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ROBERT SIEGEL, HOST:

From NPR News, this is ALL THINGS CONSIDERED. I'm Robert Siegel.

AUDIE CORNISH, HOST:

And I'm Audie Cornish. The Pacific Island of Guam is experiencing a population explosion of spiders. There are more there now than anyone can remember. To explain why, NPR's Christopher Joyce begins our story in Maryland.

CHRISTOPHER JOYCE, BYLINE: I'm standing on the front porch of my house along the Chesapeake Bay right now, 6:30 in the morning and I have found two spider webs in a matter of five minutes. It's that time of year. They're pretty busy, but if I were on the island of Guam in the Pacific, right now, I might find 70 or 80 spider webs because Guam is experiencing a spider epidemic.

Ecologist Haldre Rogers realized something strange was going on as she went roaming through the jungle on Guam and three nearby islands.

HALDRE ROGERS: While I was doing that, it appeared that there were tons more spiders on Guam than there were on other islands.

JOYCE: Rogers was a researcher from the University of Washington and she was actually in Guam looking for snakes. The brown tree snake invaded Guam over 60 years ago. They snuck in aboard boats or in the wheel wells of airplanes. And now, they're everywhere - about 2 million of them.

But the spider thing was just too bizarre to pass up. So, Rogers started counting spider webs on the islands. In the dry season, Guam had about two and a half times more spider webs.

ROGERS: And 40 times more webs in the wet season than on the nearby islands.

JOYCE: Forty?

ROGERS: Forty times. Yes.

JOYCE: One that seemed to be everywhere was a great big yellow and black critter called Argiope appena, or the banana spider.

ROGERS: And actually what we found was, their webs were much larger of Guam than they were on other islands, or 50 percent larger on Guam.

JOYCE: More, bigger, and better. This all came about because the introduced tree snake multiplied only on Guam and ate almost all the birds. There are literally just a few hundred birds left there. Since birds eat spiders, this is good news for spiders. In fact, great news because birds also ate some of the bugs spiders eat, so now there's more food for spiders.

For ecologists, Guam is now a big experiment to see what happens when a top predator disappears from an ecosystem. They suspect plants could be affected soon. Many depend on birds to spread their seeds. But Rogers wants to remind people that Guam is still a nice place.

ROGERS: The average person doesn't come across that many spiders because most people don't go traipsing around in the jungle that much.

JOYCE: The research appears in the journal, Plos One. Christopher Joyce. NPR News.

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