[ExI] In Fragmented Forests, Rapid Mammal Extinctions

BillK pharos at gmail.com
Sun Sep 29 11:31:10 UTC 2013

Conservation biologists have long known that fragmenting wilderness
can put species at risk of extinction. Over two decades, Dr. Gibson
and his colleagues have tracked the diversity of mammals on the
islands. In Friday’s issue of the journal Science, they report that
the extinctions have turned out to be distressingly fast.


After the Khlong Saeng river was dammed, David Woodruff of the
University of California at San Diego recognized that the islands in
the reservoir would be good places to study how quickly nature calls
in that debt. The islands were all formed at the same time, they were
all isolated by water and they were surrounded by a vast forest
preserve that was still brimming with biological diversity.

Between 1992 and 1994, Woodruff’s team visited a dozen islands,
setting a 150-yard line of traps on each one. Each day for a week they
visited the traps, tagged any mammals they found and released the
animals. The researchers also set the same traps in the forests
surrounding the reservoir.

Just five years after the dam was built, they could see a difference.
Several species were more rare on the islands than on the mainland.

Dr. Gibson returned to the same 12 islands in 2012 and repeated the
survey. He started on a 25-acre island. The first survey had found
seven species of mammals. Dr. Gibson spent a week checking traps on
the island and found only a single species: the Malayan field rat.

This was a startling find for two reasons. One was the drastic crash
in diversity. The other was that the Malayan field rat wasn’t on the
islands when they first formed. Malayan field rats thrive around
villages and farms and other disturbed habitats. The rats Dr. Gibson
trapped must have come from the surrounding rain forests, where they
still remain scarce. When they swam to the islands, they found
fragmented forests that they could dominate.

“I thought, ‘Wow, what if this trend holds?'” said Dr. Gibson. “And it did.”

On most of the islands, all the native species were gone, replaced by
the rats. Only on a few islands did some species still cling to
existence. Dr. Gibson surveyed an additional four islands and found
they also had just one or two species, suggesting that all the islands
were suffering massive extinctions in about 20 years.

This report reminded me of Easter Island. Rat bones are the most
prolific bones excavated on Easter Island. The settlers were eating
the rats as one of their food sources. So if the rats were eating
everything else, there was probably little food left for the humans


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