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Fragmented forests warn of ecological collapse

Humans have already felled or razed about half of the world’s forests, and much that remains has been fragmented into small pieces. Research my colleagues and I published in Science today shows that when…

The creation of a reservoir in Thailand isolated forest fragments; new research shows the devastating effect on local mammals. Antony Lynam

Humans have already felled or razed about half of the world’s forests, and much that remains has been fragmented into small pieces. Research my colleagues and I published in Science today shows that when forests become fragmented, native mammals rapidly disappear, adding to the evidence that deforestation has a devastating effect on biodiversity.

Biggest threat

The fragmentation and loss of native habitats is regarded by many ecologists as the number one threat to Earth’s biodiversity—worse than climate change, pollution and overharvesting, for instance. It’s the reason I’ve spent most of my career studying habitat fragmentation and its impacts on nature, in places like north Queensland, the Amazon and the Congo Basin.

In Science, my colleagues and I describe our most recent attempt to understand the consequences of habitat fragmentation—for native mammals in Thailand. It’s difficult not to be jolted by our findings.

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 was constructed.

Many animals drowned when the reservoir was flooded, despite the efforts of rescuers.

The larger animals—deer, tigers, elephants and the like—quickly vanished from the islands but, at least initially, a diverse assemblage of smaller mammals remained. This included native squirrels and other rodents, treeshrews and quirky animals called moonrats.

One of the native mammals - a moonrat - that disappeared following the fragmentation of the forests. Luke Gibson

Ecological collapse

But, as my colleague Tony Lynam initially discovered, the islands were a place of ecological ruin. Just a few years after the reservoir was created, native mammals had nearly vanished from any island under 10 hectares in area.

By two decades later, things were even worse. Another colleague, Luke Gibson, found the islands were virtually ecological deserts. Hardly any native mammals remained at all — even on the largest islands, which were over 50 hectares in area.

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

Malayan field rats were almost the only mammals left after 20 years. Luke Gibson

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 around the world, where they’ve killed off scores of native bird species and other vulnerable wildlife.

The exotic Malayan field rat had invaded the islands, and given its great fecundity and generalised diet — it’s practically a walking rubbish bin — it evidently helped to wipe out the remaining native mammals.

One-two punch

Our study has two key implications. For starters, it shows just how severely habitat fragmentation can affect biodiversity when the surrounding habitats are severely inhospitable. This includes not just fragments encircled by water but also those embedded in intensive agriculture, such as large monocultures of soy, sugarcane, rice and oil palm, which are hostile to most wildlife and sustain few native species.

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.

Worldwide, surviving forests will continue to shrink and be fragmented this century as we struggle to sustain a projected 11 billion people. Unless we slow the destruction, we’ll see further collapses of ecosystems.

Another native mammal that disappeared - the grey-bellied squirrel. Luke Gibson

Join the conversation

9 Comments sorted by

  1. Jenny Goldie

    population and climate activist

    This is a very important article, albeit deeply depressing and the rate of decline in Thailand is particularly concerning. It is all linked to human activity, directly or indirectly, and made worse as human numbers grow. There's a conference addressing this in Canberra on October 10,11 called "Population, Resources and Climate Change: impact on Australia's near future". The three biologists addressing the issue of population and environment are David Lindenmayer, Chris Dickman and Hugh Possingham. They speak after morning tea on the first day. You can book at

    1. Dianna Arthur


      In reply to Jenny Goldie

      Thank you Jenny.

      I guess it is the brutal truth of articles such as this that keep too many of us, hands clapped over ears, singing, "La, la, la, la, la".

    2. George Burns

      logged in via email

      In reply to Dianna Arthur


      Not to worry, Master Abbot will defund all research of this nature. Adapt to living in a hermetically sealed shopping mall and think of that as evolution in action.


  2. Rebecca Diete

    PhD Candidate at School of Agriculture and Food Sciences, University of Queensland

    I don't understand why we don't get "deniers" on articles like this when they are rampant on the climate change articles. If they can deny AGW despite the clear evidence, why not deny this? Why not say, "oh, trees die anyway. Humans are vain to think that we could influence forests and biodiversity to such an extent".

    1. Edwina Laginestra
      Edwina Laginestra is a Friend of The Conversation.

      Jack of all trades

      In reply to Rebecca Diete

      Thank goodness they are not here so a reasoned discussion can still occur. However, I suppose they aren't here yet because fragmentation is still not yet fully political - so 1. they don't have to defend their politics and 2. the title isn't a red rag to a bull.

      This may change once the developers really want the remaining large tracts of land and start lobbying hard. I think the Amazon destruction and fragmentation did have a fraught period in the 80s.

      But maybe as fragmentation is just destruction by stealth it doesn't come up on the media radar. I don't think the results here would surprise anyone really. We see it when we travel, we see it in the cities by the number of roadkill. Perhaps I'm working up to a solution, but I doubt it. Every so often a number of charities try and raise the issue and it just seems to die out again.

    2. John Foley

      Various ...

      In reply to Rebecca Diete

      If this was a pressing issue to the general public, they would be more vocal on sites such as this. There won't be a 'fragmentation tax', or any other real biodiversity issue making waves, theres no reason for them to get upset and get active.

      Plenty of people might get upset about CSG and mining, mind you. While biodiversity is only part of that, I'd guess it might be harder to dismiss when its part of larger concerns - including population growth.

      And there debate seems to have moved on from denial to simply elevating the ecomony to first and formost among concerns. In NSW it is now the primary, over-rinding consideration. As bad as it is, hubris and over-reach may bring it all undone. We will see ...

  3. John Newlands

    tree changer

    Some threatened mammals may make a comeback if road users drive more carefully and they control their cats and dogs. Yesterday I was investigating leakage in an earthen dam and to my surprise an Eastern Barred Bandicoot ran into a burrow I was examining. I'd only previously seen them as roadkill.

    Other factors may help. Expensive fuel may the temper the fad for semirural living. I speculate that road sense may be inherited by some critters. That is animals wary of cars survive to reproduce. Bottom line is the world has more than enough humans at 7.3 bn and both humans and wildlife may be better off with less.