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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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Comments

  1. Tyler James Seth
    chico
    March 11, 2013, 4:19 am

    Born and raised moved in 1999, I’ve seen spider webs in the jungle ranging 40 feet in diameter

  2. Lona Pierce
    United States
    January 23, 2013, 10:05 pm

    If this imbalance is accurate — lack of birds due to nonnative snakes is increasing the number of spiders — then it makes me wonder what would happen to the insects the spiders must be consuming, Will the flowers not get pollinated? Will butterflies go extinct there? Any study needs to see what the spiders are feasting on. Isn’t there a snake trap anyone has invented?

  3. Lynzi
    AZ
    October 16, 2012, 12:08 am

    When will we find out the results of the investigation?

  4. megan
    texas
    October 13, 2012, 2:43 pm

    The whole article is talking about all the damage these snakes of down and the research of the aftermath. I don’t really see anything talking about solving the problem. Come on if they don’t come out during the day and they’re hard to find at night make the snakes come to you. According to that one guys post an aviary full of song birds would be perfect bait. Gather an army of hippies, because I’m pretty sure they do these things for free. Set up a whole of bunch of aviary’s and arm those hippies with sticks and night vision goggle and get to killing. Problem solved your welcome.

  5. michaela
    arizona
    October 2, 2012, 10:12 pm

    the lizard the man was holding was huge i think it’s mouth could fit the man in it.

  6. Bob Hinkle
    Solon OH
    September 24, 2012, 9:25 pm

    We get what we deserve… so sad….

  7. TOP SECRET !!
    September 24, 2012, 1:59 pm

    NOOO
    dont disturb my life snake.
    I HATE U ALL SNAKE!
    hehehe!
    youre cold bloodeed animal.
    i hate u snake 4ever
    youre the king the president of earth one day!

  8. Cêsar Tepe
    San Martín -Perú
    September 21, 2012, 10:57 am

    In rice field, when is the land prepation time are there birds (Garza blanca), and the milk stage are there small birds. During de tiller stage, there are spider between 30 a 40 spider in 20 simple time evaluation red, and birds are only flying and often time in the field

  9. Greg Dever
    Palau
    September 19, 2012, 5:06 am

    In the early 80′s, when I was assigned to Guam by the U.S. Public Health Service, it was very noticeable how quiet the dawn was – there was no early morning cacophony of bird calls – very unlike other Pacific Islands. We tried to start a aviary in our enclosed back porch – finches, love birds, canaries and such. Things went well for about six months and then the snakes discovered the aviary. Within a month, all the birds disappeared. It was impossible to keep the snakes out. So we gave up. We worry that the brown tree snake will invade the rest of Micronesia and silence the dawn.

    • David Braun
      September 19, 2012, 11:01 am

      A scary thought, indeed. I wonder what’s going to happen in the Everglades as the invasive Burmese pythons continue to proliferate.

  10. cris671
    Dededo, GUAM
    September 19, 2012, 12:47 am

    All these stories about Guam & being invaded by spiders & snakes aren’t true… I have lived here for about 20 years & i’ve only seen probably 5 or 6 snakes at the most… Most of them at the zoo or as roadkill… As for spiders, they’re tiny spiders & the biggest one i’ve seen is less than 2 inches in length… If you go into the boonies aka jungle you need a friggin stick to help you on steep rocks/slopes due to water & due to not wanting to be scratched by sharp plants… Whoever funded these researchers wasted their money… & these researchers over exaggerated every information… The article title alone was disappointing… So stop making up things about Guam because everybody here shakes their head on how ridiculous these articles are…

  11. mags
    yes yes yes
    September 18, 2012, 7:02 pm

    there has def been an increase in the spider pop. in my bedroom

  12. Shirley Plastow
    Ontario
    September 18, 2012, 12:59 pm

    With the birds all gone, what are the snakes eating now?

    • David Braun
      September 19, 2012, 11:01 am

      Lizards. They have moved on to a new source of food.

  13. peter lehnert
    iron mountain, michigan
    September 18, 2012, 9:51 am

    what is the natural predator that eats the brown tree snake? maybe a nocturnal bird? do brown tree snakes lay eggs? catch a female, attach a gps transmitter under the skin and follow the snake to where it lays its eggs. do the snakes mate and lay eggs communally? find the mating and nesting patterns and locations. set traps baited with snake pheramones? to remove the males from the population? are the snakes nocturnal because there is a bird that eats them during the daylight hours?

  14. jack mangum
    texas
    September 18, 2012, 1:06 am

    Maybe sterilyzed cane toads. The hungry snakes would eat them and die, and the toads would live until they die and not be able to reproduce, so eventually they disappear too.