Much of tiger country in India is tropical dry forest. According to a study published in Biological Conservation, tigers disappear from these forests at a faster rate than any other habitat. In recent years, two such forests, Sariska in 2004 and Panna in 2009, lost their host of tigers.
Before the big cats disappeared from Panna National Park, Madhya Pradesh, Raghunanthan Chundawat conducted one of the longest running tiger studies. He alerted the park's management to the massive scale of poaching within the reserve. Not only did his pleas fall on deaf ears, his research permits were revoked. Chundawat and his team published this paper based on field notes and data of that time.
Using historical and current records, the researchers assessed the vulnerability of tigers living in different types of forests. Krithi Karanth and others had suggested that protected forests reduced the extinction risk of Indian mammals.
Chundawat and his team investigated the size of protected areas in various forest types and their relationship with the persistence of tigers. They found tigers in tropical dry forests are especially at risk. Over the past 100 years, the cats seem to have vanished from these forests at a higher rate than from any other habitat.
The researchers investigated what made tigers in tropical dry forests so vulnerable.
Large home ranges
Between 1996 and 2005, the team followed six radio-collared tigers in the 543 square kilometres Panna National Park. In 2002, the reserve boasted almost 7 tigers per 100 square kilometres The average home range of two males was close to 180 square kilometres, spanning a third of the park. While females defended an average of about 47 square kilometres of terrain. These tigers' territories were three to four times larger than in other tropical habitats.
The size of a tiger's home range varies with terrain and depends on the biomass of prey animals. In rich alluvial flood plains, a tiger can thrive within 16 square kilometres, while in the bitterly cold Russian Far East, it can span several hundreds of square kilometres.
In 2003, Panna offered a delectable choice of prey, from wild pig to nilgai and livestock. With a high 46 ungulates per sq.km. to pick from, the big cats ought to have smaller home ranges. The authors speculate that prey biomass alone doesn't determine the size of territory. Perhaps, they say, the distribution of resources like water, prey, and habitat plays a role.
The presence of floaters
Within the large domains staked by territorial males, other adult male tigers roamed for several years. The researchers suggest this may be an alternate behaviour strategy.
“If ranges are large, it is almost impossible for males to defend them exclusively,” Chundawat told Firstpost. Since Panna is isolated and unconnected to other forests, floaters or dispersing males that are unable to find suitable habitat outside the park return to its confines. “The dominant male has no choice but to accommodate these males. The Iriomote cat [a kind of leopard cat], endemic to one island, behaves in similar ways.”
Territorial males tolerated other males as long as they didn't pose a challenge. Unlike other forests, where floaters had little chance of mating, in Panna, females mated with both dominant males as well as floaters.
With many males around, cubs run the danger of being killed. Females may try to confuse the paternity of their offspring by mating with several males.
The question of size
The home ranges of all radio-collared tigers spilled outside the park, where they didn't enjoy nearly as much protection. The enormous space requirements of tigers in Panna-like forests and the modest size of protected areas in this landscape made small populations vulnerable, say the authors.
“Protected area size is not a good indicator of extinction risk,” says Chundawat. Forest type may also be a key criterion. “Tigers in a similar-sized protected area in the Terai have a much higher probability of survival than in tropical dry forests.” To conserve tigers, the area of protected dry forests has to be larger.
Increasing the size of Panna and other similar forests across the subcontinent would be the solution in an ideal world but a difficult proposition, when the reserves are surrounded by villages and agricultural fields.
“However, small forest patches embedded in these large landscapes have the potential to be good tiger habitats,” the authors say in a press release. “A series of several such areas, interconnecting populations on a landscape level, could create a larger breeding population.”
That would be one way to end Panna's isolation. Even as researchers talk of creating these stepping stones, Panna's integrity is threatened by the Ken-Betwa river linking project. The project will submerge 10 percent of critical tiger habitat and disturb 200 square kilometres of forest.
“It will not only destroy the possibility of making the Panna population viable, but also the future of other tiger habitats in the landscape,” says Chundawat. “Without the source population of Panna, there will be little chance for tigers to survive in the long-term in any of the surrounding landscapes.”