Spiders on the march

Researchers on Guam, a US island east of the Philippines, have found that the island’s jungle spider population is more than 40 times larger than that of nearby islands.
In one of the first studies to examine how the loss of forest birds is affecting Guam’s ecosystem, biologists from Rice University, the University of Washington and the University of Guam found that arachnid populations grew as much as 40-fold once invasive brown tree snakes wiped out most species of the island’s insect-eating birds.
Since the snakes reached Guam in the 1940s, all but two of the island’s dozen native bird species have disappeared. But as the birds disappeared, spiders gained ground.
“You can’t walk through the jungles on Guam without a stick in your hand to knock down the spider webs,” said lead author Haldre Rogers, a Huxley Fellow in ecology and evolutionary biology at Rice.
The scientists investigated whether the disappearance of birds led to the proliferation of spiders. Small experiments have shown a link but the Guam study is the first to examine an entire forest.
Rogers said the results showed that birds had a strong effect on spiders. “Any time you have a reduction in insectivorous birds, the system will probably respond with an increase in spiders,” she said. She suspects that as bird numbers have fallen in many places around the world, spider populations have risen to match.

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