and conservation biologist at the University of Tasmania. His research investigates many problems that are relevant to the conservation of biodiversity in the past and the present.

With thanks to

University of Tasmania and the Australian Research Council Centre of Excellence for Australian Biodiversity and Heritage (CABAH)

Rock wallabies, such as this brush-tailed rock wallaby from Victoria, never made it to Lutruwita/Tasmania. This suggests the Bass Plain was too flat for them. Photograph by Heath Warwick. ©Museums Victoria. CC BY-NC 4.0.

The failure of the koala to reach Lutruwita/Tasmania tells us that woodland on the plain was small and separated. Photograph by Richard Crook. CC BY-NC-ND 2.0.

For example, mainland Australia has many species of rock-wallabies: 17 of them, living in rocky gorges and hilltops all over the continent, including the brush-tailed rock-wallaby in Victoria. But there are no rock-wallabies in Lutruwita/Tasmania, and the fossil record suggests there never have been. Lutruwita/Tasmania has plenty of ideal rocky habitat with very similar climate and vegetation, and was once connected to Victoria – so why no rock-wallabies?

The most likely answer is that rock-wallabies evolved on mainland Australia but were not able to cross the Bass Plain, when it was exposed in the most recent ice age by falling sea levels, to reach Lutruwita/Tasmania. Rock wallabies need steep and rocky habitat for shelter, and they rarely move more than a couple of hundred metres away from such areas. Probably, the Bass Plain was just too flat for them, so they never colonised it and they never reached Lutruwita/Tasmania.

The absence of rock-wallabies from Lutruwita/Tasmania is a strong clue that the Bass Plain had very few rocky or hilly areas. This was already obvious from the topography of the sea floor. A similar way of arguing from distributions of species can help us out with things we don’t know so well, such as whether there were forests and woodlands growing between Victoria and Lutruwita/Tasmania, and how wide-spaced the trees might have been.

Victoria has lots of koalas. Many of the tree species that Victorian koalas feed on also grow in Lutruwita/Tasmania – but without the koalas. Koalas like forests where the trees grow densely enough that the canopies of neighbouring trees touch one another, allowing them to climb from tree to tree. They will come down to the ground to walk from one tree to another if they must, but when on the ground they are clumsy, slow moving, and vulnerable to predators. The failure of the koala to reach Lutruwita/Tasmania tells us that while there might have been patches of woodland on the Bass Plain, these were small and separated.

How widely separated? Possums can help answer this question. Victoria has both gliding and non-gliding possums, but the native possums of Lutruwita/Tasmania are all non-gliding. One gliding possum, the sugar glider, has been introduced to Lutruwita/Tasmania, where it is doing perfectly well. So, we know the Tasmanian environment suits them.

Gliders are like koalas. While they are comfortable in trees, they are clumsy on the ground (their gliding flaps get in the way when they try to run). They move around their habitat by gliding from tree to tree. Species like sugar gliders can comfortably glide as far as about 50 metres, especially when they launch from a tall tree. Therefore, they can move through landscapes where trees are spaced at around that distance or less. It must have been the case that over large areas of the Bass Plain trees grew more sparsely than that.

Gliders, like this squirrel glider, don’t like to leave the trees. The treeless areas of the plain were a barrier to them crossing to Lutruwita/Tasmania. Photograph by David Paul. ©Museums Victoria. CC BY-NC 4.0.

The plain was no barrier to wombats. They live in mainland Australia and can be found in Lutruwita/Tasmania and the islands of Tayaritja/the Furneaux Group. Photograph by Andy Tyler. CC BY-ND 2.0.

Grey kangaroos, like wombats, used the plain as a bridge to cross to Lutruwita/Tasmania where they can still be found today. Photograph by Moritz Lino. CC BY-NC-ND 2.0.

We can get a further clue about the spacing of trees on the Bass Plain by considering mistletoes and the mistletoebird. There are many species of mistletoes in southeastern Australia, living on eucalypts and other trees, but none in Lutruwita/Tasmania.

Mistletoes grow on the branches of a host tree. They produce small, sweet fruits that are eaten by the mistletoebird (also absent from Lutruwita/Tasmania). Mistletoes need mistletoebirds to move their seeds from one host tree to another. This happens when a mistletoebird that has eaten mistletoe fruit in one tree perches in another tree. Mistletoebirds poo while perching. Their poo is very sticky so when it lands on the branch the bird is standing on, it stays there, gluing the seed to the branch where it then germinates. The result is a new mistletoe plant growing in a different tree.

Mistletoebirds are quite mobile, so we might think they could transport mistletoe seeds across large treeless gaps between woodland patches. That is true, except that the mistletoebird poos out the seed only about five minutes after eating the fruit. In that time, it might fly several hundred metres, but probably not much further. If there are no trees for the birds to perch and poo on, new mistletoes can’t grow. The failure of mistletoes and the mistletoebird to cross into Lutruwita/Tasmania suggests that the typical spacing between trees or woodland patches was well above several hundred metres.

So, Bass Plain was mostly without trees!

Mistletoes need mistletoebirds to move their seeds from tree to tree. The absence of mistletoebirds and mistletoe in Lutruwita/Tasmania suggests there were large spaces on the Bass Plain without trees. Photograph by Patrick Kavenagh. CC BY 2.0.

Mistletoe grows on eucalypts and other trees throughout mainland Australia but not in Lutruwita/Tasmania. These trees grow in Lutruwita/Tasmania, so why was mistletoe unable to cross the plain? Pictured is drooping mistletoe (Amyema pendula) which is very common in Victoria. Photograph by Tony Rodd. CC0.

What about the vegetation in the ground layer? The kangaroos and wallabies that occur in both Victoria and Lutruwita/Tasmania – the eastern grey kangaroo and red-necked wallaby – like grassy vegetation on flat country, with patches of woodland or shrubland for cover. It’s a good bet that there was plenty of that on the Bass Plain. There are two species of pygmy possums in Victoria, and both occur in Lutruwita/Tasmania as well. These tiny possums live in heathy and shrubby vegetation with stunted trees, where they feed on nectar, fruit, and insects. There must have been plenty of that vegetation on the Bass Plain.

We can begin to see that the Bass Plain was an open and mostly treeless landscape, with scattered patches of cover. Nonetheless the vegetation was quite diverse, with areas of grassland interspersed with thickets of woody heath and shrubland. The topography of the area also suggests there were large lakes and wetlands in the middle of the plain.

If we could visit that country now, we might find it an unfamiliar and even alien landscape. Those kinds of vast and semi-wet plains just don’t exist in southeastern Australia now. But I am sure we would notice something else. This environment would have had large populations of kangaroos, wallabies, and emus living in its grasslands and thickets, and its rich ground vegetation and waterways would have been diverse and productive. There were many opportunities for hunting and for gathering.

For the people who lived on it, the Bass Plain was a good country.

Tiny pygmy possums, like this eastern pygmy possum, live in heathy and shrubby vegetation with stunted trees. There must have been plenty of this on the plain for them to cross to Lutruwita/Tasmania. Photograph by David Paul. ©Museums Victoria. CC BY-NC 4.0.

It’s a good bet there was plenty of grassy flat country with patches of woodland or shrubland for grey kangaroos to take cover in on the Bass Plain. Photograph by Greenfleet Australia. CC BY-NC-ND 4.0.