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.


A new Armageddon for amphibians?

Amphibians -- such as frogs, toads, salamanders, newts, and caecilians -- are among the most ancient of all terrestrial vertebrates.  And increasingly, they seem to be among the most imperiled as well.

Biodiversity in retreat...  many frogs are imperiled  (photo by Mike Trenerry)

Biodiversity in retreat...  many frogs are imperiled (photo by Mike Trenerry)

Why?  For one thing, amphibians, more than any other group of terrestrial vertebrates, rely intimately on water.  Their eggs dry out on land, and their larvae (such as tadpoles) are often aquatic.  Across the planet, aquatic ecosystems are being destroyed and degraded -- by pollution, dams, river channelization, introduced predators and competitors, and other maladies -- at a horrific pace.

Beyond this, amphibians seem unusually vulnerable to exotic diseases.  The water that they rely on so intimately is an excellent medium for transmitting pathogens, many of which will quickly die if they dry out. 

Many amphibians need clean water for survival  (photo by William Laurance)

Many amphibians need clean water for survival (photo by William Laurance)

Combine that trait with our hyper-mobile modern society -- where exotic organisms are suddenly being moved across the planet at lightening speed, often by accident -- and you have the makings of environmental Armageddon.

For example, a few decades ago a mysterious chytrid fungus suddenly appeared and swept like a tsunami across Australia, Latin America, and parts of Asia and Europe with deadly effect. 

In total, some 200 species of frogs and salamanders have declined catastrophically or been driven to global extinction by the fungal disease (notably, ALERT director Bill Laurance and colleagues did some of the earliest work on the pathogen's impacts).  

Now, there is a new disease threat.  Exotic viruses have appeared that seem to be targeting a range of amphibian species.  This is frightening, because viruses are usually specific to a particular species

A virus that can attack several species at once could be catastrophic for amphibians.  Normally, a virus dies out when it kills most individuals of its host species, because the host becomes so rare that the virus is no longer transmitted successfully.

But a multi-species virus is different.  One of its host species might be largely wiped out, but the virus can still thrive in a different species.  Because it persists in the environment, even when one of its hosts becomes perilously rare, such a virus can drive a species completely to extinction.

That is exactly what seems to be happening now in Europe.  At least two new types of Ranaviruses are plaguing frogs and salamanders there, causing massive die-offs in several different species of hosts at once. 

The viruses induce hemorrhaging in the frogs, creating open sores and killing their limb tissues.

Alarmingly, even reptiles might be affected; a snake that ate an infected salamander died soon afterward, apparently from the virus. 

Such events suggest that one of the most damaging features of modern humanity might be our proclivity for moving exotic organisms all around the planet.  Species have adapted to ecosystems in which a major new pathogen might come along once every few centuries or millennia.  Now they're arriving at a pace that's blindingly faster than that.

Little wonder that the amphibians -- among the most ancient denizens of the Earth -- are having a hard time surviving the onslaught.