Neutrality versus the niche

[Harry Veeder <eo200@xxxxxxxxxxxxxxxxxxx>]

Some might find this "neutral theory" interesting
from an economic perspective.

Harry  Veeder

---------


Neutrality versus the niche
According to some ecologists, you don't need to invoke adaptation to explain
biodiversity. They may sound like nihilists, but their ideas are proving
remarkably resilient.

3 June 2002

JOHN WHITFIELD

This article is from the News Features section of the journal Nature.


Vive la difference? Forget it. La difference est morte. That is the message
from the proponents of neutrality, a view of ecology that is anathema to
many in the field. Rather than focusing on how differences between species
allow them to coexist, neutrality assumes that trees in the rainforest, or
corals on a tropical reef, are basically all the same.

Ecologists have strived to explain species' patterns of distribution,
abundance and coexistence for more than half a century. The traditional
explanation is that each species is adapted to exploit a unique niche -
shady or sunny, wet or dry, and so on. But neutral theories assume that all
organisms are equal, and consider only factors such as random dispersal, the
birth and death of individuals and the total number of organisms in the
community.

The troubling thing for most ecologists is that neutral simulations can
produce ecosystems that look just like the real thing. "Neutrality starts
with assumptions that are clearly wrong, but produces patterns that match
what we see in nature," says Jonathan Levine of the UK Natural Environment
Research Council's Centre for Population Biology at Silwood Park, west of
London.

Sceptics say that recreating patterns isn't the same as understanding the
mechanisms that cause them, and complain that the assumptions of neutrality
are practically impossible to test. The debate isn't purely academic - our
ideas of how biodiversity arises will influence how we attempt to conserve
it. "We'd better figure these things out," says Stephen Hubbell of the
University of Georgia, Athens, one of the gurus of neutrality.

Put it in neutral

Hubbell, a specialist in the ecology of tropical forest trees, last year
published The Unified Neutral Theory of Biodiversity and Biogeography1,
which has become the bible of neutrality. Hubbell's neutral model considers
only the birth and death of individuals, random dispersal, the overall
population of individuals in the ecosystem, the total number of species and
the origin of new species. The models favoured by neutral ecology's other
leading proponent, Graham Bell of McGill University in Montreal, Canada, are
even more austere, leaving out the birth of new species2, 3.

Neutral models consider the ecological properties of every individual in the
population to be identical. Individuals compete for the same, fully
exploited pot of resources, but the identity of the winner of any single
contest - the tree that fills a gap in the canopy, for example - is left to
chance.


Get the balance between birth, death and dispersal right, and you get a
near-perfect recreation of natural communities. "It predicts the individual
species abundances of more the 800 tree species in a Malaysian rainforest
with just three numbers," says Hubbell of his model1. Neutral theories also
make predictions about evolutionary history, for example that rare species
will have arisen recently - although Hubbell acknowledges that these will be
difficult to test. "By the time a species is detectable it's already really
old and really abundant," he argues.

Hubbell admits to being taken aback by neutrality's success in mimicking the
natural world. "I'm very puzzled by how well it's worked, and I was really
unprepared for how theoretically rich in questions it was," he says.

If correct, neutral theory means that the species in a habitat have been
thrown together - and will come and go - at random. This insight, Hubbell
believes, might be useful in designing nature reserves. If species are
closely adapted to fit an ecological niche, communities will be relatively
stable, and hard to invade. Reserves can therefore be small. But if species
are more equal, and come and go at random, communities will be more fluid,
and bigger reserves will be needed to protect rare species from the
buffetings of chance. "My view is that most communities are open and easily
invaded," he says.

But if species are essentially all the same, does it matter if we lose one
of them? That is a dangerous line of argument, says Bell. If just 1% of
species perform a vital and specific ecological function, he explains, this
will make little difference to the statistical predictions of neutral
ecological models. But take those species away, and a previously healthy
ecosystem will be in serious trouble.


Even the staunchest supporters of neutral ecology accept that the theory has
limitations. It only applies within one level of the food web - it might
explain the diversity of trees, for example, or the diversity of herbivorous
insects, but not how the number of tree species might affect the number of
herbivorous insect species. Hubbell says that the theory is more likely to
hold for plants or microbes - where different species overlap more in the
way that they exploit resources - than animals. Moreover, it breaks down on
large spatial scales. To give an extreme example, alpine plants are clearly
adapted to different conditions from those in which lianas and mahogany
trees thrive, so would struggle to survive in the rainforest.

The big question is whether biodiversity really results from neutral
ecological processes - indicating that ecologists have overestimated the
importance of the niche - or whether the models' accuracy is a coincidence.
Some ecologists, such as Levine, believe that neutral models are most useful
as an ecological 'null hypothesis' for revealing the differences between
their predictions and real ecosystems. Others point out that niche-based
models can also produce a good description of the natural world. This year,
for instance, a team led by Jerome Chave, now at the Laboratory of
Terrestrial Ecology in Toulouse, part of the CNRS, France's national
research agency, showed that both niche and neutral models can reproduce
natural patterns of species abundance4.

A simple life

Hubbell counters that theories that stress the importance of the niche
generally have to be much more complex than neutral models to achieve
superior results. "To say that 100 or more parameters gives a better fit is
not very satisfying as a critique," he says.

The niche may still be a valid concept, Hubbell suspects, albeit not as
important as ecologists have long supposed. But Bell goes further, arguing
that there is little experimental evidence to support the idea of niches. If
plants are closely matched to their local environments, they should perform
poorly if transplanted. But that isn't what Bell found when his team tried
moving plants within Canadian forests5. "The results puzzled us - they
clearly didn't point to any powerful degree of local adaptation, and
sometimes the rarest species was the most successful in a new location," he
says.


Bell is now looking at tundra plant communities to try and work out the
scales on which the predictions of neutral theory hold. The key test, he
says, is how likely species are to be present together. If particular
species tend to occur together, it would suggest that some environmental
factor determines community composition, says Bell. But more haphazard
patterns of cohabitation would support the neutral theory.

Scale bars

Other studies have produced mixed results. One, published earlier this year
by a team including Chave and Hubbell, found that, in Panamanian and
Amazonian forests, random dispersal could explain patterns of tree diversity
between different areas at scales of between 0.2 and 50 square kilometres.
However, the neutral theory broke down at smaller and larger scales6. A
study of sawflies feeding on birch trees in Finland, which showed that
species are specialized to feeding on leaves of a particular age7, has also
been cited8 as evidence against the strongest interpretations of neutrality.


Some sceptics, meanwhile, still find it impossible to accept that neutral
theories can yield real insight into ecological processes. They also point
to the difficulty of validating Hubbell's model, given that it includes
parameters such as the rate at which new species arise, which cannot readily
be measured. Neutral models are intriguing, concludes Peter Petraitis of the
University of Pennsylvania in Philadephia, who studies rocky-shore
communities. "But they don't offer much insight into what's operating
underneath. It doesn't push us forward."

Bell senses the tide turning his way, however. "At first people thought
neutral theory was nonsense. Now they think there's something in it," he
says. Bell compares the debate to one that began more than 20 years ago
among population geneticists about whether changes in gene frequencies are
driven primarily by natural selection or by random 'genetic drift'. After
initial resistance, most researchers now accept that genetic drift can be a
significant factor. Bell suspects that it may take two decades for
ecologists to reach a similar consensus.

While the debate goes on, even those who are unconvinced about neutral
theory's validity value its stimulating effect on community ecology. "It's
forcing us to address fundamental questions and work out what we really
think," concludes Sean Nee, an evolutionary biologist and ecological
geneticist at the University of Edinburgh, UK.

References

1.    Hubbell, S. P. The Unified Neutral Theory of Biodiversity and
Biogeography (Princeton Univ. Press, New Jersey, 2001).
2.    Bell, G. Neutral Macroecology. Science 293, 2413 - 2418 (2001).
|Homepage|
3.    Bell, G. , Lechowicz, M. J. & Waterway, M. J. in . Integrating Ecology
and Evolution in a Spatial Context, (eds Silvertown, J. & Antonovics, J.)
117 - 138, (Blackwell Science, Oxford, 2001).
4.    Chave, J. , Muller-Landau, H. C. & Levin, S. A. Comparing Classical
Community Models: Theoretical Consequences for Patterns of Diversity.
American Naturalist 159, 1 - 23 (2002). |Article|
5.    Bell, G. , Lechowicz, M. J. & Waterway, M. J. Environmental
heterogeneity and species diversity of forest sedges. Journal of Ecology 88,
67 - 87 (2000). |Article|
6.    Condit, R. et al. Beta-diversity in tropical forest trees. Science
295, 666 - 669 (2002). |Homepage|
7.    Kause, A. et al. Seasonally varying diet quality and the quantitative
genetics of development time and body size in birch feeding insects.
Evolution 55, 1992 - 2001 (2001). |Article|
8.    Agrawal, A. A. & Van Zandt, P. A. The community ecology of live long
and prosper . Trends Ecol. Evol 17, 62, (2002). |Article|



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