The elimination of ground-level vegetation is bringing another of the world's tiger subspecies to the brink of extinction, according to Virginia Tech and World Wildlife Fund researchers.

The Sumatran tiger, native to Indonesia, could be the fourth type of tiger to disappear from the wild. This is due, in part, because of deforestation and the loss of thick groundcover, also known as understory cover, said Sunarto, lead scientist on a study that is the first to systematically investigate the use of both forests and plantation areas for tiger habitat.

Although tiger's prefer forest to plantation areas, the study found that the most important factor was that availability of thick ground-level vegetation which apparently serves as an environmental necessity for tiger habitat, regardless of location.

"As ambush hunters, tigers would find it hard to capture their prey without adequate understory cover," said Sunarto, who earned his doctorate at Virginia Tech and now is a tiger expert for the World Wildlife Fund-Indonesia (WWF-Indonesia). "The lack of cover also leaves tigers vulnerable to persecution by humans, who generally perceive them as dangerous."

Within forest areas, tigers also strongly prefer sites that have low levels of human disturbance as indicated by their preference for areas closer to forest centers and farther from human activity centers such as bodies of water and areas bordering plantations and towns.

Tigers occupy only around 7 percent of their historic range. Estimates place the current wild tiger populations at as few as 3,200 tigers, including only about 400 Sumatran tigers, which are listed as critically endangered on the International Union for Conservation of Nature Red List of Threatened Species.

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"These study results indicate that to thrive, tigers depend on the existence of large contiguous forest blocks," said study co-author Marcella Kelly, an associate professor in Virginia Tech's Department of Fish and Wildlife Conservation and Sunarto's graduate advisor.

The Indonesian government has set aside many areas and national parks for the conservation of endangered species but about 70 percent of tiger habitat in Sumatra, an island in western Indonesia, remains outside these protected areas. The preservation of such habitats, which requires support from government, landowners, and concession holders, is critical for conservation of the species, the study authors emphasize.

A recently published Indonesian presidential decree on land use in Sumatra points out the importance of building wildlife corridors between critical areas, where commitments from concession owners are key to successful implementation.

"Even with current legal protection for the species, tigers are not doing well in many places, especially those outside protected areas," Sunarto said. "As long as forest conversion continues, tigers will require active protection or they will quickly disappear from our planet."

The study concludes that in order to protect tigers, it is critical to stop clearing Indonesia's remaining natural forests for plantations. With adjustments in management practices on existing plantations to include more understory and riparian forest corridors, tigers could use a mosaic of forest patches across fragmented landscapes.

"We hope that plantation managers and concession owners can use the recommendations of this report to apply best management practices to further protect Sumatran tigers from extinction," said Anwar Purwoto, director of the Forest, Freshwater, and Species Program at WWF¬Indonesia.

"Ensuring that tigers are able to roam freely in natural forests and restored habitat is crucial to their survival," said co-author Sybille Klenzendorf, head of WWF's species program, who earned her master's and doctorate degrees in wildlife science from Virginia Tech. "This study is a reminder of just how important it is for us to protect the natural forests that tigers and other animals rely on."

The report was published in the Public Library of Science's online journal PLoS ONE on Jan. 23, and was a collaboration between the university and WWF, with support from the Indonesian Ministry of Forestry.