Guam has a spider population 40 times greater than its neighboring islands and an invasive snake population that has decimated bird life, leaving the forests eerily silent.
Five years ago, at a reunion party on Guam in the Western Pacific, approximately 2,492 km from the Philippines, Haldre Rogers, an associate professor in the Department of Fisheries and Wildlife Conservation at Virginia Tech, USA, who has studied Guam’s ecology for the past 22 years, witnessed a brown tree snake wrapping itself around a pig carcass and voraciously consuming it. This invasive snake species was introduced to Guam in the 1940s, likely after sneaking aboard a cargo ship. Within just four decades of the brown tree snake’s arrival, this gluttonous predator had wiped out the island’s native bird populations. Of the 12 bird species, 10 are now extinct, while the remaining two survive in inaccessible caves and urban areas.
Brown Tree Snake
Brown tree snakes preying on birds in Guam. (Photo: Science).
Today, the population of two million snakes on Guam will eat any prey they can find, including other snakes, shrews, lizards, or sometimes human leftovers. “They will eat anything,” said Henry Pollock, executive director of the Southern Plains Land Trust in Colorado. “They will even eat each other.”
Brown tree snakes are not typical predators. There are virtually no limits to what this carnivorous species is willing to swallow. They often consume prey that is 70% of their body weight, equivalent to a 60 kg person eating a small red kangaroo. However, the brown tree snake is not only voracious but also an extremely effective hunter.
With their agile climbing skills, they can locate prey even in the most hard-to-reach places. In 2021, a research team at the University of Colorado discovered a completely new climbing technique in brown tree snakes called “loop climbing.” According to them, brown tree snakes can coil around cylindrical objects, hook their tails around their heads, and sway as if climbing a coconut tree. To protect the remaining Såli birds from the brown tree snakes, conservationists have installed numerous nesting boxes and reinforced smooth metal poles that are 0.9 meters long and 15 cm wide, which the snakes cannot climb.
Over the past few decades, conservation specialists and wildlife authorities have employed every possible measure to eradicate brown tree snakes from Guam, but to no avail. These measures included searches, sprays, irritants, traps, poisons, and chemicals. Researchers have even explored the possibility of using viruses as biological weapons against the brown tree frog, intending to reduce their numbers significantly without affecting other wildlife. This method would function similarly to a fungal pathogen that was once widespread in France and Australia. However, despite numerous efforts and annual funding for control measures, authorities have been unable to eliminate the large snake population.
Spiders
Huntsman spiders the size of a hand on Guam. (Photo: BBC).
Across much of the Mariana Islands, there are relatively few spiders during the rainy season, with numbers soaring during the dry season, but Guam is different. The limestone forests on the island have spiders year-round, with webs stretching nearly continuously for kilometers. There are giant golden-bellied banana spiders, hand-sized huntsman spiders, and tent spiders filling every gap in the trees.
To accurately determine how many spiders have invaded Guam, Rogers and colleagues decided to conduct a cross-sectional survey in the island’s forests. The research team carefully navigated the rugged coral reef below while dragging a roll of tape in a straight line. They counted the number of webs along their path that still had spiders present. They found that during the rainy season, the spider population in Guam’s forests was 30 times greater than that of nearby islands Rota, Tinian, and Saipan. In the dry season, spider populations in the area typically soared, being 2.3 times greater than those in Guam.
Throughout the year, Guam’s forests are blanketed with spider webs. Researchers found 1.8 to 2.6 webs per meter in their cross-sectional survey during the rainy and dry seasons, respectively. When extrapolated across the entire forest area, the total number of spiders is estimated to be between 508 and 733 million, based on the assumption that each web has one spider.
The islands of Rota, Tinian, and Saipan do not have brown tree snakes and maintain healthy bird populations. The study results indicate that the spider population in Guam was relatively insignificant before exploding in the past few decades due to the absence of birds. This is partly due to the lack of insect competitors for the spiders. Similar studies in the Bahamas have shown that spider populations on islands are ten times higher in the absence of lizards. Since the brown tree snake’s arrival, the existence of banana spiders in Guam has become quite comfortable.
Many scientists believe that it is impossible to eliminate the large population of brown tree snakes in the forests of Guam. Currently, the brown tree snakes and the army of spiders they have created remain secure and may dominate for a long time to come.
