How Invasive Snakes Have Turned Guam Into a Spider-Infested Horror Show

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Back in the 1940s, the highly invasive brown treesnake was accidently introduced to the island of Guam. In just four decades, these snakes had wiped out 10 of 12 native bird species, with the remaining two species forced to live in small areas, protected by snake traps. The result: an island with no bird chirps — and a 40-fold increase in the island's spider population.

And as a recent research expedition has revealed, the spiders have indeed taken over the island — a frightening example of what can happen when a critical predator is removed from an ecosystem.


Start counting

The research was conducted by biologists from Rice University, the University of Washington and the University of Guam, who were among the first to study how the loss of forest birds were impacting on the island's ecosystem. Without the insectivorous birds there to bother the spiders, the arachnids have largely taken over the place.


And as lead researcher Haldre Rogers observed, spiderwebs have indeed become a ubiquitous feature of the canopy; her team had to carry sticks and constantly wipe away the webs as they were making their way through the jungle.

And dealing with all the spiderwebs was in fact part of the job. By counting spiderwebs on both Guam and the nearby islands in the Marianas Island chain, the researchers were able to get a sense of what has happened on the island, compared to the other locations where the treesnake has not made an appearance.


And to do so, Rogers and her team — with tape measures and counters in hand — scoured the spider-infested island over the course of four months, counting webs. (So, you want to be an ecologist...)

Natural experiment

Even though this is a disturbing turn of events, at least Guam is proving to be a rather remarkable "natural experiment" for the researchers. Normally, you'd have to build a large exclosure, to simulate this sort of effect. But Guam is offering the scientists a unique real-world example of an ecosystem gone wrong, the result of a large-scale unintentional disruption of a natural system.


They discovered that, depending on the location, Guam contains anywhere from two to 40 times the normal number of spiders. Their research shows just how important insectivorous birds can be to an ecosystem. The researchers worry (and predict) that other locations in which insectivorous birds are on the decline could see a similar increase in spider populations. The closing words of their study ends rather ominously: "If insectivorous birds continue to decline, we will likely be living in a more spider-dominant world in the future."

In Guam, the treesnake is so problematic that the U.S. spends more than $1 million each year making sure airplanes and cargo are snake-free as they leave the American territory. And hunting them has not gotten any easier, since they are an elusive, nocturnal predator.


Moving forward, the researchers will continue to study the island, paying particular attention to whether or not the spider population will continue to increase.

The study was published in PLOS.

Images via Isaac Chellman.