Extinction rates skyrocket after disturbances in Thailand

In a study spanning two decades, researchers from an international team witnessed the near-complete extinction of 12 species of small mammals once found on forest islands in Thailand.

The decline in populations was linked to two causes: the isolation of populations after flooding due to the construction of a hydroelectric dam, and the arrival of an aggressively invasive species — the Malayan field rat.

Lead author of the study, Luke Gibson, grew up in Davis and is now completing his doctorate at the National University of Singapore.

“Many native species cannot sustain viable populations in small habitat areas, and the invasive Malayan field rat outcompeted and displaced native species,” Gibson said. “We do not have data to differentiate between these two mechanisms (since all islands had high densities of the Malayan field rat), but it seems pretty clear that its presence was a big factor … The replacement of a group of 12 native species with one hyperabundant generalist species can increase the risk of disease outbreaks in both animals and humans.”

Gibson’s study began five years after a man-made reservoir was flooded. By that time, the Malayan field rat was already abundant on most islands.

“The Malayan field rat probably colonized islands immediately after flooding, so its impact on native species was likely immediate … 25 to 26 years after isolation, we found on average less than one individual — of all native species — per island,” Gibson said.

Sohab Arif, a third-year biochemistry and molecular biology major, also explained the impact of disturbances on animal species.

“Any kind of interference can cause disturbance in the habitat and lead to extinction of that species,” Arif said. “For example the hydroelectric dam, the individuals of a species might rely on different sources of food, but these sources are no longer available thus leading to the extinction of that species.”

Gibson said that the loss of biodiversity is the loss of an ecosystem. What the researchers found was the near-total loss of an entire group of small mammals in the tropical forest ecosystem is akin to the removal of a whole organ from the human body. Without that one piece, the rest will fall apart. In tropical forests, small mammals play important roles as seed dispersers, and thus their disappearance could greatly slow the growth of new forests.

According to Brian Todd, assistant professor of wildlife biology, biodiversity is a necessary part of an ecosystem.

“Biodiversity represents all the cogs in the important machinery of life. When we start to remove the cogs, it’s only a matter of time before the machinery begins to break down and the important services like clean air, clean water and abundant game start to disappear,” Todd said.

Gibson said to avoid further threats to biodiversity, forests should be preserved.

“About half of the world’s forests have already been destroyed. Certainly growing human populations and appetites will require further clearance to provide our food and energy, but if we want to preserve a world with diverse animal communities and without serious threats of disease outbreaks in domestic animals and agricultural crops and in humans, we have got to save a large portion of the remaining forest,” Gibson said. “The future of tropical forests — and our species — depends on it.”

CLAIRE SULLIVAN-HALPERN can be reached at science@theaggie.org.