Fragmentation of Tropical Forests Can Create "Genetic Bottleneck"
7/4/98
OVERVIEW & COMMENTARY by EE
The consequences of tropical ecosystem fragmentation and damage, to
both ecological systems and constituent biodiversity, are far from
known. A University of Georgia study indicates the disproportionate
manner in which individuals trees left standing in pastures can
dominate the reproduction in nearby remnant forests, "creating a
genetic bottleneck." This indicates that the genetic diversity of
seedlings in forest fragment may be relatively small, with severe risk
of inbreeding. Continued survival, particularly viability of
fragmented tropical rainforest, is far more complex than previously
indicated and may require new conservation strategies.
g.b.
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RELAYED TEXT STARTS HERE:
Title: Fragmentation Of Tropical Forests Can Create "Genetic
Bottleneck," According To New University Of Georgia Study
Source: University of Georgia
Status: Distribute freely accredited to source
Date: July 2, 1998
Contact: Phil Williams
philwpio@arches.uga.edu
706-542-8501
University of Georgia
Contact: Jim Hamrick
hamrick@dogwood.botany.uga.edu
706-542-1826
University of Georgia
ATHENS, Ga.--Tropical forests have been disappearing at alarming rates
for the past three decades. Farmers, ranchers and timber industries
have cut millions of acres, and only in the past few years has the
ecosystem damage become clear.
A new study by botanists at the University of Georgia now shows for
the first time that trees left standing in pastures can actually
dominate the reproduction in nearby remnant forests, creating a
"genetic bottleneck." The research indicates that the survival of
tropical forests could be far more complex than was known before and
that new approaches to conservation strategies may be needed.
"The key is to understand how much genetic movement there is between
fragments of forest," said Dr. James Hamrick. "When we lose fragments
of forest, we lose genetic diversity. Gene exchange between fragments
helps to maintain this diversity."
The study, by Hamrick and his graduate student Preston Aldrich, was
published today in the journal Science.
Genetic diversity is vital in both plant and animal communities.
Farmers have, for hundreds of years, bred crop plants and farm animals
to maintain a healthy diversity of what were, before the 20th century,
called traits. Now, with advanced techniques to determine the exact
genetic makeup of individuals, scientists understand considerably more
about how genes drift through populations.
Aldrich and Hamrick studied a tree species called Symphonia
globulifera in a little-examined premontane rain forest area in
southern Costa Rica. S. globulifera is a shade-tolerant canopy tree
with bright red flowers that are pollinated primarily by hummingbirds.
Bats disperse the seeds by eating fruits and then passing seeds on
through guano at their resting sites. Like many areas in the tropics,
the study area consisted of an area of fragmented forest with a number
of large nearby members of the species standing alone in open pasture
land. There were neither seedlings or saplings of S. globulifera in
the pastures, suggesting poor habitat quality for germination and
growth.
The question was simple: What trees are the parents of seedlings
growing in the forest fragments? Finding the answer would have been
nearly impossible even a decade ago until the invention of sensitive
techniques that allow researchers to determine the exact genetic
makeup of individual plants in an ecosystem. Even now, the problem is
daunting, since there were more than 800 possible parent pairs.
"In trying to tell who the parents are, we had to use the same
techniques used in forensic analysis to determine a child's parents,"
said Hamrick. "We were able to do this only because Preston was able
to develop the techniques for our specific genetic analysis himself."
The breakthrough came in using segments of DNA called microsatellites
as specific markers for S. globulifera. These markers allowed Aldrich
and Hamrick to determine the pedigree for a number of seedlings and
saplings in a one hectare forest fragment on their 38.5- hectare
research area. (A hectare is a metric unit of area equal to 2.471
acres.) The scientists knew the genetic composition of all the adults,
42 individuals, in the study area. The results of the genetic analysis
were startling. Out of nearly 250 seedlings studied from a single
forest fragment, some 68 percent were produced by adults in pastures -
- not from adults within the fragments themselves. Moreover, of the
seedling produced by pasture trees, 77 percent came from only two
trees. Adults left in the fragment produced less than 5 percent of the
seedlings in their own patches. The importance of the discovery lies
in the fact that the genetic diversity of seedlings in forest fragment
may be relatively small indeed. "If you looked at the number of
seedlings superficially, you might say that this is a healthy rate of
regrowth," said Hamrick. "But in truth, the effect is ecologically
unhealthy due to the potential for inbreeding in subsequent
generations." As humans know, inbreeding can expose deleterious genes.
The scientists have several theories why the pasture trees have
such an overwhelming impact on gene flow. First, there is little
competition for the pasture trees for sunlight and nutrients,
giving them superior abilities to flower and fruit. Second, the
abundance of flowers may attract more hummingbirds for
pollination. Finally, bats have easy pickings of the fruit and
take them from the pasture trees to the forest fragment, where
they eat and then pass seeds back to soil in guano.
The study has important implications for conservation and forest
restoration. And it shows that the impact of deforestation has
been far more devastating that the simple removal of individual
trees. At least in this species of tree, fragmentation has
resulted in the possibility of a serious loss of genetic
diversity in this test area.
Thus, areas that look healthy in terms of regrowth may not be
healthy at all over the long term. They may be facing serious
future problems due to problems with genetic drift and
inbreeding. American farmers already know of the problems this
can cause, since they barely avoided a near-disaster because of
over-planting of corn with too little genetic diversity in the
1970s.
Still, scientists say it is beginning to become apparent that it
will be difficult to make any kind of blanket statements about
gene movement among populations of forest species. Indeed,
botanists say we are now only beginning to understand what
happens to genetic diversity in natural populations over
time--and why.
"One of the important things this study has shown us is that the
superficial appearance of an area might not be telling you the
whole story," said Hamrick. "Each situation has unique
characteristics that make if very difficult to say that tropical
trees in a certain situation will behave this way or that. Quite
simply, our study area looked healthy, but it wasn't."
Written by Phil Williams.
Editors/Writers: James Hamrick will be out of the country until
Monday, July 6. He may be reached at the above phone number at
that time.