Prachi Thatte PhD student, National Centre for Biological Sciences, Bangalore, India
Sal, mahua, teak, tendu and several others, deciduous trees of varying heights characterize the tropical dry forests of central India. Light filters in through the uneven canopy and enables the growth of shrubs, lush grass and climbers in these forests. Such tropical dry forests constitute the largest habitat for the tiger, making up nearly 50% of its current habitat. At the same time, the tropical dry forest ecosystem also supports high human population densities.
Tropical dry forest, Panna Tiger Reserve. Photo by Rohan Jahagirdar
With fertile soil and agreeable climate, this ecosystem provides favorable conditions for agriculture and human settlement. Clearing of land for agriculture has resulted in extensive deforestation of this ecosystem over years. Human activities are considered the largest threat to this ecosystem. Panna and Sariska, where tigers recently went extinct and were re-introduced, are both tropical dry forests. Poaching was established as the main reason behind the extinctions.
Tiger in a tropical dry forest. Photo by Kaushal Patel
For better management of tiger populations in the future, it was important to identify other factors that may have contributed to the extinction. In a recent study, researchers have found that the type of forest and the size of the protected area can also contribute to the vulnerability of predator extinction.
Dr. Raghunandan Singh Chundawat and co-authors examined whether tigers in some habitats are more vulnerable to extinction than others in a study published in the journal Biological Conservation in 2016. They also investigated if the size of a protected area contributed to its vulnerability of extinction.
In order to understand the role of forest type, the authors first generated a historical tiger distribution map for the Indian subcontinent from old tiger location records. They then compared this map to the current tiger distribution. Overall, tigers have disappeared from several areas including the agriculture dominated areas of Uttar Pradesh, Northern Madhya Pradesh, Rajasthan, North Eastern Karnataka and Tamil Nadu over the last century. Their current distribution is severely restricted. The comparison also revealed that the tiger populations are disappearing at a faster rate in the tropical dry forests compared to other habitat types. This suggested that the tigers in tropical dry forests are more vulnerable to extinction than other habitats.
Historical and current tiger range. By Sanderson, E., Forrest, J., Loucks, C., Ginsberg, J., Dinerstein, E., Seidensticker, J., Leimgruber, P., Songer, M., Heydlauff, A., O’Brien, T., Bryja, G., Klenzendorf, S., Wikramanayake, E., via Wikimedia Commons
The researchers wanted to understand if the size of the protected area, along with the forest type, contributed to the vulnerability of extinction. For investigating this, the researchers used information from the Panna Tiger Reserve where tigers had gone extinct in 2008. The researchers had radio-collared tigers in Panna Tiger Reserve before they went extinct. Using the data on territory size of the collared tigers, they investigated whether the space required by tigers matches that provided by our protected areas.
The data on radio-collared tigers revealed that the territories of tigers in Panna were three to four times larger than those generally reported for other habitat types, e.g. tropical rain forests in the Western Ghats. The size of tiger territory generally depends on the density of prey in an area. So if the density of prey is lower, the tiger territory is larger. Interestingly, an earlier study by the researchers had revealed that the prey densities in Panna and the rainforests of the Western Ghats were comparable. The researchers suggest that the larger territories in Panna must be a result of ecological characteristics of the tropical dry forests like the uneven distribution of water sources, prey and suitable habitat.
If the territory size in the tropical dry forests is larger, the size of the protected areas (PAs) in this habitat should also be larger compared to other habitat types. However, the researchers found a gross mismatch between the area tigers require and the average size of the PAs in tropical dry forests. Even in Panna tiger reserve, a relatively large PA that is 543 sq. km., the home ranges of all the breeding tigers extended beyond t he park boundary and the tigers frequently ventured out of the PA. Using areas outside PA boundaries can be risky for tigers due to several reasons including exposure to organized poaching and retaliatory killing. The study suggests the lack of sufficient protected area is an important reason why tigers are disappearing at a faster rate from the tropical dry forests.
Increasing the size of the PAs in the tropical dry forests sounds like a straightforward solution. However, increasing area may not be possible given the human dominated areas that surround most PAs. The researchers hence propose having several small protected areas in the human-dominated landscape. This will create a network of interconnected PAs and help in ensuring the survival of tigers in areas that are not large enough.
Several other recent studies on tigers in India suggest that conservation focus should shift from tigers inside protected areas to incorporate surround areas and inter-connected populations. Such a landscape approach to conservation would be critical to ensure persistence of tigers into the future.
Original paper: Chundawat, R. S., Sharma, K., Gogate, N., Malik, P. K., TamimVanak, A. (2016). Size matters: Scale mismatch between space use patterns of tigers and protected area size in a Tropical Dry Forest. Biological Conservation 196, 146-153