- Finding May Cast Doubt On
Some Conservation Methods
- PRINCETON, N.J. -- A painstaking effort to track every square inch of plant
life in large patches of tropical forests has started to produce significant
discoveries in ecology. Princeton professor of ecology and evolutionary
biology Stephen Hubbell, a founder of the project, is using the research
to answer fundamental questions about what factors come into play in maintaining
the diversity of life on Earth.
- Hubbell's latest finding, reported in
the current issue of Science, overturns one of the bedrock beliefs among
ecologists about what allows tropical forests to maintain such a dazzling
variety of tree species. The common thinking was that when a trees dies
or is blown over in a storm the resulting infusion of direct sunlight,
called a light gap, allows new species to flourish and compete to fill
the open slot in the forest. The frequency and size of light gaps was,
therefore, thought to predict type and number of species present in the
- Hubbell found, however, that no such
correlation exists. Using vast amounts of data generated from the tracking
project, he showed that areas with many light gaps are no richer in species
than areas with few pockets of sunlight. The mix of species also was not
notably different. Although some species do depend on light gaps to survive,
they are such a small minority that they don't change the results.
- The finding may cast doubt on one common
suggestion for how to reconcile logging and conservation efforts in tropical
forests, said John Terborgh, co-director of Duke University's Center for
Tropical Conservation. There have been many suggestions that logging could
be allowed in forests if it mimicked the natural pattern of light gaps;
the logging, then, would promote the diversity of trees rather than harm
it. That idea is based on a theory that Hubbell has now shown to be wrong,
- "I think it's a very exciting paper,"
Terborgh said. "I think this is doing a great service to ecology."
- Hubbell's finding also may indirectly
affect other conservation efforts. One reason that the old light gap theory
failed to hold up is that the distribution of seeds in a forest is far
from ideal. Light gaps create a variation in the local growing environment,
which should, in turn, create a variation in the types of seeds that grow.
As a practical matter, however, the same old varieties tend to grow in
light gap areas because there are not enough light-loving seeds that can
take advantage of the new conditions. That lesson could have implications
for conservation efforts around the world where roads and development isolate
one section of forest from another, exacerbating the effects of poor seed
dispersal and possibly forcing the extinction of species that would normally
be good competitors.
- The light gap question is just one of
a series of issues being illuminated by a research effort that Hubbell
helped start nearly 20 years ago, an ambitious project to track the diversity
of species in tropical forests. The location that provided the data for
his Science paper is a 120-acre plot within a tropical forest on an island
in the Panama Canal. Starting in 1981, a team of researchers directed by
Hubbell, who was then at the Smithsonian Tropical Research Institute, and
colleague Robin Foster, a curator of the Field Museum in Chicago, began
identifying and tagging every tree and sapling that was at least chest
high and at least one centimeter in diameter. That is a far more detailed
level of sampling than had been done in any other study, most of which
looked at plants that were two or three times bigger and sampled just a
few acres. By the time the researchers finished the study two years later,
they had tagged 300,000 trees. The researchers have gone back every few
years to repeat the process. It takes a team of 15 people nine months to
comb through the plot and update the data.
- From its very beginning, the project
has yielded dramatic results, said Elizabeth Losos, the director of the
Center for Tropical Forest Science, an organization within the Smithsonian
that was formed to manage the research (http://www.si.edu/organiza/centers/stri/stri.htm).
For example, the researchers discovered right away that tropical forests
are not the stable, unchanging ecosystems that they were assumed to be.
In just two years, 40 percent of all the tree species had significantly
changed with relative abundance, with some dropping to extinction in that
plot and others becoming more dominant species.
- The project's success led to a series
of collaborations that eventually resulted in the creation of 15 other
120-acre sites in 12 countries, involving scientists from three dozen institutions.
- One long-term outgrowth of Hubbell's
work is his discovery of what he calls "the E=mc2 of community ecology,"
a theory that for the first time links several other seemingly distinct
theories about the abundance and distribution of species. Hubbell originally
published the theory four years ago, but is now finishing a book, to be
published by Princeton University Press, that presents the idea in an expanded
form. The theory, called the unified theory of biodiversity and biogeography,
allows scientists to calculate a single number, called the fundamental
biodiversity number, that describes a whole range of characteristics of
plant and animal communities. For example, by figuring the biodiversity
number for a particular forest, a scientist could estimate how many species
the forest is likely to contain and whether some of those species are much
more dominant than others, as well as a number of other factors.
- Hubbell's work is inspired by a deeply
felt concern for the planet's future. He worries that scientific knowledge
about ecological systems and biodiversity are not keeping pace with the
speed with which humans are harming the planet. He cites a recent study
that showed that humans consume 40 percent of all the energy that goes
into producing the Earth's biomass, a number that is way out of proportion
to humans' relative abundance among species. At the same time, biologists
don't even have a rough guess for how many species the Earth holds and
are far from knowing how those species depend on and compete with each
- "We're still in the Middle Ages
in biodiversity research," Hubbell said. "We're still cutting
bodies open to see what organs are inside."
- "I've told myself, look, I need
to spend the rest of my life fighting for this," Hubbell said. "It
may be a lost cause, but I at least want to be able to say I tried."