Spiders Take Control as Birds Fade From Guam

As bird populations plummet worldwide, will Earth become the Planet of the Spiders?

Researchers on Guam, a 30-mile-long U.S. island about 3,800 miles west of Hawaii, found that arachnid populations grew as much as 40-fold in the wake of entire species of insect-eating birds eaten into oblivion by invasive brown treesnakes. One biologist suspects spiders are multiplying also in other regions where birds are in decline.

Guam is the textbook study for what can happen to birds when an ecosystem is devastated by invasive species. After brown treesnakes somehow made their way to island in the 1940s, it took less than half a century for them to extirpate all but two of the island’s dozen native bird species. But as the birds slipped down the gullets of the insatiable nocturnal predators, spider populations proliferated. Did the fall of the birds lead to the rise of the spiders?

Biologists from Rice University, the University of Washington and the University of Guam found that Guam’s jungles have as many as 40 times more spiders than are found on nearby islands like Saipan, according to their research paper published in the open-access journal PLOS ONE.

“You can’t walk through the jungles on Guam without a stick in your hand to knock down the spiderwebs,” says Haldre Rogers, a Huxley Fellow in Ecology and Evolutionary Biology at Rice and the lead author of the study published last week.

“The results are some of the first to examine the indirect impact of the brown treesnake on Guam’s ecosystem,” Rice University said in a news statement about the research.

“The new study is the first to examine the impact of bird loss on the scale of an entire forest.”

The scientists investigated whether the disappearance of birds led to the increase in the spider population on Guam, since many birds consume spiders, compete with spiders for insect prey and use spider webs in their nests, Rice explained. “Small-scale experiments in other ecosystems have consistently shown a link between the presence of birds and the abundance of spiders, but the new study is the first to examine the impact of bird loss on the scale of an entire forest.”

The scientists compared the density of spider webs on Guam with webs on nearby Marianas Islands. Rogers said the difference between the number of spiders she and her colleagues counted on Guam and three nearby islands that still have birds “was far more dramatic than what any small-scale experiments had previously found.” The findings underscore the importance of using both observed counts and controlled experiments when attempting to predict how entire ecosystems will react to change, she said.

The highly invasive brown treesnake (Boiga irregularis), native to coastal Australia, Papua New Guinea, and islands in northwestern Melanesia, probably came to Guam as a stowaway in cargo on a ship or a plane. Not adapted to this kind of stealthy nocturnal predator, most species of native forest birds were wiped out as the snake established itself across the island. The effect of the loss of birds is still under investigation. Meanwhile, the brown treesnake has moved on to eating Guam’s lizards. (Credit: Isaac Chellman)

 

To prevent brown treesnakes from spreading from Guam to other islands, the U.S. spends more than $1 million a year searching airplanes and cargo to prevent the snakes from escaping Guam, Rice University noted. “However, the reclusive, nocturnal reptiles are extremely hard to find. Said Rogers: “The average resident or tourist on Guam will never see one, and even those who actively hunt them are hard-pressed to find one, which is one reason the snakes have been impossible to eradicate from the island.”

Related Post: Snake Plague on Guam Impacts Trees

 

Haldre Rogers. (Credit: Isaac Chellman)

Rogers first worked on Guam to lead the U.S. Geological Survey’s brown treesnake rapid response team, a small group of snake hunters charged with capturing brown treesnakes that manage to get off the island, Rice said in its news statement. “Specifically, the team’s mission is to respond within 24 hours of any sighting of a brown treesnake on any island that is served by flights from Guam.”

“When I was out there searching for snakes at night, I spent a lot of time thinking about the differences between the forests I was walking through and the forests back on Guam,” Rogers said. “The spiderwebs were just one difference. The lack of songbirds also make Guam’s forests eerily quiet during the day,” she said. By the time Rogers enrolled in graduate school at the University of Washington in 2005, she had a number of ideas for ecological field studies aimed at measuring and explaining the differences she’d observed, according to Rice.

“There isn’t any other place in the world that has lost all of its insect-eating birds.”

“There isn’t any other place in the world that has lost all of its insect-eating birds,” Rogers said. “There’s no other place you can look to see what happens when birds are removed over an entire landscape.”

One of her first experiments 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.

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, Rice 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,” Rice said.

“None of the small-scale experiments recorded that kind of increase. 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.”

In future studies, scientists hope to determine whether the loss of Guam’s forest birds is what caused spider populations to increase. (Credit: Isaac Chellman)

 

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 in many places in the world, I suspect there has been a concurrent increase in spiders.”

“With insectivorous birds in decline in many places in the world, I suspect there has been a concurrent increase in spiders.”

Rogers plans to conduct exclosure experiments on neighboring islands that still have forest birds and compare those results with observations on Guam to determine the exact links between the lost forest birds and the spider population increases.

“Ultimately, we aim to untangle the impact of bird loss on the entire food web, all the way down to plants,” Rogers said. “For example, has the loss of birds also led to an increase in the number of plant-eating insects? Or can this increase in spiders compensate for the loss of birds?”

“We’ve known for some time that the introduction of the invasive brown tree snake was a disaster for the island’s native bird populations; now we are learning about some of the consequences of that loss of avian diversity,” said George Fenwick, President of American Bird Conservancy. “This study clearly illustrates the valuable function that birds provide in controlling invertebrate numbers, and the natural balance that birds bring to our environment.”

Birds pollinate our crops, control crop pests, and, it would seem, keep spider populations from exploding, Fenwick noted in a news statement.

The research was supported by the Budweiser Conservation Scholarship through the National Fish and Wildlife Foundation, the University of Washington Department of Biology Giles Award, a Howard Hughes Medical Institute undergraduate research fellowship, a National Science Foundation Graduate Research Fellowship and grants from the National Science Foundation and the U.S. Department of Agriculture.

This post was based on news materials released by Rice University and PLOS ONE.

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Forty years in U.S., UK, and South African media gives David Braun global perspective and experience across multiple storytelling platforms. His coverage of science, nature, politics, and technology has been published/broadcast by the BBC, CNN, NPR, AP, UPI, National Geographic, TechWeb, De Telegraaf, Travel World, and Argus South African Newspapers. He has published two books and won several journalism awards. He has 120,000 followers on social media. David Braun edits the National Geographic Society blog, hosting a global discussion on issues resonating with the Society's mission and initiatives. He also directs the Society side of the Fulbright-National Geographic Digital Storytelling Fellowship, awarded to Americans seeking the opportunity to spend nine months abroad, engaging local communities and sharing stories from the field with a global audience. Follow David on Facebook  Twitter  LinkedIn