Biology Reference
In-Depth Information
with their populations and change the composition of the forest. Harald Beck, a peccary bio-
logist and Terborgh protégé, found that in areas where peccaries no longer roamed freely
because they were hunted out, there was a dramatic increase in seedling dispersal of a com-
mon palm genus,
Iriartea
, whose seeds peccaries destroy in consumption. Peruvian forests
without peccaries and jaguars look different from forests where they have been present, in
part because palms are less common in the understory when those species are present. Pec-
caries, along with rhinos elsewhere, certain other large mammals, and some birds, are able to
serve as ecosystem engineers. As such they play vital ecological roles, often disproportionate
to their numbers or biomass, in shaping the structure and species makeup of the rain forests.
These are keystone species: their demise often triggers a collapse or a dramatic change in that
system, much as the keystone in an arch is an essential piece in the design of that structure.
The loss of a keystone species often triggers a trophic cascade.
Of course, one doesn't have to purchase a ticket to Puerto Maldonado, or any tropical wild-
land, to see the effects of such a phenomenon. A walk through a deciduous forest in the east-
ern United States illustrates the ecological damage of an ongoing trophic cascade. The erad-
ication of pumas and wolves from these forests, along with their fragmentation, has led to
a proliferation of white-tailed deer. Absent their predators, deer have become a nuisance to
motorists and gardeners and a health threat, serving as hosts for the ticks that spread Lyme
disease. The change has not been good for native birds such as wood warblers, either. The
understory vegetation in many eastern forests has been greatly reduced or even eradicated
by the hungry hoofed mammals. As a result, the hooded warbler, worm-eating warbler, Ken-
tucky warbler, and Canada warbler, among other species that nest on or near the ground, have
declined because overbrowsing by deer exposes their nests and nestlings to predators.
In the rain forest, monkeys also serve as a great example of keystone species in the canopy.
I mentioned to Sue one day as we were scanning the canopy for primates, “Your saki mon-
keys are probably safe from hunters because of their cryptic behavior . . . But what about
spider monkeys? Don't hunters consider them a delicacy? Do we know what happens when
a forest loses its fruit-eating monkeys?”
“Spider monkeys are often the first to be hunted out in most rain forests,” Sue responded.
“It seems animals that eat mostly fruit—spider monkeys, macaws, parrots—all have tasty
meat the hunters go for.” Then Sue mentioned the work of a World Wildlife Fund postdoctor-
al student, Gabriela Nuñez-Iturri, who compared seedling presence and abundance in areas
where spider monkeys and other fruit eaters are still common and protected with that in areas
where these species have disappeared. At sites where primates had been hunted for thirty to
forty years, seedlings and small juvenile trees whose seeds are dispersed by the larger prim-
ates were reduced by nearly 50 percent, whereas the seedlings of wind- and gravitydispersed
plants became 284 percent more common. Her study showed that the composition of seed-
ling and small juvenile tree species that ultimately regenerate future forests differ markedly
