One of the first experiments she had in mind was to investigate all those spiderwebs, which are much less plentiful elsewhere in the Marianas.
"I certainly wasn't the first to notice the incredible number of spiders in the jungles on Guam, but we were the first to quantify the difference between Guam and nearby islands," Rogers said.
Historically, if ecologists wanted to study how insects reacted to the absence of birds, they would build an "exclosure," a covering designed to keep birds out of their study area. Rogers said most exclosures cover a few branches of one tree, and, in rare cases, an entire tree. Building structures large enough to exclude birds from an entire forest simply isn't affordable, so the brown treesnake has effectively set the stage for experiments that ecologists couldn't otherwise do.
To find out exactly how many spiders were on the island, Rogers' team grabbed a tape measure and spent four months hiking through jungles counting spider webs, as a proxy for spiders. She and study co-authors Janneke Hille Ris Lambers and Josh Tewksbury of the University of Washington and Ross Miller of the University of Guam found that spiders were between two times and 40 times more plentiful on Guam than on neighboring islands.
Rogers said the results were a surprise, because they were several times more than would have been predicted from simply scaling up the numbers from small-scale exclosure studies.
"None of the small-scale experiments recorded that kind of increase," Rogers said. "It suggests that the small-scale experiments had gotten the interaction correct -- there is an increase in spiders when you lose birds -- but they may have underestimated the effect size."
Rogers said the result "shows that birds have a strong effect on spiders. Anytime you have a reduction in insectivorous birds, the system will probably respond with an increase in spiders. With insectivorous birds in decline
|Contact: Jade Boyd|