Up to 40 Times More Spiders Recorded in Nearly Birdless Guam

Invasive brown tree snake.

The other day, I pondered what a world without birds would look like. Coincidentally, a new study has just come out revealing that the loss of birds on Guam may have led to a frightening uptick in the amount of spiders on the island.

It’s actually one of the first large-scale looks at what could happen if a native bird species is wiped out by an invasive species.

The brown tree snake was introduced to Guam back in the 1940s. Over the last half century, the amount of brown tree snakes has grown exponentially and wiped out all but two of the native bird species in the process. (The last two bird species are being safely protected in a highly trapped area of Guam to prevent brown tree snakes from taking over.) Without the presence of insect-eating birds, the number of spiders has also exploded.

Here’s more from one of the authors [via National Geographic]:

“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 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 [University] 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.”

By comparing the density of spiders on Guam with nearby islands whose birds weren’t devastated by brown tree snakes, the researchers found that up to 40 times more spiders live on Guam.

This finding definitely points to the importance of birds in keeping the number of insects down, but it would also be interesting to see what sort of domino effect this might have on the rest of the ecosystem. Are there more insects in general? How is this having an effect on the plants? Is the spiders’ prey being diminished?

It’s still a bit early, although they expect to delve further into how the loss of birds on the island also play into seed dispersal. Now, if you ever want to experience what a birdless world might look like, all you have to do is go to Guam.

Researcher Haldre Rogers said that upon initial comparisons of the two islands (Guam without birds and another island with birds) was that Guam had many more spiderwebs. Another notable difference was that the lack of songbirds made the Guam forest eerily quiet during the day.

Hopefully, this study about the effects of diminishing bird populations will encourage other places to put more provisions into place that would protect some endangered or threatened species.