looked like a monstrous pregnancy bulging from a tree trunk was
revealed to be a nest filled with termites. Cohn-Haft recognized a plant
known as a melastome. He turned over one of its leaves and tapped on the
stem, which was hollow. Tiny black ants poured out, looking as ferocious
as tiny black ants can look. The ants, he explained, protect the plant from
other insects in return for receiving free lodging.
Cohn-Haft grew up in western Massachusetts, as it happens not far
from where I live. “Back home, I thought of myself as a general
naturalist,” he told me. He could name most of the trees and the insects
he came across in western New England, in addition to all of the birds. But
in the Amazon it was impossible to be a generalist; there was just too
much to keep track of. In the BDFFP’s study plots, some fourteen hundred
species of trees have been identified, even more than in Silman’s plots, a
thousand miles to the west.
“These are megadiverse ecosystems, where every single species is
very, very specialized,” Cohn-Haft told me. “And in these ecosystems
there’s a huge premium on doing exactly what you do.” He offered his
own theory for why life in the tropics is so various, which is that diversity
tends to be self-reinforcing. “A natural corollary to high species diversity
is low population density, and that’s a recipe for speciation—isolation by
distance,” he explained. It’s also, he added, a vulnerability, since small,
isolated populations are that much more susceptible to extinction.
The sun was starting to sink, and in the forest it was already twilight.
As we were heading back toward Camp 41, we came upon a troop of ants
following a path of their own just a few feet from ours. The reddish-brown
ants were moving roughly in a straight line that led over a (to them
especially) large log. They marched up the log and then down again. I
followed the column as far as I could in both directions, but it seemed to
go on and on and on, like a Soviet-style parade. The column, Cohn-Haft
told me, consisted of army ants that belonged to the species Eciton
burchellii.
Army ants—there are dozens of species in the tropics—differ from
tuis.
(Tuis.)
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