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Law
of the jungle
What
makes a successful species
Forget all about ingenious
adaptations and well-tailored
lifestyles. What makes a successful
species is good luck.
Aiming to understand rainforest
trees? Assume they’re all the
same. Snails, shrimps and other
sea-floor fauna? Set the
distinctions aside. The key to
explaining biodiversity, says
ecologist Steve Hubbell, is to
ignore it.
It’s a startling idea. If Hubbell
is right, abundant species such as
mallard ducks haven’t “earned”
their success by being somehow
fitter or better than rarer species
such as white-headed ducks. Instead,
species wander erratically between
dominance and extinction the way
drunkards reel down a street. It’s
not survival of the fittest-it’s
survival of the luckiest. Yet even
in this indeterminate and nihilistic
world, Hubbell, a plant ecologist at
the University of Georgia, has found
a sort of order-a way to predict how
many species, and in what numbers,
any given habitat will support.
Harvard biologist Edward O. Wilson
says ecologists will some day rank
Hubbell’s contribution among the
most important of the past half
century. In the meantime, though, a
lot of people are scratching their
heads trying to figure out how such
an obviously wrong assumption can
work so well. “I think a lot of
people would like to just call him a
crackpot and ignore him,” says
palaeobiologist Doug Erwin of the
National Museum of Natural History
in Washington DC. “But that will
be hard.” Hubbell’s work drives
straight to the heart of the science
of ecology. For all the attention
paid to biodiversity, ecologists
still can’t answer the most basic
question about it. Take any plot of
land or patch of sea.Why is this
species common and that species
rare?
You only need step into the wild to
grasp the size of the problem. With
up to 300 tree species in a single
hectare of tropical forest, and 100
types of coral and 500 species of
fish in one stretch of reef,
biodiversity is almost unfathomably
complex. Or is it?
When Hubbell set out to understand
tree diversity in a tropical forest
in Costa Rica back in the 1970s, he
certainly expected it to be complex.
A host of factors could have
determined the mix of species and
the range of abundances he saw:
variable soil conditions, uneven
distribution of the trees’
pollinators, predators and other
associates, and differences in the
risk of infestation and disease, to
name a few.
But before wading into all that,
Hubbell decided he’d better be
clear about his starting point: what
would a forest look like if none of
those factors mattered? What if
chance were all that counted.
Hubbell created a computer model of
the forest that used just a single
rule-one that came to him as he
stared up at the canopy. “I
realised there was no more room to
fit trees in there,” he says, so
he decreed that each new tree could
grow only in the vacancy left by
another’s death. Hubbell used the
computer’s random number generator
as the hand of fate, dealing death
and bestowing offspring to all
individuals with equal probability.
The results completely took him by
surprise “I got patterns that
looked very similar to the patterns
I was seeing in the forest,” he
says. In other words, it looked as
though randomness ruled in nature
herself.
Most biologists were dismissive.
Sure, they said, maybe identical
trees could produce patterns like
those in nature, but real trees
aren’t identical.
For decades, ecologists have viewed
the characteristic features of a
species as “niche specialisation”-adaptation
to a way of life that is uniquely
its own. In their thinking, having a
niche of your own is crucial to
survival-and finding a bountiful
niche is the key to success, because
you can exploit a bigger portion of
the world’s resources.
Last year Hubbell finally published
a book-length treatise entitled The
unified neutral theory of
biodiversity and biogeography.
Hubbell’s core achievement is a
formula that for any given place and
life form-trees, grazing animals,
fish or whatever-predicts how many
species will coexist and in what
abundances.
The whole complex formula hangs on
just four simple variables, each
detailing a real and, in principle,
measurable aspect of the
environment. The easiest way to
appreciate these factors is with an
example-Hubbell’s favourite is
trees on Barro Colorado island in
Panama. The first variable is the
total number of trees in the area
under study. This number reflects
the overall suitability of that
environment for trees-its soil,
sunlight, climate, pests and so on.
The second is the total number of
trees in the ecological
“universe”-in this case, the dry
tropics of Central America or
thereabouts. This represents the
pool from which immigrants will
arrive and new species be born. The
third variable is the rate at which
individuals immigrate to the study
area from elsewhere in this
universe. In this example, it’s
the rate at which wind and birds and
bats carry seeds to Barro Colorado.
The fourth and final number is the
rate at which new species emerge
within the larger universe.
Together, these four factors
describe a biodiversity equilibrium
in which new species arrive at the
same rate as others become extinct.
The main lesson from Hubbell’s
theory is that success begets
success. Species lucky enough to
rise to prominence will persist
longer and produce new species more
frequently and are more likely to
sire a long line of descendant
species. That means the more
abundant species should tend to be
older as well, says Hubbell-another
testable prediction that is certain
to attract interest.
Some critics scoff that by tuning
the four factors you can make
Hubbell’s model fit any pattern,
random or not. But Jim Brown, an
ecologist at the University of New
Mexico in Albuquerque, says many
theories before Hubbell’s have
aspired to explain far less with
much more flexible formulae. Brown
regards Hubbell’s theory as just a
stepping-stone toward a more
complicated theory, which he says
will eventually have to take
species’ differences into account.
But others say Hubbell’s formula
already captures something true
about natural systems and is ready
for use
The biggest oddity so far, however,
seems to be how little is left to
explain. How can a theory throw away
everything biologists have ever
learned about species, yet explain
what its competitors can’t? “We
have to work that out,” says
Brown. Hubbell sees this question as
a major challenge for the future. In
the meantime, ecologists may just
have to embrace the theory first and
ask the questions later.
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