Biology Reference
In-Depth Information
distributions. In their natural habitats some of these species may be, like the golden-fronted
bowerbird, easy to locate and observe but may nevertheless trick the biologist into treating
them as “common” when they are really just “obvious,” an important distinction. The real
revelation for some species, such as the six-wired bird of paradise, is how narrow their elev-
ation distribution is—just a vertical sliver of the mountainside—thus making them globally
rare because of their restricted range. By contrast, the golden-mantled tree kangaroo is twice
rare—it is both geographically rare (confined to a few mountain uplands in New Guinea's
northern coastal ranges) and population rare: hard to find even in its favored habitat.
Birds of paradise of one species or another once inhabited all of the island's area below
timberline. But their distinct distributions reveal a clear pattern of rarity. Ten species are rare
because they are endemic, confined to tiny mountain uplands or offshore islands. Some, such
as the black sicklebill and pale-billed sicklebill, are naturally rare or uncommon, with very
low population densities. They are the largest members of the bird of paradise family (the
Paradisaeidae), and these outliers tend to be widespread but nowhere common. Others, in-
cluding the yellow-breasted bird of paradise, appear to prefer very specific habitats, being
absent from some sites and found in others.
Although the main purpose of the Foja expedition was not to parse rarity but to discover
new species and recover lost ones, it nevertheless confirmed that even in a place without a
historical presence of humans, rarity is a common phenomenon. In this region, with its tiny
human population confined to the lowlands, birds of paradise are not threatened by hunting.
Thus, we can see their populations at what biologists call carrying capacity—densities one
would likely see that reflect what the habitat can support. From Bruce's field notes it became
clear that some, including the manucodes and the lesser bird of paradise, are naturally com-
mon within their range and easily encountered. Others, such as the pale-billed sicklebill, are
uncommon and elusive. By contrast, the remarkable black sicklebill, which has been hunted
out in many parts of New Guinea, turned out to be present in fair numbers in the Foja up-
lands, where no hunting takes place. The big surprise was the absence of the superb bird of
paradise, which is perhaps New Guinea's most common and widespread mountain-dwelling
species. It is known in virtually every mountain range in New Guinea except for the Fojas.
The last observation is truly puzzling and requires further study. But the other sightings,
or lack of them, show that even in perhaps the most remote spot in the tropics, the rules of
rarity still apply. Many species—be they birds of paradise, tree kangaroos, frogs, butterflies,
or palms—naturally occur at low population densities and hence are rare. Viewed the other
way around, there may be something about the very nature of rain forests that prevents some
species from becoming superabundant. The Fojas are rich in species, so perhaps the sheer
number of species in the ecosystem leads to a novel mechanism at work—a kind of diffuse
competition in which the interactions of many species keep others in a limited range or at a
low density. We will revisit this potential structuring force in the Peruvian Amazon in the
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