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Ular brown tree snake  Bisa Berarti juga merugikan  di Guam 

Ular brown tree snake  datang dapat  mewujudkan hal-hal buruk yang dapat terjadi ketika menjadi spesies invasif  di mana mereka hanya  memiliki sedikit  predator . Tapi penelitian baru menunjukkan bahwa dampak tidak langsung menjadi lebih luas  , kemungkinan  mengubah distribusi pohon dan mengubah ekosistem yang sudah rusak .....read more

Brown Tree Snake Could Mean Guam Will
Lose More Than Its Birds

Date:

August 10, 2008

Source:

University of Washington

Summary:

Brown tree snakes have come to embody the bad things that can happen when
invasive species show up where they have few predators. But new research
suggests that indirect impacts might be even farther reaching, possibly
changing tree distributions and altering already damaged ecosystems.

........................

In the last 60 years, brown tree snakes have become the embodiment of the
bad things that can happen when invasive species are introduced in places where
they have few predators. Unchecked for many years, the snakes caused the
extinction of nearly every native bird species on the Pacific island of Guam.

A variety of other damage has been directly attributed to brown tree
snakes, including large population losses among other native animal species in
Guam's forests, attacks on children and pets, and electrical power outages.

But new research by University of Washington biologists suggests that
indirect impacts might be even farther reaching, possibly changing tree
distributions and reducing native tree populations, altering already damaged
ecosystems even further.

"The brown tree snake has often been used as a textbook example for
the negative impacts of invasive species, but after the loss of birds no one
has looked at the snake's indirect effects," said Haldre Rogers, a UW
doctoral student in biology.

"It has been 25 years since the birds disappeared. It seems to me the
consequences are going to keep reverberating throughout the community if birds
are fundamental components of the forest," she said.

Birds typically make up a small part of the life of a forest, but they are
important for pollination, spreading seeds around the forest and controlling
insects that feed on plants. Guam, an island 30 miles long and 5 to 15 miles
wide about 3,800 miles west of Hawaii, lost most of its native birds after the
brown tree snake was introduced by accident from the Admiralty Islands following
World War II. The snake has few predators on Guam, so its population density is
quite high – estimated at more than 3,000 per square mile – and some
individuals there grow to an unusual size of 10 feet long.

Before introduction of the brown tree snake, Guam had 12 species of native
forest birds. Today 10 of those are extinct on Guam, and the other two species
have fewer than 200 individuals. Though Guam has some non-native bird
populations, few other birds moved in when native species died out, and none of
them live in the forest. That leaves few birds to consume tree seeds and then
drop them away from the trees.

That could have two possible negative impacts on the native forests, Rogers
said. First, some plant species need birds to handle their seeds to ensure
effective germination. In addition, seed predators and fungi that kill seeds
are often found in high density directly beneath a parent tree, so the trees
rely on birds to disperse seeds beyond the range of those negative effects. If
native birds performed those functions on Guam, tree populations could suffer
from the loss of birds. It appears 60 percent to 70 percent of tree species in
the native forests are dispersed, at least in part, by birds, she said.

To test the effects of the loss of native birds on seed distribution,
Rogers devised seed traps that look a bit like satellite dish receivers, with
tubing bent into a circular shape and covered with fine mesh screen-door
netting. She set 119 traps beneath and near Premna obtusifolia, or false elder,
trees on Guam and the nearby island of Saipan, which does not have brown tree
snakes. For each tree sampled, she set two traps directly beneath the tree's
canopy, two about 3 feet away, three at 16 feet, three at 33 feet and seven at
65 feet.

On Saipan, Rogers and her colleagues found seeds in nearly every trap at
each distance, though more seeds were found in closer traps and fewer farther
away. However, on Guam the seeds appeared only in traps directly beneath the
canopy. What's more, most of the farther-dispersed seeds from traps on Saipan
had the seed coats removed, a factor that could speed seedling germination and
the growth of new trees and something that likely could only be accomplished in
the gut of a bird. None of the seeds found on Guam had seed coats removed.

In addition, the scientists randomly selected points in native forests on
Guam, Saipan and two other nearby islands, Tinian and Rota, searching for
seedlings of a tree called Aglaia mariannensis and each seedling's most likely
parent, the closest adult of that species. On Guam all seedlings were found
within 16 feet of the nearest adult tree, most within 6 feet. On the other
islands the nearest adult trees were found two to three times farther away from
the seedlings.

"These findings could have global implications, since forests in areas
that have had a decline in bird populations instead of outright extinction
might show effects similar to those in the forests of Guam," Rogers said.

She notes that recent studies show bird populations are declining
worldwide, and that as many as 25 percent of U.S. species face the threat of
extinction.

Rogers presents her data August 8 at the Ecological Society of America
meeting in Milwaukee. Co-authors are Joshua Tewksbury and Janneke Hille Ris
Lambers, both UW assistant professors of biology.

Further research, Rogers believes, could turn up other indirect impacts the
brown tree snake has had on Guam. For example, she notes anecdotal evidence
that there is a substantially higher spider population on Guam than on other
nearby islands, and she speculates that could largely be because the native
bird population has been decimated.

But the biggest indirect impact, she said, could be altered seed scattering
that in turn might, in the near future, transform the remaining forest from a
diverse mixture of tree species to clumps of trees of the same species,
separated by open space. That could have serious consequences, including
extinction, for plant and animal species that still live in the forests.

"It seems logical that if there are no birds then seeds are not able
to get away from their parent trees, and that is exactly what our research
shows," Rogers said. "The magnitude of difference between seed
dispersal on Guam and Saipan is alarming because of its implications for Guam's
forests, and for forests worldwide experiencing a decline or complete loss of
birds."







Story Source:

The above story is based on materials provided by University
of Washington. Note: Materials may be edited for content and length.

http://www.sciencedaily.com/releases/2008/08/080808090313.htm

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