Skip to navigation | Skip to content

'One-two punch' decimates small mammals

Chiew Larn Reservoir

The study was carried out in the Chiew Larn Reservoir in southern Thailand (Source: Antony Lynam)


Ecologist Professor Bill Laurance, of James Cook University, and colleagues, report their findings from a case study today in the journal Science.

"It's an interesting example of what can happen when you get a synergism between two different environmental changes happening at once," says Laurance.

"It's sort of a one-two punch."

Researchers have long known that fragmenting habitat can be harmful to biodiversity.

Chopping up habitats isolates native species from each other and their smaller population size makes them more vulnerable to threats.

Laurance and colleagues studied the impact of fragmentation in an area of southern Thailand that has some of the most diverse forest in the world.

"You would have to go to places like the Amazon to find species diversity comparable with what you get in Southeast Asia," says Laurance.

About 25 years ago an area in the middle of the study area was flooded for the Chiew Larn hydroelectric reservoir, resulting in 100 forest islands, which proved perfect places to study the impacts of fragmentation.

"They seem like natural experiments because they're fragments of forests surrounded by a hostile landscape -- water, in this case," says Laurance.

'Catastropic loss'

The research team used live trapping to sample small mammals on 16 of the islands, first between five and seven years after the flooding, and again 25 to 26 years after.

"We found catastrophic loss of mammal diversity across all the islands," says Laurance, who has been studying fragmented forests in place such as North Queensland, the Amazon, Borneo and Africa for many years.

The researchers found that there was near-total loss of small mammals in fragments under 10 hectares, within five years of flooding, and in areas up to 60 hectares, within 25 years.

They found the average time for half the species to disappear was 13.9 years.

"You're talking about an extremely diverse and specialised native small mammal fauna just collapsing," says Laurance.

Invasive rats

Laurance says the collapse was probably partly driven by the invasion of the habitat fragments by the Malayan field rat.

The rat does not occur naturally in the area but is associated with villages and agricultural areas.

"It reinforces the notion that fragmentation can really be a very corrosive ecological process," he says.

Laurance says given the role of small mammals in the ecology -- from seed dispersal to pollination -- there would likely be flow-on effects to rest of the ecosystem.

"There are probably parallel changes happening in lots of other components of the ecosystem such as bird and reptile and plant communities. We just don't know it hasn't been studied."

Laurance says the research comes at a time when there is a "tsunami" of forest fragmentation and degradation taking place in Southeast Asia.

"Systems, when they get fragmented, become vulnerable to so many changes and sometimes they are things we just don't anticipate," he says.

"It underscores the need to maintain large intact systems."

Tags: ecology, mammals