The accidental introduction of a snake species to Guam several decades ago almost completely wiped out the Pacific island’s forest birds. Now the lack of birds is causing Guam’s spider population to explode.

A new study shows Guam’s jungles have 40 times more spiders in the wet season than the jungles of the nearby islands of Rota, Saipan and Tinian, where birds are still around to eat spiders.

During the dry season, Guam — a U.S. territory about 1,500 miles south of Tokyo and 3,700 miles southwest of Hawaii — had more than twice as many spiders as nearby islands did.

Haldre Rogers, lead author of the study published this month online in the journal PLOS ONE, and fellow researchers measured sections of jungle about 65 to 100 feet long and counted the number of spider webs with spiders in these areas. They used the same type of forest — limestone — for the study on each island.

"When we did all these surveys on the other islands, it didn’t take any time at all. You got your transect tape and you counted spiders, and there weren’t very many and it was very quick," said Rodgers, a faculty fellow at Rice University. "Then we got to Guam, and each survey just took forever because there were so many more spiders."

Webs woven by one species — Argiope appensa, or garden spider — were also 50 percent larger on Guam, the study found. This suggests the garden spider is able to keep its webs up longer or build larger webs when birds aren’t around, Rogers said.

Rogers said she conducted the study after anecdotally observing that Guam had more spiders. But she didn’t expect the research to show such a large difference.

Guam’s forest birds were almost completely eliminated by the brown tree snake, a reptile native to Australia and the Solomon Islands which doesn’t have any natural predators on the island. It’s unknown how brown tree snakes arrived on Guam, though most likely they stowed away on a cargo ship after World War II.

Hawaii officials have long been concerned the snakes might sneak onto cargo leaving Guam and ravage avian species the way they have on Guam.