Biology Reference
In-Depth Information
we mean by “rare,” though? By what measure is a rhino or tiger considered rare? Most bio-
logists 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 topic, 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, sug-
gested 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 superabund-
ant 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 but-
terfly 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 pat-
tern 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 ex-
ample, 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
