SOME 25 metres above the forest floor in YasunĂ National Park in
eastern Ecuador, Gabriel Grefa balances precariously on a tree branch thinner
than his wrist. He wears no safety rope. His right arm is wrapped around the
tree trunk, and in his hands he holds the end of a pruning pole. Struggling to
control its 12-metre length, he guides the pole up into the canopy overhead.
Somewhere up there, hidden from view by foliage, this tree is in bloom.
Down below, Katya Romoleroux watches with binoculars, directing Grefa until
the pruner’s blade rests against a flowering branch about a metre and a half
long. “Alli! (There!)” she calls. With a snip, the branch comes crashing down.
As Grefa shinnies back down the trunk, Romoleroux bags the branch with a
satisfied smile. She has coveted this specimen since she first noticed the
blossoms a week ago. This afternoon, back in the laboratory, she will discover
which species of tree this is—if she’s lucky.
Romoleroux is from the Pontifical Catholic University of Ecuador in Quito,
and Grefa is her assistant. Both are botanists—but botany in the Amazon
requires some unique skills. “We use smell a lot,” says Romoleroux, putting her
nose to a tree trunk that Grefa has just lanced with the tip of his knife. “This
one smells like a legume.”
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Rainforest trees form a dense canopy, so very little light penetrates to the
understorey and mature trees usually have none of the low, easy-to-reach
branches that make botany softer going in temperate forests. Hopeful collectors
must be prepared to climb—or settle for what they can see and smell from
the ground and scour the forest floor for fallen leaves. A few less patient ones
have even been known to blast away at the canopy with shotguns in the hope of
bringing down a flowering branch.
But even with a prize specimen of tree blossom in hand, the going is still
slow. Romoleroux is one of the senior researchers on an ambitious project to map
and identify every tree on a 50-hectare study plot in YasunĂ—about
a quarter of a million individuals in all. With just two hectares
completed—an area of just 40 by 500 metres— Romoleroux and her
colleagues have had to classify more than 700 species of trees, including at
least five species and possibly a genus that are new to science.
The big question, of course, is why such an astonishing number of species
flourish there. For most plant and animal groups the Amazon jungle, especially
the western part just below the Andes, is more diverse than anywhere else on
Earth. Over the past three decades, ecologists have published reams of papers on
the subject. But they can’t agree about the answer or even the right way to
estimate the total number of species.
One of the key figures in the debate is Terry Erwin, a tropical entomologist
from the Smithsonian Institution in Washington, DC. He is at work just 7
kilometres down the road from Romoleroux in YasunĂ National Park. And, as
it turns out, he is facing even bigger problems in identifying his species.
Working with his colleague Jonathan Coddington, Erwin is carrying out a
census of insects and spiders. For the past three years, they have been studying
one small plot in the vast forest using a simple but staggeringly
labour-intensive technique. A hundred times during each trip to the park, they
spread a sheet of nylon three metres square just above ground level, then spray
a fog of biodegradable insecticide into the canopy and collect everything that
falls. Back home in Washington, they sort and identify every individual in 14
major groups of insects and spiders—nearly a million and a half specimens
so far.
Eighty per cent of the insect species in a typical sample have never been
seen before by entomologists, Erwin says. He has catalogued the beetles in just
18 of his 900 samples, and already he has 1348 species. Even among the obscure
insect group called treehoppers, the researchers have found 740 species in a
single year’s samples.
But these tallies still underestimate the true diversity, since they do not
include ground-dwelling species or those that live under the bark of trees where
they die without falling onto the collecting sheet. In all, Erwin estimates, a
single hectare of rainforest here may harbour between 50 000 and 60 000 species
of insects and spiders—compared to just 20 000 in the whole of Britain.”
The number of species is unbelievable,” says Erwin.
And this is still only the beginning. A similar hectare just a few tens of
kilometres away will have a vastly different set of species. The spider fauna
can change completely within 100 kilometres in Amazonia, says Coddington—a
bigger turnover than you would see travelling 1000 kilometres in North
America.
Diverse theories
One evening in Quito, Ecuador’s capital and the nearest large city to
YasunĂ , Erwin, Coddington and a half-a-dozen of their colleagues take a
break from collecting insects. They settle down in a hotel appropriately named
La Floresta (The Forest), drink the local lager, smoke roll-up cigarettes and
discuss the central problem of why such staggering diversity is found in the
Amazon.
One old suggestion is that the Amazon is a kind of paradise: lack of a harsh
winter or a severe dry season have left species free to evolve more and more
intricate adaptations for finding food, escaping being eaten, and competing for
resources. Another possibility is that as the Andes eroded, they exposed a
greater variety of soil types, allowing plants to specialise. Or perhaps the
unusual shape of the Andes—with long fingers of lowland rainforest trapped
in valleys between high mountain ridges—gave just the right degree of
isolation, so that species evolved differently in each valley but gradually
spread out to overlap with their cousins, just as Darwin’s finches did in the
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One of the most popular theories, however, holds that during the last ice
ages, when much of the Earth’s water was frozen into glaciers, less rain fell
over Amazonia. As a result, the rainforest shrunk into isolated pockets, or
refugia, in the wettest parts of the region. By the time the glaciers receded
and the rainforest became continuous again, evolution had created separate
species in these scattered patches. Amazon diversity now is a result of the
gradual intermingling of once isolated remants of forest.
Erwin and his friends all seem to accept the logic of this hypothesis.
Unfortunately, evidence is thin on the ground. Pollen fossils from the eastern
Amazon show that the rainforest did shrink during the ice ages, but no one yet
knows whether the central Amazon also fragmented.
For years, the best evidence for this scenario came from the distribution of
living species. Some regions of Amazonia—such as eastern
Ecuador—have more than their share of species found nowhere else. These
clusters, it was argued, contained species that had never spread from the
refugia where they evolved.
In 1990, however, biologist Bruce Nelson of INPA, Brazil’s national institute
for Amazon research in Manaus, took a closer look at the botanical data and
found that the clusters of species merely marked places where biologists had
collected most intensively. Not surprisingly, rare species turned up more often
in places where ecologists had searched harder. That was all. Evidence for the
refugium theory disappeared in a puff of statistics.
At the same time, however, researchers have new evidence that a similar sort
of draught-induced fragmentation might have occurred even after the ice-age
glaciers had receded. At first glance, water shortage is about the last thing
anyone would expect inside the Amazon rainforest. The air is permanently hot and
heavy with moisture. Even in the dry season, the frequent thundery rainstorms
are fierce and furious, and in the rainy season they’re nothing short of
spectacular. But strange as it may sound, large parts of this vast green forest
live on the edge of drought, getting barely enough water to make it through the
year. And with drought could come gigantic fires that could break up the forest
and create refugia to speed the creation of more and more different species.
Far away on the opposite end of the Amazon, a little way south of the
Atlantic port city of Belém in Brazil, one researcher has been worrying
about drought almost from the first time he set foot in the forest. Ecologist
Dan Nepstad from the Woods Hole Research Center in Massachusetts knew that in
some parts of the Amazon, at least, the dry season can bring many consecutive
days without serious amounts of rain. But the trees go on transpiring water from
their leaves, and the temperature—which drives evaporation from the
soils—is unfailingly hot throughout the year. When Nepstad did the sums to
try to balance water going in and coming out, he found that there should be many
days in the dry season when the trees are struggling. So why, he wondered,
should the canopy stay a perpetual, healthy green?
One way, he reasoned, would be for the trees to tap water from deep down in
the soil. But how? Rainforest trees, which can grow more than 50 metres tall,
are stabilised by massive, buttressed roots that spread out sideways, not
downwards. So most people had assumed that nothing much happened more than a
metre or so below the surface.
Then, in 1990, Nepstad and his colleagues decided to dig some pits and see
for themselves. To their surprise, no matter how deep they went, they kept on
finding roots—in one case, down to 21 metres below ground. The roots were
delicate—just a few millimetres in diameter—but they were myriad,
more than enough to solve the water mystery.
But Nepstad still wasn’t happy. If you work out how much water the soil can
hold at different depths, there are still some years when the trees would have
to suck up every bit of water for several metres down to supply their needs.
What if there was an exceptionally dry year? Would there be enough roots deep
enough to tap the soil reservoir and keep the canopy green? And if not, could
massive fires begin?
It isn’t easy to start a fire in the rainforest. The forest floor has plenty
of leaf litter to supply fuel, but the dense, closed canopy keeps sunlight out
and moisture in so the litter is usually too wet to burn. “You can go in after
three months without rain and throw kerosene on the forest floor, and a match,
and the fire doesn’t go anywhere,” says Nepstad.
But fire is not as unlikely as it seems. If the roots run out of water and
the canopy begins to parch, leaves start to fall and the intense equatorial
sunlight will flood in. The litter can then dry out in a matter of days, turning
the forest from green oasis into tinderbox.
Indeed, four times in the past 1500 years there have been huge droughts, and
perhaps fires, in the Amazon basin, according to archaeological and climatic
evidence compiled in 1994 by Betty Meggers of the Smithsonian Institution. Each
of these, Meggers suggests, was caused by a drastic version of a regular
climatic disturbance in the Pacific, known as El Niño, which reverses
winds and currents, cutting rainfall in the Amazon basin.
Nepstad and his colleagues are now trying to work out exactly how close the
forest is to the edge. Their first experiments, in 1992 and 1993, produced some
worrying signs. Throughout both years, they kept track of the water budget in a
patch of virgin rainforest near Paragominas in the eastern Amazonian state of
Pará. Every two weeks, they set out before dawn to climb preselected
trees and measure the leaf water potential—the amount of tension in the
leaf’s vascular system, which measures how hard it is to pull water up from the
soil into the canopy.
Later in the day, they counted the number of leaves on certain branches and
climbed down into their pits to record the amount of water available at
different depths in the soil.
By the end of the 1992 dry season—which, fortuitously for the
experiment, turned out to be unusually dry with only 60 per cent of the normal
rainfall—the soil water was all but exhausted right to the bottom of their
pits, 8 metres down. Up in the canopy, 15 per cent of the leaves had fallen.
Smaller trees in the understorey layer were showing distinct signs of stress as
well: their leaf water potential reached extraordinary levels as they battled to
pull water up from the deep soil.
Unlike the canopy trees, however, they did not shed leaves. Understorey trees
hold onto their leaves tenaciously because they compete fiercely for the limited
light that breaks through the canopy, and can’t afford to throw away leaves
without good reason.
Even so, the 15 per cent overall leaf loss was enough to push the forest over
the edge. For three days before the rains came to the rescue, the litter was so
dry that a chance spark could have set it alight. Elsewhere in Paragominas that
year, many ranchers reported fires in portions of their virgin forest.
If the forest comes this close to burning after only a moderate drought, how
much would it take to send the whole place up in smoke? Nepstad and his team are
working on this problem by deliberately restricting the amount of rain falling
on a patch of forest to see how far they can push it. They have already
completed a pilot project in which they used a system of low-level roofs and
gutters to channel rainwater away from a 10 by 10 metre plot. Now they are
scaling up to start work on a system to control the rain falling on a hectare of
forest.
They also plan to poison a few trees to force them to drop their leaves and
check the effect on the flammability of the litter. And they plan further
experiments in the south, at the boundary between forest and savanna, to see
whether lack of rain could allow the savanna to creep northward into the
forest.
Back in the Hotel La Floresta in Quito, Erwin and his friends consider the
possibility that fire may be a cause of the forest’s diversity. A year ago last
July, when El Niño had almost dried up the rainy season in YasunĂ,
they certainly saw signs of a forest close to catastrophic conflagration. “You
could walk through the forest and see the leaves wilting,” says Erwin. “Some
looked like cooked lettuce.”
But no one in the room is prepared to put all their money on that
theory—or any other—when so little is still known about the Amazon
rainforest. Coddington thinks it likely that many of the explanations may be
partly right. “My prejudice is that biodiversity is complex, and people who want
simple answers are going to be wrong.”