As a schoolboy growing up in the spectacular wilderness of southwestern India's hill country, known as Malenad, I was enchanted by tigers. The many tiger-themed rituals in our Hindu culture fueled my fascination. During the autumn Dasara festival celebrating the triumph of good over evil, for example, muscular Huli Vesha men, their bodies painted in patterns of ochre, white and black, mimicked the cat's graceful movements as the dancers moved to the crescendo of drumbeats. It was an electrifying spectacle. But the unfolding reality around me was grim: livestock farmers and sport hunters were killing off the last wild tigers, and loggers were relentlessly felling the rich forests for timber. By the time I was a teenager in the early 1960s, I had given up dreams of ever seeing a tiger in the wild.
A few years later, however, an apparent miracle intervened. Responding to the rising clamor from conservationists, India's then prime minister Indira Gandhi implemented strict conservation laws and established several protected wildlife reserves. Tiger conservation gained global momentum in the decades that followed. Many countries banned legal tiger hunting and attempted to reconcile the deep contradictions between the tiger's need for forests and human demands on its habitat. India did better than most tiger nations: although it harbors only 20 percent of the remaining tiger habitat today, India shelters 70 percent of the world's tigers—no mean feat, given pressures from its 1.2 billion humans, persistent poverty and the growing industrial economy.
Despite those conservation initiatives, however, tiger populations have continued to blink out across Asia. Just two centuries ago wild tigers roamed across 30 Asian countries, from the reed beds of the Caspian Sea to the conifers of Russia, from India's woodlands to Indonesia's rain forests. That once vast range has collapsed by 93 percent, confined to a handful of countries. And populations with reasonable chances of recovery occupy an even smaller area—less than 0.5 percent of the historical tiger range.
The fate of these 40 to 50 tiger clusters, known as source populations because only they are large enough to sustain reproduction, hangs in precarious balance. Most are isolated and surrounded by hostile human landscapes. Like patients under intensive care, these source populations need close monitoring. Yet even after long-running conservation efforts, such focused tiger monitoring is the exception rather than the rule. As a result, scientists have a poor understanding of how wild tigers are actually faring. The traditional methods for surveying tigers are at best sufficient for determining where in Asia they still roam; they cannot reliably estimate how many individuals remain. Indeed, many of the tiger numbers bandied around by conservationists in the media have little solid evidence to back them up.
In recent years my colleagues and I have made significant inroads into the problem of how to count these elusive felines. By combining camera-trap technology that snaps shots of animals as they pass by with software that identifies specific individuals and sophisticated statistical analyses that can estimate full population sizes from samples of tiger photographs, we have painted a far more accurate picture of several tiger populations. The challenge going forward is getting conservation agencies to apply these improved surveillance methods to track the fate of the source populations across their range.
An Elusive Subject
Determining how many tigers exist, and where, is a formidable task because the cats are scarce, secretive, far-ranging and distributed across an immense geographical area. For decades these traits rendered useless attempts begun by officials in India, Nepal, Bangladesh and Russia in the 1960s to census tigers by counting tracks. The officials assumed that just as human fingerprints are unique, so, too, are tiger paw prints. Thus, they thought, they could count every tiger by counting tracks. But in truth, these methods fail because tracks can be difficult to differentiate and can go undiscovered. In India, clouds of unreliable data generated using this flawed census approach gave the impression that tiger numbers were rising and bred a deep complacency about conservation, even as risks to tigers rose. But while the officials were engaged in their misguided paw-print counting, rapid scientific progress in the fields of ecology, photography, computer programming and statistics were giving rise to new methods capable of accurately tallying tigers.
As a graduate student at University of Florida in the 1980s, I busied myself learning these novel approaches. I was determined to enter the secret world of tigers so that I could learn about their behavior and better understand how they were faring in the wild, particularly those in Nagarahole National Park, one of the reserves in Malenad where tigers had staged a comeback following Gandhi's conservation mandates. In 1990 I got my chance, working with the Wildlife Conservation Society to carry out the first ever radiotelemetry study of tigers in India. By keeping close tabs on a few individuals, I would be able to glean insights into tiger behavior that would inform efforts to count as well as conserve them.
I recall the cool, bright morning of January 29 when I sat five meters up in a Randia tree with a dart gun, waiting for a 220-kilogram tiger that other team members were chasing my way using a cloth funnel. From my perch I spotted a flicker of sunlit gold 50 meters away in the dense brush. The tiger was calmly padding toward me. As his shoulder, and then flanks, came into my crosshairs, I squeezed the trigger. The red-tailed dart flew through the air to sting his thigh, eliciting a mild growl. We soon found him lying sedated under a shady tree and outfitted him with a special collar containing a fist-size transmitter that would broadcast radio signals that I could pick up using a handheld antenna, allowing me to locate him at any time. A couple of hours later the tiger—now labeled T-04—wandered off to join three other tigers I had collared earlier in the 645-square-kilometer reserve.
Over the next six years radiotelemetry revealed the nuances of tiger behavior by enabling me to spend less time searching blindly for tigers and more time observing them. More important, this approach exposed where the cats wandered. The resident tigers I tracked in Nagarahole had home ranges of about 18 square kilometers for adult females and 50 square kilometers for adult males. Tigers are territorial, and adults steer clear of one another unless they are mating. These small range sizes suggested that tiger population density in protected parks such as Nagarahole might be higher than previously thought.
The telemetry work also revealed in greater detail than ever before what the Malenad tigers eat by leading me to the stinking carcasses of the prey they killed. Together with the even smellier scats I collected, these data showed that tigers typically kill one large prey animal a week, consuming two thirds of it over a period of three to four days before moving on. Ultimately the diet findings implied that depletion of wild prey by human hunters was a decisive factor in driving historical tiger decline and suggested ideas for how best to recover the species now.
By 1993 I had also figured out how to estimate numbers of the tiger's chief prey—deer, wild pigs and wild cattle—in a given area. I started with a sampling method, developed by American wildlife biologists, that involves two surveyors walking stealthily along transects—straight, narrow, 3.2-kilometer-long trails that I cut through the forest. The surveyors count all the prey animals they see on their walk and measure a given animal's distance from the transect line using a range finder. From these counts and distance measurements, one can estimate the total number of prey animals, accounting even for animals that were missed during the count.
Looking at my results—the first such data from Asia—I was astonished by the abundance of prey in the protected reserves of Malenad. These forests now harbored 16 to 68 wild ungulates (the mammal group that includes deer, pigs and cows) per square kilometer, densities higher than those in the richest East African savannas. This was good news for tigers: India's reserves, though relatively small compared with the parks of North America or Africa, could still support a lot of big cats. From such estimates of prey availability, biologists could begin to guess how many tigers any forest in Asia could potentially support.
But by the mid-1990s tigers in India's reserves came under intensified poaching pressure from organized criminals catering to burgeoning demand for tiger body parts from newly rich Chinese consumers. Conservationists needed to assess the scope of their impact by getting accurate counts of tigers in key populations. How many tigers actually remained? How many were being lost or gained every year? Did tiger numbers naturally fluctuate? Did their densities vary from region to region?
Ready for Their Close-ups
To answer these questions, I hoped to identify and count tigers in what was a new way at the time, using photographs shot automatically by camera traps placed along trails. The traps were electronically triggered by tigers (and other animals) walking past them. I would identify each tiger based on the unique stripe pattern on its flanks. The camera traps would allow me to spy on many more tigers than the radiotelemetry permitted. Still, I realized that my traps would photograph only a subset of the tigers in the populations I was studying. To correct for this shortcoming, known as imperfect detection, I needed to be able to estimate the size of the full population by extrapolating from the number of animals I managed to photograph.
My search for the appropriate statistical method for this situation led me to James D. Nichols of the U.S. Geological Survey's Patuxent Wildlife Research Center in Maryland. Nichols is an expert in what are known as capture-recapture models, which rely on numbers of identifiable individuals caught in repeated surveys to address the problem of imperfect detection. Imagine a jar of marbles of equal size. You grab a few, label them, then dump them back into the jar. Then you take another handful. Some are labeled; some are not. From the frequency of recaptures of labeled individuals, the models can estimate the average probability of detecting any given individual and then the total population size.
I had to fine-tune this generic model to solve the particular problems that tiger biology and field logistics posed. Whereas each marble is just as likely to get caught as any other, the same does not hold true for tigers. Because tigers have different home ranges and preferred paths, camera traps located in any area differ in their chances of capturing each individual. Tiger movement can vary by season and by the age and sex of the animals, thereby affecting capture rates. Some tigers may get spooked by the camera flash and avoid the trap next time. And unlike marbles in a jar, tiger populations experience births, deaths, and movement of individuals in and out of the area. I had to sample the population repeatedly but do so within a short period of 30 to 45 days to ensure that the numbers did not vary too greatly. Unfortunately, many expensive tiger surveys still ignore this precaution and produce inflated numbers as a result.
My camera-trap studies showed that population densities could range from 0.5 tiger per 100 square kilometers to 15 tigers per 100 square kilometers. Why, I wondered, did they vary so widely across habitats? In 1967 wildlife biologist George Schaller surmised from his observations of tigers in India's Kanha National Park that a tiger annually takes 10 percent of all prey animals available in its territory. If, as my early telemetry studies indicated, a tiger kills roughly 50 prey animals a year, then it needs some 500 ungulates in its territory to produce sufficient prey for it to consume. I speculated that prey densities might explain the huge variations in tiger densities.
To test this idea, between 1994 and 2003 I ventured beyond Malenad to estimate tiger and prey densities in reserves across India with diverse habitats ranging from mangrove swamps to evergreen forests. My results, published in 2004, confirmed the predicted ratio of one tiger to 500 prey animals. They also supported my hunch that overhunting of prey animals by local hunters, not tiger poaching for international markets, was the primary driver behind the historical collapse of the tiger range over the past 200 years. Determining the main cause of the decline was essential because it suggested that the key to combating the decline was preventing villagers from hunting the tigers' preferred prey through effective local patrolling, as opposed to catching tiger traders in faraway places.
Building on those density data, I expanded the annual monitoring of tiger populations from Nagarahole to other important reserves in Malenad in 2004. When camera-trap surveys are repeated year after year, they can capture population increases or decreases as well as numbers of individuals lost (from deaths and dispersal) and gained (from births or immigration). Such comprehensive, real-time understanding of tiger population changes offers the only means of providing a rigorous audit of successes or failures of efforts to secure and recover tiger populations.
Manually comparing each new tiger photograph with thousands of previous ones to identify individuals was tedious and slow. But pattern-matching software called ExtractCompare, developed by mathematician Lex Hiby of Conservation Research in England, enabled me to automate and speed up the identification process starting in 2000. (This versatile software identifies not only live tigers but also tiger skins seized from poachers, which greatly helps to secure criminal convictions.)
Twenty-five years of camera trapping in Malenad has created one of the largest systematic photographic databases of wild tigers, with 8,843 images of 888 individuals on record. Every season I document about 250 individual tigers concentrated in reserves that together span some 4,000 square kilometers. Some individual tigers appear year after year in the surveys, whereas most are detected in only one or two seasons, indicating high rates of turnover in the tiger population. The population of 400 to 450 tigers in the Malenad landscape is possibly the largest in the world now. My observations suggest that there are five times more tigers here than there were 50 years ago—a tribute to the efforts of local governments and conservationists.
Results from these long-term studies demonstrate for the first time how healthy tiger populations function in the wild. Well-protected tiger populations, such as the one in Nagarahole, are not static. Their densities naturally fluctuate from a low of seven tigers per 100 square kilometers to a high of 15 tigers per 100 square kilometer over longer periods. Even such a high-density tiger population loses an average of 20 percent of its members annually. Natural violence—killing of cubs by males, injuries sustained while fighting or hunting, followed by starvation—inflicts substantial losses. Killings by farmers who are defending their livestock and poachers who are supplying the black market for tiger parts—activities that occur even around protected reserves—also contribute to mortality rates. But because prey is abundant on these reserves, the number of new tigers born more than compensates for these losses. The surplus animals try to disperse and settle in new areas. These findings mean that instead of fretting over deaths of individual tigers, as conservationists often do, our goal ought to be to focus on populations as a whole. Rather than using our limited resources to try to eliminate all the threats tigers face everywhere across their range, we should target our efforts on sustaining those source populations with the greatest potential to recover and expand.
Through the 1990s and early 2000s I focused on understanding how tiger source populations function and are affected by human pressures. Yet these relatively secure populations are themselves embedded in wider landscapes that are less tiger-friendly. What is happening to tigers that live not in the reserves that house the source populations but in these surrounding “sink landscapes,” so named because they absorb the surplus tigers produced by the breeding source populations?
My camera trapping in Malenad revealed long-range dispersals of newly grown up tigers: male tiger BDT-130 migrated more than 180 kilometers from Bhadra to reach Anshi-Dandeli in 2008; another male, BPT-241, moved more than 280 kilometers from Bandipur to the forests in the Shimoga district in 2011. Many other tigers traveled between adjacent reserves. These data suggested that sink landscapes allow animals from different source populations to mate, which helps to maintain healthy levels of genetic diversity. Thus, an important aspect of sustaining the source populations is maintaining habitat connectivity through sink landscapes to permit tigers to disperse.
To obtain a fuller picture of where tigers live, I decided to expand my assessment to monitor landscapes beyond 4,000 square kilometers. But the camera-trap surveys that worked well in smaller reserves were impractical and expensive to use over such large areas. Landscape-scale tiger surveys must necessarily use methods that involve searching for less direct signs that are more readily encountered than the animals themselves, namely tiger spoor—tracks and scats—data that can establish where tigers are present but not how many there are.
In 2006 I initiated a habitat-occupancy survey of tiger signs across the entire Malenad landscape of 38,350 square kilometers. The results showed that tigers inhabited about 14,076 square kilometers, or 66 percent, of the 21,167 square kilometers of suitable habitat available to them, which means that tiger populations do have plenty of room to expand. My findings additionally revealed that those areas with the highest tiger densities also had higher prey densities and restricted levels of human access, bolstering the notion that a key to saving tigers is ensuring that human hunters do not compete with them for prey animals.
In an ongoing collaboration between the Wildlife Conservation Society–India Program and the Indian Statistical Institute, my colleagues and I are exploring how tiger abundance measured at reserves using intensive and expensive methods such as camera trapping can be integrated with extensive and cheaper scat and track data from wider landscapes to yield better estimates of tiger numbers across even wider regions and countries. We hope the work will provide new insights into how to enhance tiger survival across the species' range.
Photographic capture-recapture and large-scale occupancy modeling are now used to estimate tiger numbers and range in several countries across Asia. (Scientists who study other elusive carnivores with unique body markings, including African wild dogs and wolverines, are also employing these approaches.) Yet on the whole, although the science of tiger population assessment has rapidly progressed, its adoption by governmental and nongovernmental conservation agencies has not, whether because of a lack of understanding of or comfort with the new methods or because the old methods cast a more flattering light on their efforts.
A recent example illustrates just how insidious reliance on outdated tools is. In April the WWF and the Global Tiger Forum announced to great fanfare that the planet's wild tiger population was at last on the rise, numbering 3,890 individuals. These groups aim to increase the number of tigers to 6,000 by 2022. But their tally, based on official estimates, relied on flawed methodologies, including the use of statistically weak extrapolations from tiger photographs and field counts of spoor. And their goal for population growth far exceeds what one would expect to realize on the basis of studies carried out using the rigorous techniques described here. Furthermore, apart from the increases in tigers in a few reserves in India and parts of Thailand, there are no convincing data to show that populations are recovering in the rest of Southeast Asia or Russia. Indeed, countries such as Cambodia, Vietnam and China have lost their viable tiger populations in recent years—losses masked by any single global tiger number.
Speculative tiger numbers for countries and regions undermine efforts to save tigers by distracting conservationists and the public from what should be our top priority: guarding and growing the source populations. In a way, the overall number of wild tigers, if we could even get an accurate count, may not matter. The source populations are the ones we need to monitor vigilantly, using the best science available to track their numbers. Only with reliable counts can we set realistic goals for future growth, develop suitable strategies for meeting those goals and measure the impact of our conservation efforts.
History shows that scientific progress can stall from lack of understanding, institutional inertia and political considerations for decades or even centuries. But as the world enters into the sixth mass extinction of wild species, we simply cannot afford to divorce conservation practices from sound science if we are to have any hope of saving a wildlife icon like the majestic tiger.