The Kingdom of Rarities (2 page)

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, why should biologists study rarity, the rhinos rather than the roaches? The obvious academic response is “Because we know so little about them.” Rephrasing the question, though, brings into focus a profound and central riddle of nature: Why, wherever you land, do you always find a few superabundant species and a multitude of rare ones?

One of the first lessons in community ecology—the science of how species interact in nature—is the prevalence of rarity at any locale in the tropics. Sweep a forest plot with a butterfly net, identify all the trees in that tract, scan those trees for singing birds, and you'll find the same result: many individuals of a few species and a lengthy list of singletons. This pattern holds from the forests of Madre de Dios, Peru, to Mondulkiri Province, Cambodia. Even though rare species occur everywhere, we still know too little about how they fit into the big picture of our wild menagerie. But some intriguing answers have emerged regarding, for example, the roles various rare species play in shaping the form and functioning of ecosystems and how ecosystems are affected as particular rare species are lost.

Attention to rarity can raise vital questions: Are all rare species, for example, by definition on the verge of extinction? Have all species that are currently rare been historically rare? Which species common now are likely to become rare? Greater clarity on these fundamental issues will help shape our response to saving wild nature. Will species that are common now become rare as a result of changing climate? For example, how will egg-laying sea turtles find nesting sites when sea levels rise, and how will moisture-dependent frogs lay eggs when rain forests face prolonged droughts, in some cases by the middle of this century? When the microclimate at the summit of Mount Udzungwa in southern Tanzania changes in a profound way, will the African violet—ancestor of the familiar houseplant—and the Udzungwa partridge disappear, or will they be able to adapt to the new conditions?

During the 1980s, leading biologists began to suggest that we were in the midst of the sixth great extinction event in the history of Earth. And in 1995, Stuart Pimm, one of the fathers of modern conservation biology, calculated that the current rate of species extinctions was as much as 1,000 times the normal background extinction rate. If so, newly rare species may face different, and more serious, problems from those encountered by species that have
historically been rare—another major reason for exploring rarity in the natural world.

Beyond the extinction crisis, some scientists refer to our current epoch, the Holocene, as the Anthropocene or the Homogenocene, terms that describe two aspects of a new ecological state that is still poorly understood. The first refers to our period, wherein the human footprint extends everywhere in nature. The second refers to another kind of affront in which certain species have spread or been introduced by humans far beyond these species' original range and, as a result, natural habitats around the world, full of invasive species, begin to resemble one another. Being rare in this brave new homogenized world, as we'll see in the case of Hawaii, could mean something much different from when these same species first appeared in relative isolation. Rarity is not just a condition of nature; it is a condition that can be—and has been—imposed on species by human activity, all too often sending them on the road toward endangerment and extinction. In short, viewing the natural world through the lens of rarity can bring certain facts and species traits to our attention that we might otherwise overlook. Understanding these facts and traits may in turn provide insights that can help us save species from the current state of environmental deterioration.

Many conservation biologists target “saving rare species” as the ultimate aim of their work. Yet rarity, as a phenomenon in nature, can take many forms, not only among species, although that is central, but also in the building blocks of the natural world: genes, populations of species, habitats, assemblages, and ecological and evolutionary phenomena. Species, with few exceptions, are made up of populations distributed across the landscape. Saving only one population of each rare species simply as a token gesture would be of little ecological value, especially where those species play a role in maintaining a given ecosystem's integrity. So an essential goal is to conserve multiple populations of species and the genetic, ecological, and behavioral features that these building blocks contain. Conserving dispersed populations and their genetic variability
gives species a better chance of adapting to and persisting amid changing conditions, such as a rapidly changing climate or invasion of their homeland by introduced species.

Buried within the species extinction crisis is another, less publicized calamity: the increasing rarity of species populations. These losses of populations, as well as in some cases entire species, have led biologists to sound warning after warning. The eminent biologist E. O. Wilson, for example, pronounced in a speech in early 2000 that “biodiversity cannot afford another century like the last one. We are about to lose thousands of species a year, especially in rainforests.” Wilson could have extended the depth of the problem, if risking the simplicity of his message, by adding a phrase whose meaning has gone unnoticed by the general public: we have been losing
populations
of species faster than we have been losing species themselves.

These two concerns—rarity of species and paucity of particular populations—merge when it comes to those species whose entire earthly existence is represented by a single population, as a result of either natural forces or human encroachment. Who are these singleton species, and how many of them are now close to the abyss of extinction?

In 2003, several colleagues and I put together a paper for the
Proceedings of the National Academy of Sciences
to address this question, name those species, and suggest how their imminent extinction might be prevented. Our work on the paper, which was published in 2005, sparked the scientific basis for this book, an interpretation of the evolutionary and contemporary aspects of rarity. We focused our effort on a subgroup of relatively well known but threatened vertebrates, our fellow creatures with backbones—birds, mammals, reptiles, amphibians (fishes are yet to be analyzed). We postulated that certain of these species were already so uncommon that they would be extinction's next dodo birds unless action were taken to prevent their disappearance.

To begin, we turned to the gold standard for evaluating rarity of wild species, the International Union for Conservation of Nature
and Natural Resources (IUCN) and its famed Red List of Threatened Species, which ranks species on the basis of sizes of remaining populations. The IUCN assigns the category “endangered” or “critically endangered” to species whose numbers have plummeted toward extinction. We then went a step further. “Let's name the rarest of the rare, those species whose entire global range is limited to one population at a single site,” my colleague John Lamoreux suggested. He was proposing that we limit our survey to such species as the Bloody Bay poison frog, which hails from the last patches of rain forest on the island of Trinidad, the only place on Earth where it can be found.

Once a species such as the Bloody Bay poison frog is restricted to a single dot on the map, if one or another of several catastrophes strikes—if the spot is plowed, burned, flooded, drained, paved, polluted, or overrun with pigs, rats, or other invasive species—the threatened species that lived in that dot is gone: vanished forever. Rarity then becomes the precursor to extinction or, at least, its preexisting condition. Alternatively, if you save the place, you save the rare species—conservation in black-and-white.

Our results provided some new insights and a number of surprises. First, despite there being 20,000 species on the IUCN Red List, only 800 species found at 600 sites (some species shared the same site) met our criteria. Second, half of the species limited to a single site turned out to be amphibians. Third, many single-population species were restricted to isolated mountaintops. A botanist on our team, George Schatz, cautioned the vertebrate specialists against any euphoric notion that saving the world's rarities might be as easy as saving some isolated mountaintops where few people live. “Remember,” he warned, “the 250,000 or so vascular plant species have yet to be evaluated for levels of threat. At least 10 percent of these are known only from the single site where they were first collected.” There is a joke among field biologists that rarity is partly a natural phenomenon and partly the result of some less energetic biologists failing to wander far enough from the road or the field
station in surveying their specialty. There may be an ounce of truth to that, but the idea that the populations of many plant species, and the insect species they host, could be so few only reaffirms the important role of rarity, especially in the tropics.

The next question for our group of biologists was which rare species or place we should try to save first. This exercise drew us to a global map and triggered much debate. “Here.” Mike Parr leaned over northern South America to point out the location of a mother lode of rarities. His pen tip lingered on a massif that stood by itself in northern Colombia, the Sierra Nevada de Santa Marta. The solitary giant sat about 42 kilometers from the Caribbean coast and about 115 kilometers from where the sawtooth eruptions of the northern Andean chain began. Santa Marta in Colombia, like Mounts Kilimanjaro and Udzungwa in Tanzania, Mount Cameroon on the border of Nigeria and Cameroon, and Mount Kinabalu in Sabah, Malaysia, are but a few of the dozens of solitary mountains in the tropical belt that are hotbeds of natural rarities. Why this might be so was one of the questions I wanted to investigate.

“Here is where I want to go next,” I said, pointing to the Zapata Swamp on the island of Cuba. Considered the Cuban version of the Everglades, this freshwater swamp is home to the Cuban crocodile, the Zapata wren, the Zapata rail, and two species of hutia (a guinea pig–like rodent) found nowhere else in Cuba, the Caribbean, or anywhere else. In the same swamp are the only robust populations of several Cuban birds—the Cuban sparrow, Fernandina's flicker, Gundlach's hawk, and the blue-headed quail-dove—proving that rarity is not confined to tropical mountains or even rain forests.

As we populated the map in front of us and delved into the causes of rarity for the 800 species that met our conditions, we saw another insight into rarity confirmed. Some of these species had likely always been rare, such as the 13 frog species sharing the same genus and the same mountaintop in Haiti, the Massif de la Hotte; others on the list had been made rare by human activities. Some species had been much more common during an era when
the climate was different from what it was during our mapping project—colder, hotter, drier, wetter. They were now climate refugees. Some species had been doing fine at a single site until rats arrived on their island. We realized that we had to consider all the different causes of rarity to better understand which species would be likely to persist without much conservation effort. We needed to know which species had always been rare but were now facing even lower numbers, a more limited range, or a new invader.

Some of the more promising places to look for the causes of rarity and of patterns of rarity and abundance are where there are no people. A remote mountainous region of New Guinea with no history of human visitation, the locale of chapter 2, offers a good venue to investigate the extent of rarity under natural conditions. By comparing what we discover there with what is found in other ranges where local tribes have access, we can begin to answer several fundamental questions about how rarity is created and what pattern exists where humans have had no perceivable influence. New Guinea also offers a rewarding glimpse of how extreme isolation and active geology can lead to rarity and a narrow range of resident species. In contrast, another area with low human activity, the Peruvian region of Madre de Dios, the locale of chapter 3, illustrates a condition that exists for many tropical rarities, from jaguars to canopy trees—a wide range of species living at extremely low densities.