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MOST people visit Australia鈥檚 lush Wet Tropics rainforest to gawp at the
gorgeous greenery. But in 1997, Christopher Schneider and Thomas Smith arrived
with quite a different plan鈥攁nd their eyes firmly on the ground. Shuffling
through the forest, the two men spent four long weeks staring down at the leaf
litter, watching for the flick of a golden lizard tail. With some frantic
lunging and a few strained muscles, by the end of their stay the duo had nabbed
over 100 lizards.

It鈥檚 an odd way to rewrite evolutionary theory, but that鈥檚 precisely what
Schneider, from Boston University, and Smith, of San Francisco State University,
hope to do. By capturing and studying lizards and other animals the scientists
are debunking the old ideas about rainforest biodiversity. What makes these
places so rich, they say, is not their history or geography, but their scabby
edges. The idea is that new species need a kick-start, and they get that when
adventurous animals, darting between peripheral tree patches and dense forest,
discover new niches and adapt to them. Ecological variety, in other words, is
the true spice of life.

If these scientists are correct, then as well as rewriting the textbooks,
they will change the way conservationists work. But testing their theory will
take some sweat. Over the next three years, Schneider, Smith and their
colleagues plan to trudge through jungles on three continents measuring animals,
drawing blood for DNA analysis and analysing the terrain. When all is done, they
hope to find out whether ecology drives biodiversity. The idea dates back to
Charles Darwin, but it has never been thoroughly tested.

Darwin had a keen eye for ecology. He presumed that natural selection favours
organisms that adapt to particular ways of living, from a finch blessed with a
good beak for cracking seeds to a moth endowed with a sinewy tongue for slurping
nectar. In 1942, building on Darwin鈥檚 observations, Harvard University biologist
Ernst Mayr proposed that new species emerge when a natural barrier, like a river
or mountain, pops up, splitting a population into separate geographical
neighbourhoods. Prevented from mating, the divided populations gradually
diverge, acquiring distinctive looks and habits, even if the environments they
occupy remain unchanged.

Along the way, these populations become reproductively isolated鈥攗nable
to mate with their former peers, even if chance should bring them together.
Thanks to an accident of geography, the ancestral crowd has become two (or more)
different species.

Rainforests seem to be the perfect landscape for speciation driven by
geography. For three decades, many scientists have believed that at times when
the climate was cool and dry鈥攕uch as the Pleistocene ice age more than 10
000 years ago鈥攖he tropical rainforests shrank while the grasslands that
thrive in cool weather expanded. Animals left inside the forest fragments would
have diverged into new species, and then rejoined under an expanding tropical
canopy as the Earth gradually warmed. The result, so the theory goes, is
tropical rainforests of exceptional biodiversity.

But even as textbooks adopted this Pleistocene refuge hypothesis, some
scientists remained unconvinced. In 1996, for example, geologist Paul Colinvaux
of the Smithsonian Tropical Research Institute in Panama reported that pollen
trapped in sediment cores from Amazonian lakes showed that the nearby tropical
rainforest has remained intact for at least 40 000 years鈥攚ell into the
time the Pleistocene ice age might have shrivelled the forest. Moreover,
Colinvaux says there is no hard evidence for widespread savanna grasslands
during this time.

Meanwhile, ecologist John Endler of the University of California, Santa
Barbara, has argued that simply separating animals is not enough to generate new
species. Instead, he contends, some sort of ecological change鈥攁n unusually
wet spot of forest, say, or a new predator鈥攊s required to ignite the
process of natural selection by prompting an animal to adapt and evolve
independently of its peers. He says the rainforests鈥� amazing diversity may have
arisen long before the Pleistocene.

And that鈥檚 precisely what Schneider, along with University of Queensland
biologist Craig Moritz and others, have found. Comparing the mitochondrial DNA
of lizards, frogs, birds and mammals in Australia鈥檚 Wet Tropics, the scientists
conclude that the populations diverged from an ancestral population
thousands鈥攐r even millions鈥攐f years before the chilly Pleistocene.
What鈥檚 more, in a new study Schneider and colleagues report that even groups of
lizards separated for millions of years by mountains have changed very
little鈥攁nd mate freely if they bump into one another. According to
Schneider, this is because the lizards still share similar predators, food and
environments鈥攋ust on opposite sides of the Black Mountain Corridor.

If geographic isolation didn鈥檛 cause the rainforest鈥檚 booming biodiversity,
what did? The researchers went looking for an ecological explanation. And they
found one. They zeroed in on the rainforest edge鈥攖he ecotone, or
transition zone, where patches of open forest blur into dense jungle. In
Australia鈥檚 Wet Tropics rainforest, for instance, lizards living just a few
hundred metres apart vary tremendously, depending on whether their home is open
forest or the thick rainforest itself.

Lizards scrambling under the open trees are strikingly smaller, with shorter
limbs and larger heads鈥攑roportions that help them live fast and love
young, before watchful birds swoop down and nab them, says Schneider. By
contrast, lizards lazing inside the rainforest canopy enjoy more shelter from
birds, so they can afford to grow bigger and mate later. Despite the fact that
these two groups of lizards share the same geographic region and may be
exchanging genes, they are clearly diverging, says Schneider.

In fact, the researchers have found that rainforest rats, bats and birds all
show the same pattern鈥攇eographically separated populations often look
alike, while neighbouring populations in subtly different habitats vary. In
Cameroon, for instance, where grassy fields gradually give way to rainforest,
birds living in the ecotone are far more diverse than those deep inside the
forest. Smith鈥檚 team looked at black-bellied seedcracker finches and discovered
a motley crew along the forest edge.

Three types of finch, each with a unique bill and genome, dominate this
ecotone鈥攁nd each type neatly corresponds to the seeds available in its
sliver of the forest. 鈥淚n some cases, you have all three forms occurring
together,鈥� says Smith. That鈥檚 a cue that subtly different, yet neighbouring,
habitats have created an unusual blend of species, he adds.

The picture is similar for another of Cameroon鈥檚 birds, the greenbull.
Greenbulls that live deep in the forest consistently resemble each other, even
when they live more than 600 kilometres apart, with natural barriers in between.
But the differences between these birds and those in the ecotone are striking.
What鈥檚 more, birds living on the forest edges have a wide variety of wing
lengths and bill sizes, depending on their diet and vulnerability to predators.
Smith鈥檚 preliminary gene studies support this pattern, with greenbulls along the
ecotone diverging more than those inside the forest.

Endler predicts that the findings of Smith, Schneider and their colleagues
will make a big splash, since they offer long awaited evidence for
ecology-driven speciation. 鈥淧eople will have to consider ecological factors at
last,鈥� says Endler. He agrees with Smith and the others that geographical
changes simply accelerate population differences sparked by shifting habitats.
Harvard University evolutionary biologist Kerry Shaw also calls the new work
compelling: 鈥淭he data suggest we need to look at ecological explanations for
speciation much more seriously than we have in the past,鈥� she says.

Indeed, the idea has implications way beyond the rainforest. A growing number
of studies in other habitats highlight the role of ecology in speciation. For
instance, Dolph Schluter of the University of British Columbia in Vancouver,
Jeffrey McKinnon at the University of Wisconsin in Whitewater and their
colleagues have shown that freshwater and marine stickleback fish prefer to mate
with peers from the same environment鈥攅ven if those peers live on the
opposite side of the Pacific Ocean and so are more genetically distant than fish
in alternative habitats at home. This suggests that fish living a world apart,
yet in the same ecological niche, do not diverge much. And Daniel Funk of the
University of Arizona in Tucson has found that recently separated beetle
populations are more likely to split into new species if they encounter
different plants to feed on. They will take the evolutionary plunge more often,
says Funk.

Not everyone is convinced, though. University of Kansas ornithologist
Townsend Peterson lauds the quest to quantify the part that ecology plays in
speciation, but suggests that its starring role will fade as researchers study
more animals and environments. Peterson and his colleagues tracked 37 pairs of
bird species living in forested mountains separated by a stretch of arid land in
southern Mexico.

Although members of each pair flocked to similar habitats in opposite
mountains, all had diverged to form distinct species since they were separated
by the arid barrier 100 000 years ago. 鈥淭hat suggests adaptive speciation thanks
to ecological change is probably the exception and not the rule,鈥� says Peterson.
He estimates that only 1 in 50 speciation events occur because of shifting
habitats. If he鈥檚 right, could these still be driving all the biodiversity of
rainforests? That, says Peterson, remains to be seen.

In fact, Smith, Schneider, Moritz and a fourth colleague, Robert Wayne from
the University of California, Los Angeles, have just launched an ambitious
programme to gauge the roles of ecology and geography in speciation worldwide.
Armed with a $2.6 million grant from the National Science Foundation, the
researchers will spend the next three years tramping through tropical
rainforests in Australia, South America and central Africa. In each region, they
will scoop up mammals, birds, reptiles and amphibians. To unravel the source of
all this steamy speciation, the team will compare how these animal populations
vary, both in genes and appearance, across a variety of habitats that occur not
just at rainforest edges, but also in continuous forest, across rivers and on
mountains.

The scientists plan to use this information to test three popular models of
rainforest biodiversity: the Pleistocene refuge hypothesis; the riverine barrier
hypothesis, which suggests that emerging rivers separated rainforest species,
causing them to diverge; and the montane isolate model, which holds that periods
of low rainfall stranded animal populations on different parts of rainforest
mountains, where they subsequently diverged. Each model assumes that some
geographic barrier abruptly divided animal populations and drove the evolution
of new species. And lastly, the research team will test their data on a model
that suggests ecology drives rainforest speciation鈥攔egardless of
geographic barriers. 鈥淔inally, we have the right kinds of field and lab
techniques to ask these questions,鈥� says James Patton, a colleague from the
University of California at Berkeley.

Even so, Shaw points out that tracing evolutionary history can be a tricky
business. To turn back the clock, scientists build phylogenies, or evolutionary
family trees, of animals based on their mitochondrial or nuclear DNA. Some DNA
stretches mutate rapidly, making them good tools for differentiating among
species. The challenge, notes Shaw, is choosing DNA stretches that most reliably
reflect an animal鈥檚 evolutionary history, since genes in a single species can
mutate at very different rates. Smith, Schneider and their colleagues hope to
solve this problem by studying a variety of molecular markers鈥攁nd by
homing in on characteristics closely related to evolutionary fitness, such as
the sexy croon of a bird or the hungry jaw of a lizard.

Their findings could do more than solve theoretical riddles. Until now,
conservationists have focused on preserving blocks of rainforest as hot spots of
diversity. They have paid little attention to the ragged forest border, says
team member Todd Schafer of San Francisco State University鈥檚 Center for Tropical
Research. Pipelines, buildings and other hallmarks of human life often crop up
just outside the rainforest edge鈥攕mack in the middle of the ecotone.
鈥淢aybe that鈥檚 not okay,鈥� he says. The team has linked up with the World
Resources Institute in Washington DC, which will draft and promote rainforest
conservation policy based on the research.

Already, conservationists are taking note of the emerging focus on ecology
and the rainforest edge. Conservation International in Washington DC has
revamped its rainforest strategy to focus not only on big blocks of forest, but
also on the surrounding landscape. That includes maintaining the forest edge,
says Conservation International鈥檚 Claude Gascon.

The idea, he says, is to preserve the processes that spur genetic variation
in rainforest species, rather than simply saving a collection of isolated
animals teetering on the brink of extinction. Gascon thinks other conservation
organisations, as well as governments, will follow suit. 鈥淢ost people are
beginning to realise that just saving isolated rainforest regions isn鈥檛 going to
make a big difference to biodiversity in the end.鈥�

But rainforest conservation may just be the start. Smith predicts that this
research will shed light on the makings of biodiversity worldwide鈥攆rom
birds swooping along the Swiss Alps to squirrels scampering near the lakes of
northern Minnesota. After all, mountains, wetlands and temperate forests all
have their own ragged edges and ecologically diverse zones. 鈥淭hese kinds of
ecological patterns are very common,鈥� says Smith.