Guam's Wildlife Threatened by Invasive Snake

Sometime shortly after the end of World War II, an unwelcome predator found its way to the island of Guam: the Brown Tree Snake (Boiga irregularis). There is no way to know exactly how or when the snake arrived on Guam. The most likely explanation is that after the war ended, snakes from the Admiralty Islands (located off the northwest coast of Papua New Guinea) repeatedly found their way onto US military cargo ships bound for the new US Pacific Headquarters on Guam.

When the stow-away snakes arrived on Guam, they found what to them must have been an island paradise. There were no predators on the island that could challenge them and there was an abundance of native fauna on which the invading snakes could prey: birds, mammals, and lizards. Guam’s native species had evolved in the absence of predatory snakes—the only snake species native to Guam is Ramphotyphlops braminus, a tiny, blind snake that feeds on arthropods. When coiled up, Ramphotyphlops braminus can fit on the top of a nickel. As a result, Guam’s local fauna was ill-prepared to cope with the aggressive, predatory 3–6-foot Brown Tree Snake.

The snakes’ presence went undetected until the early the 1950s, when people began noticing them around the Naval Port in central Guam. The snake population quickly expanded from there. By the early 1960s, the Brown Tree Snake had established itself over more than half of the island and by 1968, it had colonized all of Guam to varying degrees and densities.

Wherever the snakes went, populations of native birds crashed. A wave of bird disappearances spread out across the island following the path of the Brown Tree Snake. The snake's toll was particularly heavy on Guam’s forest-dwelling birds. Before the introduction of the brown tree snake, there were 11 native species of forest-dwelling birds on Guam. Today, nine of those species no longer survive on the island. A few species, such as the Guam Flycatcher and the Guam Rail, exist only in captivity.

The decline of Guam’s native occurred, for the most part, two and a half decades ago. But the ecological impact of the Brown Tree Snake does not stop there. A team of scientists from the University of Washington led by doctoral student Haldre Rogers have recently suggested that an onslaught of secondary ecological impacts brought on by the Brown Tree Snake are now surfacing.

Birds often play important roles within their communities and their presence in a habitat influences the flora around them by providing seed dispersal and insect control. If birds suddenly disappear from a community as they did 25 years ago on Guam, there may be significant vegetation changes that follow.

To find out if Guam’s forests are being impacted by the absence of native birds, Rogers and her colleagues set up numerous seed collection stations around False Elder (Premna obtusifolio) trees. They positioned seed collection stations directly beneath the canopy of the trees and also set up additional seed collection stations at distances of 3, 16, 33, and 65 feet from each study tree. They repeated this setup in two locations: Guam (where the Brown Tree Snake had extirpated most native birds) and Saipan (a nearby island that does not have Brown Tree Snakes).

After collecting seeds from their study stations, they found that on Saipan, seeds were dispersed outward from the tree. They found more seeds close to the tree and fewer further from the tree, but seeds were present in nearly all of the traps placed around the tree. On Guam, they seeds were present only in the traps that were located directly under the parent tree.

Rogers and her team also noticed another interesting difference between the seeds in the two study locations. The farther-dispersed seeds collected from trays on Saipan did not have seed coats. Rogers reasoned that these seed coats were removed as the seeds passed through the guts of the birds that dispersed the seeds. Such removal of the seed coat can facilitate germination. On Guam, because no birds were present to disperse the seeds, the seeds remained encapsulated in seed coats.

The story of the Brown Tree Snake and its impact on Guam’s wildlife is far from over. The snake has had an alarming impact on the islands birds and there is still much to learn about the after effects of this invasive snake’s arrival on the island.

References:

• Fritts TH, Leasman-Tanner D. 2001. The Brown Treesnake on Guam.
• Stricherz V. 2008. Brown Tree Snake Could Mean Guam Will Lose More Than Its Birds. University of Washington.

Top photo: Photo © Isaac Chellman / University of Washington. Brown Tree Snake (Boiga irregularis).
Bottom photo: Photo © Haldre Rogers / University of Washington. Over 100 traps like this one were placed under or near trees to collect information about seed dispersal on Guam and Saipan.