Australasian Science Magazine

The flooding of Chiew Larn Reservoir created scores of small islands like the one in the foreground. Photo: Tony Lynam

By William Laurance

Native mammals are disappearing rapidly as an aggressive invader takes over in forests fragmented by a hydroelectric dam.

Take a glance around you right now and chances are that any native vegetation you see will be in small, isolated patches of what was formerly an expanse of natural habitat. This, of course, is the process of habitat fragmentation, one of the most devastating ways that humans are transforming the Earth.

Many ecologists think that habitat fragmentation is the number one threat to Earth’s biodiversity – worse than climate change, pollution and over-harvesting, for instance. It’s the reason I’ve spent most of my 30-year career studying habitat fragmentation, and its impacts on nature, in places like the rainforests of northern Queensland, the Amazon and the Congo Basin.

Working in such exotic places can produce some exciting moments. In northern Queensland, for instance, I’ve twice stepped on venomous red-bellied black snakes while spotlighting at night. And I’ve been stalked by jaguars in the Amazon and chased by angry forest elephants and cobras in the Congo.

Despite these occasional dramas it’s a privilege to work in places where nature still reins. But these places are becoming all too rare.

In a recent article in Science, my colleagues and I have described our latest attempt to understand the consequences of habitat fragmentation. It’s difficult not to be jolted by our findings about what is happening to native mammals in Thailand.

What makes our Thai study special is that, in effect, we watched nature collapse right before our eyes. The habitat fragments we studied were actually islands created 25 years ago when a large hydroelectric reservoir known as Chiew Larn was constructed in central Thailand.

When the reservoir was first flooded, many creatures perished despite the belated efforts of wildlife rescuers. In one heart-rending video, rescuers attempted to save a sambar deer as it tried to swim across the reservoir, but the animal was too big to pull into their boat and it eventually became exhausted and drowned.

The Chiew Larn islands range from less than 1 hectare to about 60 hectares in area. Because the islands were quite small, large animals such as deer, tigers and elephants quickly vanished, either dying or escaping into the forests that surround the reservoir.

At first a diverse assemblage of smaller mammals remained on the islands. This included treeshrews, various native squirrels and other rodents, and quirky animals called moonrats – which aren’t rats at all but primitive insectivores the size of a ringtail possum. Imagine a giant, pale hedgehog with hair instead of spines and you wouldn’t be too far off the mark.

Ecological Collapse

However, as my colleague Tony Lynam initially discovered, the islands were a place of ecological ruin. Tony is an Aussie from Perth who was studying for his PhD at the University of California, San Diego. The Chiew Larn project was the focus of his doctoral research.

Tony used baited live-traps to capture and census the small mammals. Upon capture, each animal was given a small ear tag, examined and measured, and then released. Just 5 years after the reservoir was created, Tony found that native mammal species had virtually vanished from any island less than 10 hectares in area.

Things were even worse two decades later. Luke Gibson, a PhD student at the National University of Singapore, returned to the islands to census small mammals there using the same live-trapping methods that Tony had used. Luke found that the islands were virtually ecological deserts. Hardly any native mammals remained at all, even on the largest islands.

However, one species was found on all of the islands, and often in great abundance. It was an invader – the Malayan field rat.

The Malayan field rat is a lot like the common black rat – a highly adaptable species that lives near human settlements, agriculture and other severely disturbed environments. Black rats have been introduced inadvertently to islands all around the world, where they’ve killed off scores of native bird species and other vulnerable wildlife.

The Malayan field rat had evidently invaded the islands, either swimming from the mainland or perhaps being carried accidentally in the boats of local residents, who sometimes harvest firewood on the islands.

The field rat is an insidious invader. It reproduces rapidly and thus became abundant on the islands. And it has a highly generalised diet—it’s practically a walking rubbish bin.

Either by directly fighting with the native mammals or perhaps by simply eating everything in sight, the field rat seems to have contributed in a big way to their demise. The species losses at Chiew Larn are as bad, or worse, than those seen in any fragmented ecosystem in the world.

One–Two Punch

It’s not much fun to study ecological train wrecks like the Chiew Larn islands, but it’s worthwhile if we can learn something from them. For those interested in nature conservation, our study has two big implications.

For starters, our research shows just how severely habitat fragmentation can affect biodiversity when the surrounding habitats are hostile to wildlife. This includes not just fragments encircled by water, like those at Chiew Larn, but also those embedded in intensive agriculture such as large monocultures of soy, sugarcane, rice and oil palm, which are unfriendly to wildlife and sustain few native species. Unfortunately, intensive agriculture is expanding rapidly in many parts of the world.

In addition, our study reveals that environmental synergisms – the one–two punch of interacting ecological threats – can be devastating. The mammals we studied suffered not only from the deleterious effects of population isolation, but also from the invading rat competitor.

This is a big concern because foreign species are invading fragmented and human-disturbed ecosystems across the planet. Some of those invaders – such as rats, cats, foxes, fire ants, brown tree snakes and fire-promoting weeds, to name just a few – can completely disrupt ecosystems and devastate their biodiversity.

The take-home message from all this is: don’t fragment native habitats if you can possibly avoid it. Too often it leads to a collapse of wildlife populations. Sometimes, as in the case of native mammals on the Chiew Larn islands, the loss of bio­diversity is almost complete.

Humans have already felled half of the world’s forests, and much of the remainder has been degraded or fragmented into small pieces. Surviving forests will continue to shrink and be fragmented this century as we struggle to sustain up to 11 billion people on Earth. We can only stop this by taking a strong stand for nature. Otherwise, before we know it there won’t be much nature left.

William Laurance is a Distinguished Research Professor and Australian Laureate at James Cook University.