The biggest and slowest don’t always bite it first

13 04 2021

For many years I’ve been interested in modelling the extinction dynamics of megafauna. Apart from co-authoring a few demographically simplified (or largely demographically free) models about how megafauna species could have gone extinct, I have never really tried to capture the full nuances of long-extinct species within a fully structured demographic framework.

That is, until now.

But how do you get the life-history data of an extinct animal that was never directly measured. Surely, things like survival, reproductive output, longevity and even environmental carrying capacity are impossible to discern, and aren’t these necessary for a stage-structured demographic model?

Thylacine mum & joey. Nellie Pease & CABAH

The answer to the first part of that question “it’s possible”, and to the second, it’s “yes”. The most important bit of information we palaeo modellers need to construct something that’s ecologically plausible for an extinct species is an estimate of body mass. Thankfully, palaeontologists are very good at estimating the mass of the things they dig up (with the associated caveats, of course). From such estimates, we can reconstruct everything from equilibrium densities, maximum rate of population growth, age at first breeding, and longevity.

But it’s more complicated than that, of course. In Australia anyway, we’re largely dealing with marsupials (and some monotremes), and they have a rather different life-history mode than most placentals. We therefore have to ‘correct’ the life-history estimates derived from living placental species. Thankfully, evolutionary biologists and ecologists have ways to do that too.

The Pleistocene kangaroo Procoptodon goliah, the largest and most heavily built of the  short-faced kangaroos, was the largest and most heavily built kangaroo known. It had an  unusually short, flat face and forwardly directed 
eyes, with a single large toe on each foot  (reduced from the more normal count of four). Each forelimb had two long, clawed fingers  that would have been used to bring leafy branches within reach.

So with a battery of ecological, demographic, and evolutionary tools, we can now create reasonable stochastic-demographic models for long-gone species, like wombat-like creatures as big as cars, birds more than two metres tall, and lizards more than seven metres long that once roamed the Australian continent. 

Ancient clues, in the shape of fossils and archaeological evidence of varying quality scattered across Australia, have formed the basis of several hypotheses about the fate of megafauna that vanished during a peak about 42,000 years ago from the ancient continent of Sahul, comprising mainland Australia, Tasmania, New Guinea and neighbouring islands.

There is a growing consensus that multiple factors were at play, including climate change, the impact of people on the environment, and access to freshwater sources.

Just published in the open-access journal eLife, our latest CABAH paper applies these approaches to assess how susceptible different species were to extinction – and what it means for the survival of species today. 

Using various characteristics such as body size, weight, lifespan, survival rate, and fertility, we (Chris Johnson, John Llewelyn, Vera Weisbecker, Giovanni Strona, Frédérik Saltré & me) created population simulation models to predict the likelihood of these species surviving under different types of environmental disturbance.

Simulations included everything from increasing droughts to increasing hunting pressure to see which species of 13 extinct megafauna (genera: Diprotodon, Palorchestes, Zygomaturus, Phascolonus, Procoptodon, Sthenurus, Protemnodon, Simosthenurus, Metasthenurus, Genyornis, Thylacoleo, Thylacinus, Megalibgwilia), as well as 8 comparative species still alive today (Vombatus, Osphranter, Notamacropus, Dromaius, Alectura, Sarcophilus, Dasyurus, Tachyglossus), had the highest chances of surviving.

We compared the results to what we know about the timing of extinction for different megafauna species derived from dated fossil records. We expected to confirm that the most extinction-prone species were the first species to go extinct – but that wasn’t necessarily the case.

While we did find that slower-growing species with lower fertility, like the rhino-sized wombat relative Diprotodon, were generally more susceptible to extinction than more-fecund species like the marsupial ‘tiger’ thylacine, the relative susceptibility rank across species did not match the timing of their extinctions recorded in the fossil record.

Indeed, we found no clear relationship between a species’ inherent vulnerability to extinction — such as being slower and heavier and/or slower to reproduce — and the timing of its extinction in the fossil record.

In fact, we found that most of the living species used for comparison — such as short-beaked echidnas, emus, brush turkeys, and common wombats — were more susceptible on average than their now-extinct counterparts.

Read the rest of this entry »




“Overabundant” wildlife usually isn’t

12 07 2019

koalacrosshairsLate last year (10 December) I was invited to front up to the ‘Overabundant and Pest Species Inquiry’ at the South Australian Parliament to give evidence regarding so-called ‘overabundant’ and ‘pest’ species.

There were the usual five to six Ministers and various aides on the Natural Resources Committee (warning here: the SA Parliament website is one of the most confusing, archaic, badly organised, and generally shitty government sites I’ve yet to visit, so things require a bit of nuanced searching) to whom I addressed on issues ranging from kangaroos, to dingoes, to koalas, to corellas. The other submissions I listened to that day were (mostly) in favour of not taking drastic measures for most of the human-wildlife conflicts that were being investigated.

Forward seven months and the Natural Resources Committee has been reported to have requested the SA Minister for Environment to allow mass culling of any species (wildlife or feral) that they deem to be ‘overabundant’ or a ‘pest’.

So, the first problem is terminological in nature. If you try to wade through the subjectivity, bullshit, vested interests, and general ignorance, you’ll quickly realise that there is no working definition or accepted meaning for the words ‘overabundant’ or ‘pest’ in any legislation. Basically, it comes down to a handful of lobbyists and other squeaky wheels defining anything they deem to be a nuisance as ‘overabundant’, irrespective of its threat status, ecological role, or purported impacts. It is, therefore, entirely subjective, and boils down to this: “If I don’t like it, it’s an overabundant pest”. Read the rest of this entry »





Using ecological theory to make more money

1 12 2014

huge.9.46974Let’s face it: Australia doesn’t have the best international reputation for good ecological management. We’ve been particularly loathsome in our protection of forests, we have an appalling record of mammal extinctions, we’re degenerate water wasters and carbon emitters, our country is overrun with feral animals and weeds, and we have a long-term love affair with archaic, deadly, cruel, counter-productive and xenophobic predator management. To top it all off, we have a government hell-bent on screwing our already screwed environment even more.

Still, we soldier on and try to fix the damages already done or convince people that archaic policies should be scrapped and redrawn. One such policy that I’ve written about extensively is the idiocy and cruelty of the dingo fence.

The ecological evidence that dingoes are good for Australian wildlife and that they pose less threat to livestock than purported by some evidence-less graziers is becoming too big to ignore any longer. Poisoning and fencing are not only counter-productive, they are cruel, ineffective and costly.

So just when ecologists thought that dingoes couldn’t get any cooler, out comes our latest paper demonstrating that letting dingoes do their thing results in a net profit for cattle graziers.

Come again? Read the rest of this entry »





Sustainable kangaroo harvests

10 11 2011

When I first started this blog back in 2008, I extolled the conservation virtues of eating kangaroos over cattle and sheep. Now I want to put my academic money where my mouth is, and do some kangaroo harvest research.

Thanks to the South Australia Department of Environment and Natural Resources  (DENR) and the commercial kangaroo harvest industry, in conjunction with the University of Adelaide, I’m pleased to announce a new scholarship for a PhD candidate to work on a project entitled Optimal survey and harvest models for South Australian macropods based at the University of Adelaide’s School of Earth and Environmental Sciences.

DENR is custodian of a long-term macropod database derived from the State’s management of the commercial kangaroo harvest industry. The dataset entails aerial survey data for most of the State from 1978 to present, annual population estimates, quotas and harvests for three species: red kangaroo (Macropus rufus), western grey kangaroo (Macropus fuliginosus), and the euro (Macropus robustus erubescens). Read the rest of this entry »