Snake's Impact on Guam Appears to Extend to Flora
Monday, August 11, 2008
One of the most infamous examples of what can happen when a nonnative species is introduced into a new environment involves the brown tree snake -- a voracious, semi-venomous species that in less than 50 years all but destroyed bird life on the northern Pacific island of Guam. Introduced inadvertently from the South Pacific just after World War II, apparently on a cargo ship, the snake has killed off 10 bird species on the island and is in the process of wiping out the remaining two.
The virtual extermination of Guam's birds has been bemoaned for decades, but new research suggests that the damage to the ecology of the narrow, 30-mile-long island did not stop there.
The hundreds of thousands of snakes, researchers say, are now changing the way Guam's forest grows and will most likely cause substantial thinning and clumping of trees in the years ahead. In addition, the snakes appear to be indirectly responsible for an explosion in the spider population.
Guam, which is 3,800 miles west of Hawaii, did not have predatory snakes before the brown tree snakes arrived, and as a result the birds were not afraid of such creatures and not prepared for the onslaught. The snakes have few natural predators on the island and have at times climbed electric poles in their search for young birds, causing power outages.
"The brown tree snake has often been used as a textbook example for the negative impacts of invasive species, but after the loss of birds no one has looked at the snake's indirect effects," said Haldre Rogers, a University of Washington doctoral student in biology who presented her findings last week at the Ecological Society of America's annual meeting.
Rogers, who first went to Guam in 2002 as part of a U.S. Geological Survey "rapid response team" in a bid to keep the snakes from spreading, said she has studied tree growth on Guam and neighboring islands and has found "amazing" differences.
Without birds, which eat the seeds of certain trees and then spread them in their droppings, those trees are losing out to others that do not depend as much on bird middlemen. The seeds of the trees that relied on birds are now falling mostly near the trunks of the parent trees, where they are more likely to be spoiled by fungus and less likely to grow into healthy trees. The result, Rogers said, will either be the loss of some tree species or the clumping of those trees in isolated patches.
"It seems inevitable that the forest will change and some of the native species will lose out," she said. Birds typically make up a small part of the life of a forest, but they are important not only for spreading seeds but also for pollinating flowers and controlling some insects that feed on plants.
To test her theory, Rogers built traps of fine-mesh screen-door netting to collect falling seeds. She set 119 traps beneath and near Premna obtusifolia, or false elder, trees on Guam and on the nearby island of Saipan, which does not have the snakes. She set two traps directly beneath each tree's canopy, two about three feet away, three at 16 feet, three at 33 feet and seven at 65 feet away.
On Saipan, she found seeds in nearly every trap -- though, not surprisingly, there were more seeds closer to the trees. But on Guam, there were no seeds at all beyond the canopy of the trees. Most of the bird-dispersed seeds on Saipan also had their outer coverings removed, something that most likely occurred in the gut of a bird. Bare seeds germinate faster and result in speedier growth of new trees. None of the seeds on Guam had their coverings removed.
Rogers' team also randomly searched for seedlings of the Aglaia mariannensis (or mupunyao) tree and each seedling's most likely parent on Guam, Saipan and two other nearby islands, Tinian and Rota. All the Guam seedlings were found within 16 feet of the nearest adult tree -- most within six feet. On the other islands, the adult trees were two to three times as far from the seedlings.
Gregory Butcher, director for bird conservation at the National Audubon Society, said Rogers's research is consistent with the widely held view that birds provide "invaluable ecological services," especially in spreading plant seeds.
"We know that if birds are not present to fulfill that role, there often is nothing else that can do it," Butcher said. "The implications are severe, and that's why we consider bird conservation to be so important."
Rogers said other recent studies have shown a steady decline in bird populations, and that up to one-quarter of American species are threatened with extinction.
The population of brown tree snakes peaked on Guam in the mid-1980s, but the island remains one of the most snake-infested places on Earth -- with as many as 3,000 per square mile, some of them 10 feet long. The snakes, which are nocturnal and will eat adult birds as well as eggs, can harm young children but are not considered a danger to others. With most of the birds now gone, they live on rodents and lizards, the endangered Mariana fruit bat, and sometimes inhabitants' garbage.
Island officials continue to work to control the explosion, but Rogers said it is no longer possible to eradicate the snakes. Officials concentrate on making sure they are not on cargo ships leaving the island.
Rogers said Guam also appears now to be swarming with spiders, and her team -- which receives public, private and academic funding -- plans to survey that population as well as doing more research into the effect of birdlessness on trees. The spiders used to be controlled by the birds, she said, but now they, too, have few predators. Although a few nonnative birds have come to Guam, she said, the numbers remain small.
"Unfortunately, Guam is a laboratory of sorts for what happens when an invasive species brings major change," she said. "You can't really see it yet, but it appears that the indirect consequences for the forest can be as important as the direct consequences we saw on the bird population."