Conservation news

Study finds logging helps black rats invade tropical rainforests, push native species out

  • Black rats have lived on the island of Borneo for decades, but mostly stayed around urban areas until recently, when they started to enter the edges of the rainforest.
  • A team led by researchers with Imperial College London tracked the movements of more than forty rats from four different species, including invasive black rats and three native species.
  • The researchers found that of all the species they studied, black rats (Rattus rattus) appear to have the strongest preference for disturbed habitats associated with logging.

A new study shows that in addition to degrading and destroying habitat that wildlife depend on, logging continues to impact forest inhabitants well after the damage is done by helping invasive species like black rats displace mammals native to tropical rainforests.

Logged forests have denser undergrowth and thus provide more cover on the forest floor for rats. Meanwhile, fallen wood harbors insects, providing more rat food, as well.

While these changes to the forest can be good for many small mammal species, invasive and native alike, black rats favor these post-logging conditions more strongly than native species, according to the study, published in the journal Biotropica.

Black rats (Rattus rattus) first began their inexorable spread across the globe via European ships in the 1600s. Their arrival in new areas can cause problems on a number of levels, as they often carry new diseases that infect native species and have even been known to cause bird extinctions.

Rats usually avoid mature forests, which contain higher proportions of large trees, meaning there’s less low-level cover, while a forest floor covered with leaves is too noisy for rats to scurry around on without attracting predators.

To learn more about how and why black rats begin invading a forest, a team led by researchers with the Imperial College London studied how rats behave when travelling through forest habitats in Sabah, a Malaysian state on the island of Borneo.

Attaching a spool to a rat. Photo by Oliver Wearn.

Black rats have lived on Borneo for decades, but mostly stayed around urban areas until recently, when they started to enter the edges of the rainforest. The team tracked the movements of more than forty rats from four different species, including invasive black rats and three native species.

The researchers found that of all the species they studied, black rats appear to have the strongest preference for disturbed habitats associated with logging.

“Logging creates micro-environments that black rats love, helping them move in,” study co-author Dr. Rob Ewers from the Department of Life Sciences at Imperial College London said in a statement.

“This could be bad news for native mammals who might not be able to compete with black rats for food and resources. It’s also bad for the forest, as many small mammals are important seed dispersers, helping rainforest plants to grow, and they are also prey for larger animals.”

Dr. Ewers said that there are ways to minimize the amount of new black rat-friendly habitat created by logging.

“For example, trees being felled often pull down neighboring trees, leaving behind lots of deadwood that is perfect for black rats,” he said. “Clearing vines that connect trees and accurately aiming where felled trees will land would reduce this debris.”

In order to determine exactly what kinds of forest conditions each rat species preferred, the team attached a spool of thin cotton to the backs of the rats they had caught with a glue that will eventually dissolve. As the rats ran off into the forest, the cotton reeled out behind them, snagging on the forest floor and allowing the team to follow the exact trail taken by each individual.

Rattus rattus showed the clearest preference for heavily disturbed habitats and frequently left the forest floor to travel through the understory and midstory forest strata.

“This behavior may enable R. rattus to effectively utilize the complex three-dimensional space of the lower strata in degraded forests, which is characterized by dense vegetation,” the researchers wrote in the Biotropica study.

This “behavioral flexibility” on the part of R. rattus — the ability to operate in both terrestrial and arboreal space — is what probably facilitates its invasion into degraded forests, the researchers added.

“Human activities that generate heavily disturbed habitats preferred by R. rattus may promote the establishment of this invasive species in tropical forests in Borneo, and possibly elsewhere.”

The researchers said their next task will be to track how quickly R. rattus is colonizing the rainforest and what the impacts will be. “We need to understand more about what happens when and if black rats start to replace native small mammals in the forest,” said Dr. Ewers.

Müller’s rat, a species of rodent in the family Muridae, in the forest. Photo by Oliver Wearn.

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