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Live from Tasmania, for now

By Jack Ashby, on 8 April 2011

This week I’ll take a break from my delayed account of last year’s fieldwork because I’m back in Tasmania out in the field with the University of Tasmania’s School of Zoology.

Rejoining the project I was on last year, looking at the ecosystem effects of the massive crash in the Tasmanian devil population, this field trip is slightly less glamorous than trapping the devils, partly because they are practically extinct here up in the northeast of the island, where contagious cancer first appeared 15 years ago. What we’ve been doing is counting sultanas – it doesn’t actually involve setting eyes on a single animal (apart from millions of ants), but intriguing all the same.

The project is investigating whether prey species, particularly brush-tailed possums, have changed their feeding behaviour since the devil population crashed. Devils are their main native predator and they are cautious when foraging away from trees that they can escape to. The question is, are they willing to take more risks with no devils around? Not actually no devils, but the ranger in Mt William National Park, where we were sampling, said there used to be 3000 devils there, now there are 30. That’s a 99% decrease since Devil Facial Tumour Disease (DFTD) appeared.

We placed two containers holding a set volume of pebbles and 100 sultanas at each site – one at the base of a tree (where possums feel safer because they can escape) and one out in the open. What density of sultanas were they willing to reach before they stopped searching? This is called the “giving-up density”. The more sultanas they ate the more work is required to find them, and the greater risk endured. If an animal is trying to avoid a predator it gives up early and doesn’t eat many sultanas.

The prediction would be that where there are lots of predators and the risk is high, the possums will eat more sultanas in the container at the base of the tree than the one in the open. If they have changed their behavior since the devils declined, we should expect that the possums eat similar numbers from both containers. This trip was working in a diseased area; other data are being collected from disease-free regions for comparison.

This is just one strand of research into the effects of the potential extinction of a top predator. Others include looking at how numbers of both prey species and other predators (including feral cats) are reacting, and whether those changes are affecting the parasite ecology of the species involved.

Australia has the worst recent history of mammal extinctions in the world – at least 27 since European settlement. Tasmania itself lost the thylacine – once the island’d biggest predator – of which much has been written. But how much has the natural history of this continent been affected by these extinctions? The Tasmanian devil’s sudden crash has been monitored closely, from a massive number of angles including the above. That’s the way it should be.

We’ll probably never know the extent to which the animals’ behaviour, ecology and even entire ecosystem structures we see today represent the natural state of affairs, before European colonisation wiped out tens of species (not just mammals), or the arrival of aborigines 50,000 years ago resulted in the disappearance of a huge suite of animals, including elephant sized wombats, giant kangaroos and marsupial lions. Surely losing the thylacine had at least as much an effect on Tasmanian mammals as losing the devil?

How unnatural is Australia’s natural history? Very few, if any, recent extinctions were monitored for their effects, and the kind of data for before and after comparison were never collected – this form of ecology is a fairly recent science. In this study, the starting point for the investigation is before DFTD arrives in an area. That’s the only option available and the one that should be used. But in truth, the time from which the ecosystem began to be changed was much earlier – from when humans arrived.

Dodos are famous because they are the first species in history which man realised he caused the extinction of. Of course we’ve been wiping things out for millennia. Only in recent decades have we begun to investigate how our extinctions have affected the world around us. For those long gone we’ll never know how their presence changed things.

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