The Uncommon Menagerie

Riding on an elephant's back offers a privileged, if distorted, perspective on the natural world. Wildlife species that seem large and scary at eye level, such as rhinos and tigers, appear as miniaturized versions from this elevated vantage. My well-trained mount, Kirti Kali, plowed boldly through the dense twenty-foot-tall grasslands of Chitwan National Park in lowland Nepal, scattering spotted deer and wild boars in our path. On elephant-back one feels invincible. As we emerged from the tall grass into an open area, my driver, Gyan Bahadur, calmly steered Kirti alongside a rare greater one-horned rhinoceros—a dangerous species that, locally, tramples and kills several villagers a year. The two-ton female and her young calf continued grazing peacefully on the floodplain. The rhinos seemed oblivious to our presence because we had spent months habituating these aggressive creatures to close contact. As long as we remained on the elephant, rather than approaching on foot, the mother rhino would remain unfazed and we would stay in one piece.

My jungle wanderings also warped my perspective on how uncommon these animals had become. By 1988, at the end of my initial five years of research, I had recorded thousands of observations of Chitwan's 370 one-horned rhinos and had photographed, identified, and named nearly every one. Yet seeing them every day made me forget their global rarity. At the time, only about 1,500 survived in the wild worldwide; all but those in Chitwan roamed in Kaziranga National Park in northeastern India. Although the one-horned rhino's numbers have slowly increased since then—in 2012 there were over 2,900, distributed among twelve populations—this species remains among the most endangered large mammals on Earth. Its status during my initial study raised several questions: Had the ancestors of this rhino, a diverse ancient lineage, always been rare during their evolutionary history? Or is the rarity of the one-horned rhino a relatively recent phenomenon, triggered by habitat loss and poaching for the mythical qualities of the rhino's horn?

My ecological study took an unexpected turn when I asked Gyan to maneuver Kirti Kali into an ideal spot for a photograph. I raised my camera to capture an exquisite panorama: the rhino cow and calf in the foreground, perfectly framed by the Annapurna range and Mount Dhaulagiri to the north. Then I noticed some clumps of low trees that spoiled the picture's composition. Copses of a species called bhellur (Trewia nudiflora) stood out like archipelagoes in the midst of the grassland. I asked Gyan why the trees had assumed this pattern. He took a break from smoking a cigarette rolled in a jungle-leaf wrapper to answer my silly question. "Oh, it's the work of gaida," he said matter-of-factly, using the Nepali word for "rhino" and gesturing toward the tree islands. "Those are old rhino latrines."

Rhinoceroses return to the same places time after time to deposit their dung—not out of tidiness but because these communal latrines allow solitary animals living in dense vegetation to exchange vital data, via scents within the dung, about their whereabouts and sexual activity. The sheer size of the dung piles, sometimes dozens of meters long, and the dense stands of Trewia trees that sprang from them were a revelation to me. All the more so because when I first arrived in Chitwan, I had wondered how this giant herbivore could have even a minor influence in cropping the lush vegetation—the wall of green grass surrounding me—which was recharged each year by the summer monsoon.

The answer lay buried in the dung. By voraciously consuming Trewia fruit and defecating intact seeds in latrines scattered throughout the floodplain, the rhinos could rapidly convert the world's tallest grasslands into Trewia forests. Countervailing the rhino-dispersal effect were the annual floods, which wash away and bury Trewia seedlings, and the annual natural fires, which incinerate much of the previous year's crop. But some of these seedlings obviously survived to become tree islands. What remained as an indelible imprint for me was the staggering potential of rhinos to reshape their surroundings, implying, in this case, that ecological impact does not always reflect numerical abundance.

It would be a stretch to say that sifting through rhino dung or musing while on elephant-back triggered my fascination with rarity. But my observations of these rhinos, and observations that I and others had recorded of another globally rare denizen of their neighborhood, the tiger, made me wonder: What if more biologists fanned out to study in depth not the common mongoose or the ubiquitous spotted deer but members of Chitwan's uncommon menagerie—great hornbills, Gangetic dolphins, gharial crocodiles, sloth bears, and Indian bison? How might one's perspective on the natural world change? What novelties, complexities, and even counterintuitive elements might emerge, and what adventures lay in store for the pursuer of these rarities?

As a scientist, I knew that the interplay of rarity and abundance is central to understanding patterns of nature as well as understanding the idea of dynamic ecological balance. What do we mean by "rare," though? By what measure is a rhino or tiger considered rare? Most biologists would apply the term to a species that occupies a narrow geographic range, has a low abundance, or exhibits both traits. Often this label stems from a comparison of an uncommon creature with others that share its habitat or taxonomic group, but it can also be viewed in absolute terms. For example, sticking with rhinos, the greater one-horned rhinoceros is rare from a global perspective, with fewer than 3,000 individuals, but it's relatively common in comparison with the highly endangered Javan rhinoceros, of which fewer than 50 remain, and those restricted to one locale. In this book, I draw mainly on examples of rarity among mammals, birds, and plants—the creatures I know best. But the condition of rarity transcends appearance and taxonomy. Whether an organism has a backbone, a beak, pincers, or petals or is covered by scales, fur, feathers, or fins, the same rules apply—occupying a limited space geographically and exhibiting low population densities guarantees a place in what I call the Kingdom of Rarities.

The simple truth is that many, many species on Earth are rare, but few people other than biologists are even aware of this fact. A leading ecologist on the subject, Kevin Gaston, suggested an astonishing asymmetry of life on Earth: as few as 25 percent of the world's species, such as robins, rats, and roaches, may account for 90 to 95 percent of all individuals on Earth. But if Gaston's estimates are correct, as much as 75 percent of all species on Earth may be drawn from the ranks of the rare. It's a stunning idea to contemplate.

If relatively so few individual organisms on Earth make up the rare,...