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WORLDWIDE FOREST/BIODIVERSITY CAMPAIGN NEWS

Fragmentation of Tropical Forests Can Create "Genetic Bottleneck"

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Forest Networking a Project of Ecological Enterprises

http://forests.org/

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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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.

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