The Silent Forests of Guam: How Bird Losses are Warming a Forest

A Mariana Fruit-dove, extinct on Guam, but alive in the San Diego Zoo 

(Photo by Dick Daniels, carolinabirds.org via Wikimedia Commons)

The Mariana Fruit-dove—a vibrant creature decorated with what looks like multicolored puffs of spray paint across it chest and crest—is just one bird of many on the forested island of Guam that will never again be spied through a birder’s lens. The pigeon disappeared famously along with many other native birds in the wake of an invasion by brown tree snakes after World War II. Only now are scientists starting to piece together the effects—among them a thinning forest canopy increasingly riddled with holes, like Swiss cheese, the researchers say.

Over the next four years, ecologists from Rice University and the University of Guam will be investigating how this thinner canopy might be linked to the disappearance of the island’s birds. The US territory, which lies at the southernmost tip of the Mariana island Archipelago, once held 12 avian species, but ten were decimated by waves of voracious brown tree snakes, brought in unintentionally on ships during island reconstruction after the war.

By the mid-1980s, many birds were ghosts, no longer characteristic fixtures in the island’s trees. The Mariana Fruit-dove, the Guam Flycatcher, the Rufous Fantail, the Cardinal Honeyeater, and the White-throated Ground-dove were among the fallen.

The researchers looked last year at the impact of this loss on rising spider populations in Guam—finding 40 times as many spiders there as on islands close by, where birds still kept the population in check. But then another piece of the puzzle fell into place when lead researcher Haldre Rogers noticed gaps in Guam’s tree cover that she hadn’t seen elsewhere. “I noticed that there seemed to be a lot of gaps [in the trees] and that the pioneer tree species–such as papaya and sumak–were difficult to find on Guam, compared to nearby islands,” she said to the Smithsonian blog, Surprising Science.

Their hypothesis is that these fast-growing ‘pioneer’ trees aren’t filling up the gaps because the birds aren’t there to spread their seeds around. “Small birds eat a lot of the small seeds, and quite a few of the pioneer trees—the ones that grow best in open gaps with full sunlight—are small-seeded,” said Rogers in a press release. “Without birds to move their seeds to these sunny spots in the forest, these quick-growing trees may be less likely to germinate or grow to their full size.”

The result is a forest that opens up its cool, emerald depths to the sun, since the remaining slow-growing trees can’t fill the space as rapidly. As patches are laid bare the forest could turn hotter than it used to be, the researchers say, naturally a problem for creatures and trees that thrive in the coolth.

The ecologists are going to test their idea by clearing small patches in Guam’s cover and that of other nearby islands that are still bird-rich, and tracking how quickly the openings on Guam are repopulated by pioneer plants compared with those on other islands.

“It’s very important to understand the implications of those [bird] declines,” said one researcher, Amy Dunham, in the press release. “The situation on Guam—which is tragic—provides us with a unique opportunity to see what happens when all seed-dispersal services provided by animals are lost from an entire ecosystem.” For now, the researchers’ hunch hints at the untold ramifications of small mistakes made long ago.