Could disease be driving extinctions of Australian mammals?

Across the vast savannas of northern Australia, mammal populations are collapsing.  Areas that once sustained healthy populations of native mammals are now ecological deserts, virtually devoid of life.  Three researchers from James Cook University, Sandra Abell, Penny van Oosterzee, and Noel Preece, believe that deadly pathogens might be partly responsible for this ongoing calamity:

Brush-tailed bettong -- a critter we don't want to lose.

Brush-tailed bettong -- a critter we don't want to lose.

One third of all mammal extinctions worldwide have occurred in Australia.  Here, 24 mammal species have been wiped out since European arrival -- and that number is still rising.

Loss of habitat, altered fire regimes, and predation by feral cats are all implicated in the recent mammal declines. The role of disease, however, is an understudied but likely contributing factor.

Disease can be deadly for wildlife.  For instance, facial tumor disease is rapidly killing off populations of the Tasmanian Devil.  Trypanosomiasis, introduced by black rats brought by sailing ships, drove the demise of two native species of rainforest rats on Christmas Island, in the Indian Ocean.

Tasmanian Devil with facial tumors -- not a pleasant way to go.

Tasmanian Devil with facial tumors -- not a pleasant way to go.

Now key populations of the Northern Bettong, an attractive wallaby-like animal endemic to north Queensland, is crashing possibly to extinction.

And just this week, endangered Saiga antelopes in Uzbekistan in Asia were reported to have suddenly collapsed, due to the combined effects of climate change and normally harmless bacteria that have evidently become lethal pathogens to the stressed animals.

Saiga Antelope -- another victim of catastrophic disease?

Saiga Antelope -- another victim of catastrophic disease?

We and colleagues have formed a multi-disciplinary group to tackle these declines.  We call ourselves the North Australia Wildlife Decline Disease Investigators.  Our acronym, NAWDDI, rhymes with "naughty".

Our group combines top wildlife ecologists and experts in wildlife disease, including one of the world’s leading experts in wildlife pathogens, Dr Peter Daszak, Director of the EcoHealth Alliance.

NAWDDI is being guided by hard-won lessons learned from the front line of the battle to save imperiled species.

One example is the Brush-tailed Bettong.  Thought to be secure in southwestern Australia, 90 percent of its population has vanished alarmingly over the last decade.  Valiant attempts to understand and halt its decline are providing valuable new insights for conservationists.

NAWDDI is working on a variety of fronts -- from advising on global policy to developing field protocols to make disease investigation a standard practice in researching declines of wild populations.

Disease must be considered early as a potential cause of the rapid and severe mammal declines in northern Australia.  We know that virulent pathogens have caused widespread extinctions or declines of many species worldwide -- from frogs, to Hawaiian birds, to African ungulates and apes, to North American bats.

Pathogens such as the chytrid fungus have driven at least 200 frog species to extinction.

Pathogens such as the chytrid fungus have driven at least 200 frog species to extinction.

Early detection saves money and time -- and could help us avoid the anguish of having to watch helplessly as charismatic mammals like the Brush-tailed Bettong follow the path of three of its sister species to extinction.


Why we simply must have predators

ALERT member John Terborgh is a scientist of enormous stature, whose many accomplishments include a rare MacArthur 'Genius' Award.  Here he tells us why predators are so crucial for the Earth -- a lesson with big implications for understanding nature and our future:

Amur Leopard -- just 70 left in the wild today.

Amur Leopard -- just 70 left in the wild today.

Forty-five years ago, three leading ecologists asked a question of child-like simplicity, “Why is the world green?”

We take a green world for granted, yet this deceptively simple question goes to the very heart of how ecosystems work.

The world is green, the trio argued, because predators limit the numbers of herbivores, thereby protecting plants and allowing them to flourish.

At the time this idea was floated, other ecologists were busy discovering that plants manufacture a potent arsenal of chemical compounds to deter herbivores -- everything from deadly toxins to chemicals that make them hard to digest. 

Far from being helpless, these other ecologists argued, plants actively defend themselves, and this keeps herbivore numbers down.  Under this scenario, predators aren't really needed to keep the world green.  

Who is right?  In theory, it should be simple to find out: just remove the predators from an ecosystem and see what happens. 

Wolves -- widely persecuted but trying to make a comeback in parts of North America and Europe.

Wolves -- widely persecuted but trying to make a comeback in parts of North America and Europe.

But it's one thing to remove little predators such as insects and spiders.  Getting rid of big predators, such as wolves, lions, or jaguars, is a far taller task.  To perform such an experiment properly, one would need to fence off huge areas, some with predators and prey, some with prey alone, and others with neither. 

The cost of such an ambitious experiment would be so high that, to this day, nobody has tried it.  As a result, ecologists are still arguing about why the world is green.

An Accidental Experiment

But purely by accident, an unplanned experiment in Venezuela created the right conditions to test the 'Green World' hypothesis.  There, a massive expanse of forest flooded by a hydroelectric dam created hundreds of artificial islands.  The smallest islands were barely the size of a tennis court; the biggest, at hundreds of hectares, would span a dozen large golf courses.

My students, colleagues, and I studied these islands for 14 years.  And what we found is enormously exciting -- and scary.

The largest islands had enough habitat to support both predators and their prey.  But as island size declined, fewer and fewer species of predator remained.  Once below 10 hectares in size -- equivalent to about 20 football fields -- the predators vanished entirely. 

The Harpy -- king of eagles.

The Harpy -- king of eagles.

At this point you have an ecosystem with a few species of herbivores -- particularly howler monkeys, an iguana, a type of tortoise, and leaf-cutter ants -- but nothing to eat them.

With little to keep herbivores in check, did these islands stay gloriously green, or collapse ecologically?  

Ecological Collapse

What we observed was ecological chaos.  Herbivores attacked foliage in all parts of the forest. The first plants to be killed were seedlings, cut up and carried away by leaf-cutter ants, while howler monkeys and iguanas defoliated trees and vines in the canopy.

Without predators, leaf-cutter ants can decimate a forest.

Without predators, leaf-cutter ants can decimate a forest.

Small saplings were the next to disappear under the herbivore onslaught, followed by larger saplings and woody vines.  Finally, after a decade or more, big canopy trees began to die, standing leafless, ghost-like.

By the end of our study, the once-verdant forest was a degraded tangle of shrubs and vines -- a pathetic vestige of their original diversity.

And while herbivores had won, they ultimately brought about their own self-destruction.  In the end, the decimated islands barely sustained any life at all.

The trio of ecologists -- the architects of the 'Green World' hypothesis -- were right.  We need predators to keep our planet verdant and healthy, and to maintain biodiversity.

Alternative States

The 'alternative state' of a predator-free island is alarming.  Equally worrisome is that we can get there in a variety of ways.  Killing off predators is one way -- and we humans are very good at doing that.

Another way is flooding an ecosystem with nutrients, a process called "eutrophication".  This can happen, for example, if we carelessly use farming fertilizers, which then leach into waterways and other ecosystems.  Biodiversity depends on a balance, and tip that balance too far one way and nutrient-loving species dominate while excluding many other species.

Depressing 'alternative states' can arise for other reasons too, such as disrupting natural fire regimes or introducing exotic species that fundamentally change ecosystems.

But for me, the widespread decimation of predators is the most worrisome way that we are disrupting the natural world.  For that reason, I strongly support efforts to 're-wild' ecosystems -- to reintroduce big carnivores such as wolves, grizzy bears, and tigers to regions where they once held sway.  

Arctic fox in the summer.

Arctic fox in the summer.

Rewilding sizable parts of the world will not be easy.  There are many who will object -- out of fear or ignorance or potential risks to their livelihoods. 

But the world needs predators.  I have been studying nature for a long time, and one thing I have learned is that biodiversity utterly depends on them.

Bringing the science we need to maturity and explaining it to policymakers and the public is a vital goal for our next generation of conservationists.


Are Australia's mysterious mammal declines spreading?

ALERT has previously reported on the enigmatic and alarming population declines of mammal species across northern Australia.  Now, it appears the declines could be even more widespread than previously suspected.

Bye-bye beautiful bettong?

Bye-bye beautiful bettong?

To date, the declines of smaller and medium-sized mammals, such as quolls, bandicoots, and native rodents, have been mainly documented in monsoonal forests and woodlands across Australia's top end -- such as those at Kakadu National Park, where the declines have been best studied

Additional research -- including interviews of Aboriginal communities in remote areas of northern Australia -- have shown the declines to be widespread in nature, extending over many thousands of kilometers. 

Now, recent research -- which has yet to be published -- suggests that the declines might even extend to northeastern Australia, to the wet tropical region of far north Queensland. 

In this area, live-trapping and camera-trap studies by Sandra Abell-Davis of James Cook University suggest that the tropical bettong, an endangered wallaby-like marsupial, may also be declining sharply.

Abell-Davis studied three areas that had formerly been live-trapped for the bettongs, using identical trapping methods.  She found that, on average, bettong numbers had fallen by more than 80%.

Abell-Davis emphasizes that her findings, while alarming, are still provisional.  She wants to trap more extensively for the bettong and use novel genetic analyses, to see if its numbers have fallen elsewhere in the region.  In fact, she is looking for volunteers to help with this important field work. 

A number of possible drivers have been suggested for northern Australia's mammal declines, with feral cats and altered fire regimes being among the leading suspects

Other suggested possibilities include foreign pathogens, changing rainfall regimes, overgrazing, foxes, and the introduced cane toad, which produces toxins deadly enough to kill virtually anything that eats it.

Whatever is going on, it raises frightening prospects for an Australian continent that has already suffered massive extinctions of its native mammals.

The mystery of Australia's small mammal declines

Across the top end of Australia, from Cape York to the Kimberly Mountains, populations of smaller marsupials and native rodents are collapsing.  What's going on?

Tapping into traditional knowledge to help solve a mystery (photo by Ian Morris)

Tapping into traditional knowledge to help solve a mystery (photo by Ian Morris)

Several studies in recent years have documented the declines.  A few have shown major changes in a single location, such as Kakadu National Park.  Others have compiled scattered information over a much broader area; one innovative study used interviews with Aboriginal communities to piece together evidence (find study here).

The bottom line: many species that used to be common have become vanishingly rare, or have geographic ranges that are collapsing. 

What is causing all this?  No one is sure.  Some suspect it could be severe overgrazing by livestock, especially during droughts.  During such times the vegetation can become badly damaged--as we're seeing at present in many areas

Others think altered fire regimes are the cause.  And yet others suspect predation by feral cats, with a possible role for lethal cane toads for certain predatory mammals.

Whatever the cause, this might be a new biodiversity crisis for the Land Down Under, which has already lost more native mammal species than any other continent.  


Across the planet, big trees are in trouble

We all know that big animals such as elephants, rhinos and tigers are in trouble, but it turns out that the fate of our largest and oldest trees is just as dire.

Everywhere you look, big trees are hurting (photo by William Laurance)

Everywhere you look, big trees are hurting (photo by William Laurance)

All across the world, big trees are suffering.  They are being cleared for agriculture, felled by loggers, and are dying as a result of habitat fragmentation, exotic pests and pathogens, altered fire regimes, and severe droughts. 

Big trees are adapted for longevity and stability--two things in short supply in our rapidly changing world.

I first wrote about the dire fate of big trees in early 2012, in New Scientist.  That was followed by papers in Science in 2012 and Conservation Letters in 2013, in collaboration with leading ecologists David Lindenmayer and Jerry Franklin. 

Now there's more evidence of the vulnerability of big trees, from the Amazon.  Plinio Sist and colleagues have just found that many big trees are being damaged during selective logging operations there, and die soon afterward.  This is on top of the big trees that are actually being harvested.  The post-logging wave of tree death has a serious impact on the carbon storage and ecology of the forest.

It's increasing looking like big trees are an important barometer of Earth's environmental 'health'.

-Bill Laurance