Research Article: Karanth, K. U., & Nichols, J. D. (1998). Estimation of tiger densities in India using photographic captures and recaptures. Ecology, 79(8), 2852–2862.

Blog Author: Shreya Ray

Key Takeaways:

  • Tiger conservation faced significant challenges in the 1990s, as there were no reliable methods at the time to estimate tiger population numbers.  
  • It was important to form a deep understanding of tiger ecology to devise conservation strategies. There were many testable hypotheses about tiger movement, dependence on prey, habitat, and interspecific interactions with other carnivores, such as leopards.
  • Using the fact that stripes can identify individual tigers, Dr. Ullas Karanth and Dr. James D. Nichols carried out their pioneering work to estimate wild tiger populations using a photographic capture-recapture framework. 
  • Through the use of camera-trap photography, and capture-recapture models, the authors were successful in generating tiger numbers for Kanha (28), Nagarhole (27), Kaziranga(22), and Pench(5) reserves.
  • Tiger density was highest at Kaziranga (16.8 individuals/ sq. km) and lowest at Pench (4.1 individuals/ sq. km). This pattern was attributed to variable levels of habitat, prey, and human pressures in these areas.
  • Conservation efforts such as effective anti-poaching operations, relocation of human settlements, and elimination of forest product exploitation have shown a significant positive impact on tiger populations.

 

Tigers are charismatic animals with striking orange and black stripes, powerful muscles, and fierce reputations. They are the largest of the cat family members and are known for their excellent hunting abilities. Tigers are native to Asia and are found in various habitats, including forests, grasslands, and swamps. With few individuals remaining in the wild, they are considered endangered. Wild tigers are under threat due to overhunting, poaching, and habitat loss. In the journey of tiger conservation, one of the most prominent challenges till the early 1990s was to count tigers reliably. Field surveys and forest cover maps had been successful in indicating where the tigers are distributed, but the puzzle of wild tiger numbers remained till the mid-1990s, due to a lack of suitable field methodologies. Most tiger counting methodologies failed as they did not take three main characteristics of the species into account: scarcity, extensive range, and secretiveness. 

Previous studies suggested that tiger populations are dependent on numbers of prey species such as chital, sambar, wild pig, or in rare cases, small mammals. According to these studies, tigers usually prefer larger prey species if available. If both large and medium-sized prey are available in the population, there is a higher chance of coexistence with sympatric species such as leopards. However, these predictions could only be tested by estimating tigers and their prey species.

In India, tiger population estimation was previously carried out using track counts by identifying individual tiger pug marks. This failed to produce reliable estimates because of erroneous assumptions. GPS telemetry could have been a good alternative but was limited by the high cost and effort involved.

In a quest to solve the puzzle of tiger numbers, Dr. Ullas Karanth  from the Centre for Wildlife Studies and Dr. James D. Nichols from University of Massachusetts Amherst started their  pioneering work using photographic capture-recapture to estimate tiger numbers. They  conducted their work in four highly preserved reserves representing significant tiger-prey populations: Kanha(900 sq. km), Nagarhole (644 sq. km), Kaziranga (488 sq. km), and Pench (290 sq. km) national parks. The authors  wanted to develop suitable sampling designs for camera trapping tigers and optimize statistical models to estimate tiger population size and density. Finally, they estimated prey density in all study sites to understand the impact of prey populations on tiger numbers.

Tiger ecology was carefully taken into consideration while conducting the fieldwork. Tigers travel along forest roads and trails, leaving scat, scrapes, scent deposits, and tracks to communicate with each other. Camera trap locations were selected based on the availability of such signs at particular sites, such as the intersection of trails. The date, time, and location of each capture were noted. Tiger stripe shape and position were compared between photographs, and this data was analyzed using the program CAPTURE to compute tiger abundance. Such intensive field and statistical exercises were successful in producing reliable tiger numbers. Kanha had the maximum estimated number of individuals (28), followed closely by Nagarhole (27) and Kaziranga (22). However, these numbers were starkly different from Pench, which was estimated to have only five individuals. Density estimates also reflected these numbers, which predicted a density of only 4.1 tigers/100 sq. km in Pench, while estimates for other regions varied between  11.5 to 16.8 tigers/ 100 sq. km. The authors attributed these patterns to variations in habitat, prey, and human disturbances in these areas.

Kaziranga reported the highest density of large prey at 16.9 animals/sq. km, followed by Pench (11.0 animals/ sq. km), Nagarhole (8.7 animals/sq. km), and Kanha (4.5 animals/sq. km). While these numbers are reflected in tiger populations in Kaziranga, Nagargole, and Kanha, Pench had a different result. Karanth &  Nichols  predicted low tiger numbers to be a product of poaching pressure in this region during the time. Their data also reflected predictions on tiger-leopard coexistence based on the presence of large and medium-sized prey species. They also concluded that moist deciduous forests and tropical alluvial grasslands provided optimal habitats for tigers to attain the highest densities.

Tiger populations in India were in a dire state during this time  due to overhunting and habitat loss. Conservation efforts such as effective anti-poaching operations, relocation of human settlements, and elimination of forest product exploitation have had a significant positive impact on tiger populations since. While conservation efforts are vital for the population’s long-term survival, it is equally important to measure the success of these efforts. This requires counting tigers with robust field and statistical methods, as proposed by the authors. This study in the late 1990s paved the way for a new era of tiger studies in India,  guiding many aspiring biologists with theory, field techniques, and statistical designs.

Key words: tiger; abundance estimation; density estimation; camera traps; capture–recapture models; felids; India; Panthera tigris.

You can access the original article here.