Spiders Take Over Forests in Guam

What if a lack of insectivorous birds led to an incredible boost in spider populations in a single environment? Apart from being the worst place imaginable, how else would an ecological niche be affected? Biological researchers from Rice University in Texas, the University of Washington and the University of Guam have not only found that these spidery conditions exist in Guam but that jungles on the island territory contain as much as 40 times the amount of spiders as any other island nearby.

With the birds are away, the spiders have come out to play. So how was it that multiple bird species just disappeared from the country? It all started with the brown tree snake. Native to coastal areas in Australia, Papua New Guinea and the islands in northwestern Melanesia, the nocturnal brown tree snake is believed to have entered Guam by way of hitching a ride inside cargo planes sometime in the 1940s. Once on the island, brown tree snakes met no natural predators and over time began to take over.

In no time many bird species began to die off because of the snakes, and after only five decades all but two of the island’s dozen native bird species had been wiped out. Never before had research been conducted that looked at the implications caused by an invasive species in an entire forest. By studying this situation in Guam, this team was able to get a first-hand look at the effects an invasive species has on a whole island environment.

“There isn’t any other place in the world that has lost all of its insect-eating birds,” said Haldre Rogers, a Huxley Fellow in Ecology and Evolutionary Biology at Rice University and the lead author of the study. “There’s no other place you can look to see what happens when birds are removed over an entire landscape.”

Without the birds, the jungle grew eerily quiet and the spiders quickly got to work. In no time, webs accumulated over the jungle canopy—filling in all opens spaces, leaving anyone choosing to walk through unable to do so without the aid of a stick to help clear a path. The problem has gotten so severe that all steps are taken to ensure that brown tree snakes do not leave the island. Every year the United States spends over $1 million to search airplanes flying out of Guam to ensure the snake does not leave.

Yet, even with these measures in place, the average traveler will have a hard time locating the nocturnal reptiles. It is partially for this reason that the problem of eradicating the snakes has become so difficult. And now that the snakes have moved on to preying on lizards, one wonders what effects this new shift will have in the future. Researchers can be sure that their work here is nowhere near finished.

According to Rogers, these results “show 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.” The study shows that the environment is a fragile interconnected device with precise and individually working functions. When something is out of sync another will falter and then another and another until the whole finds a different way of equalizing. But what exactly be the result of such a correction can hardly be imagined.  Sometimes the adjustment can be subtle; other times, however, spiders may take over.


Photo Credit: sofia.usgs.gov/virtual_tour/pgfernforest.html

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