Snake

The widespread impacts of invasive species

Guam island is an unincorporated territory of the U.S in the North Pacific Ocean. It is about 5,800 miles (9,300 km) west of San Francisco and 1,600 miles (2,600 km) east of Manila. Around mid of the 16th century, it was a territory claimed by Spain, and it remained part of it till the end of the 19th century, after which it became part of the U.S. On December 12, 1941, it was occupied by the Japanese, but on August 10, 1944, it was taken back by the U.S allied forces of world war II and used as a significant air and naval base for the squadrons of bombers that attacked Japan near the end of the war. The second world war ended In 1945, leaving many sad stories behind. One is the story of invasive brown tree snakes on Guam island. 

As stated by the U.S department of state, the brown tree snake Boiga irregularis was introduced on this island from its native home Papua New Guinea during the 1940s through U.S. military transports after World War II. Case Study: Brown Tree Snake (state.gov). As reported by the U.S govt, Boiga irregularis was first sighted on the island of Guam in the 1950s. What is the brown tree snake? | U.S. Geological Survey (usgs.gov). Since the native forest on Guam evolved without these predators, the jungle provided ample food for brown tree snakes. There was no natural predator of the snake, which allowed the snake population to grow without any checks. Currently, the population of these snakes on the island is more than two million, with densities of up to 5,000 individuals per square kilometer (or 13,000 per square mile). The large population of these snakes affected the Guam ecosystem directly and indirectly.

Direct effects of snake invasion on the island: The brown tree snakes feeds on rodents and lizards but prefer birds. The indigenous birds of Guam evolved without these predators and therefore lacked protective behavior. As a result, they become easy prey for the snakes. The intrusion of the snake in the mid-1940s caused the complete loss of ten of the 12 native forest bird species and functional extirpation of the remaining two species. The extinct birds from this island include a species of kingfisher, Guam flycatcher Myiagra freycinet, endemic to the island. It also eliminated many non-native bird species from the island. The snakes also affected the island’s budget. The snake brown tree snake crawls along the electric cables and often causes power outrage every four days. According to the reports of U.S. departments of state, power outrage due to these snakes costs 1 million USD annually. Case Study: Brown Tree Snake (state.gov). In 1990 Nonindigenous Aquatic Nuisance prevention and control act was directed to develop a program to control the snake on the island. Control of brown tree snakes includes fumigation of cargo with methyl bromide, and other measures were taken. It costs about 2.5 million USD annually.

Indirect effects of snake invasion on the island:  The direct impact of snake intrusion on the island is on its fauna. But the indirect impact is even more dangerous. The invasive snake damages the flora of the island. The frugivorous birds are the means to disperse plant seeds. A study shows that scattered seeds are far less than those beneath the parent canopy on Guam (without birds), whereas seeds were more broadly dispersed on other islands with birds. Removal of frugivorous birds reduces the seed dispersal distance. The seed dispersal also helps the seedlings to escape the natural enemies like herbivore insects, fungal pathogens, and the mammalian predators associated with the parent trees. The same study also suggests that the probability of germination of seeds passed through the gut of birds was two to four times higher than the seeds not ingested by the birds. All of them together cause a decline in the regeneration of the forest.

Conclusion: The invasion of brown tree snake Boiga irregularis on Guam island paints an unpropitious picture of the island. The loss of native bird species caused slow forest regeneration, negatively affecting carbon storage. The invasive predators cause a widespread impact on the flora, which is largely unrecognized. The mutualistic disruption of plants and animals due to invasion is likely operating globally and needs to be addressed.        

References:

1. Savidge, J. A. Extinction of an island forest avifauna by an introduced snake. Ecology 68, 660–668 (1987).

2. Wiles, G. J., Bart, J., Beck, Jr R. E. & Aguon, C. F. Impacts of the brown tree snake: patterns of decline and species persistence in Guam’s avifauna. Conserv. Biol. 17, 1350–1360 (2003).

3. Rogers, Haldre S., et al. “Effects of an invasive predator cascade to plants via mutualism disruption.” Nature communications 8.1 (2017): 1-8.

References:

1. Savidge, J. A. Extinction of an island forest avifauna by an introduced snake. Ecology 68, 660–668 (1987).

2. Wiles, G. J., Bart, J., Beck, Jr R. E. & Aguon, C. F. Impacts of the brown tree snake: patterns of decline and species persistence in Guam’s avifauna. Conserv. Biol. 17, 1350–1360 (2003).

3. Rogers, Haldre S., et al. “Effects of an invasive predator cascade to plants via mutualism disruption.” Nature communications 8.1 (2017): 1-8.

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