By Rob Anderson
LSU Assistant Professor of Biological Sciences Kyle Harms was a principal
collaborator on a recent landmark study that
revealed that nature encourages diversity
among trees.
The study, which examined seven tropical
forests from around the world, was conducted by 33 ecologists from 12
countries. The results were
published in the journal Science. According
to Harms, the research revealed
that nature encourages diversity by
selecting for less common trees as trees
mature.
"Ecologists have debated for decades over
whether there is something of ecological
value to species diversity," said Christopher
Wills, a professor of biology at the University
of California, San Diego, who headed the
study "We
found that in forests throughout the
New and Old World tropics, older trees are
more diverse than younger ones. In
other words, diversity is actually selected
for as each of the forests mature. This means
diversity does indeed matter and is
an essential property of these complex ecosystems."
The study was conducted on seven undisturbed
forest plots, or tropical forest observatories,
maintained and studied by
research institutions in Borneo, India,
Malaysia,
Panama, Puerto Rico and Thailand under
the coordination of the Panama-based Center
for Tropical Forest Science of the Smithsonian
Tropical Research Institute.
In
addition to the Smithsonian Tropical Research
Institute, the institutions that manage
the tropical forest observatories included
in this study are the Indian Institute
of Science, Royal Forest Department of Thailand, University of Peradeniya of
Sri Lanka, University of Puerto Rico
and the Forest-Research-Institutes
in Peninsular Malaysia and Sarawak.
The forest plots, two from the Americas and five
from Asia, are themselves diverse.
They range from dense and species-rich wet
rainforest to drier and more open forest that
is often swept by fires. Even so, all the forests
show the same pattern of increasing local
diversity as trees age.
"Each forest in our study is a highly dynamic
community," said Harms. "We found
that the diversity of each local area increased
regardless of the species that were present.
This is because trees that were locally common tended to die more often
than those that were locally rare, giving a
survival advantage to rare species."
The effect was even seen within species, Harms
added.
"If a species was common in one part of a
plot and rare in another, its death rate was
higher where it was common," he said.
These
effects are strong enough to increase local
tree species diversity as trees age in all
seven of the forests, Harms said. The
researchers cite three possible reasons for the increase in
diversity, all of which are likely to play a role.
First,
rare species may be at an advantage because the animals, fungi, bacteria and
viruses that prey on them are less likely to cause damage when their hosts
are rare.
Second,
the rare species may be at an advantage in competition for certain physical
resources because individuals of the same species tend to share more similar
resource requirements than individuals of different species.
Third,
rare species would be
at an advantage when tree species have direct, positive influences on one
another, because trees of rare species are, on average, surrounded by a high
proportion of trees that are different from themselves.
The
scientists point out that none of these
processes can
operate in monoculture forests where
the individual trees are all of one particular species.
Such forests are highly susceptible to diseases, and each individual is
in direct competition with other individuals like itself.
The
three diversity-enhancing processes are also likely to be absent from badly
damaged forests. When
forests are clear-cut, the soil is rapidly eroded, depleted of
nutrients, and the "invisible world" of insects,
bacteria and fungi that help to sustain
diversity largely disappears.
The authors point out that their study suggests
that tropical forests that have been
damaged slightly by carefully managed selective logging, for example, should
soon regain their former levels of diversity
provided the damage has not been severe or
long-continued.
"If you damage a forest a little bit, the
forest can recover," says Wills.
"Even damaged
ecosystems can be restored to their former
diversity through natural processes if they
are allowed to do so.’’
Harms
and Wills agree that the new study
points the way for further detailed investigations
of the processes by which forest diversity is maintained and raises new
questions and lines of research for ecologists
and forest managers to pursue.
”Are the same processes operating in temperate
forests?" Wills asked. "How
much damage can
a forest sustain before its
diversity begins to decline? Are
other complex ecosystems like coral reefs also
selected for increased diversity? This paper
provides insights into a dynamic and evolving
natural world and shows that diversity is not
just an esthetic ideal but is also an important
property of natural ecosystems."
In addition to UCSD, LSU and the institutions
that managed the tropical forest
observatories, other institutions with
researchers who contributed to the collection and analysis of data for the
study include Harvard University, University of Alberta, University of
Minnesota, University of Georgia, Nanyang Technological University in
Singapore, Thai National Park Wildlife and Plant Conservation Department,
Field Museum, Florida State University, Osaka City University in Japan, University
of Illinois, Thammasat University in
Thailand and Tunghai University in Taiwan. The study was financed by grants
from the National Science Foundation and
the Center for Tropical Forest Science of the
Smithsonian Tropical Research Institute.