Extinctions and Loss of Species from Guam: Mammals
While a variety of mammals have been introduced on Guam, the only native mammalian inhabitants are of the Order Chiroptera (bats). Three species were found prior to 1968, although precipitous declines were noticed even at this time. Two of these species were fruit bats, the Marianas fruit bat, (Pteropus mariannus) and the little Marianas fruit bat (Pteropus tokudae). Both species were listed as endangered in 1984 by the U.S. Fish and Wildlife Service.
This declaration may have come too late for the smaller fruit bat species, P. tokudae. This bat was less common than P. mariannus and has not been sighted since one was collected in 1968. Endemic to Guam, this species is now believed to be extinct. Unfortunately, much remained to be learned about what was happening to Guam's wildlife at this time, and the brown Treesnake had not yet been identified as a causative agent. Consequently, little is known about the decline and disappearance of P. tokudae.
More information is available for the remaining species, P. mariannus. This species does persist on Guam, although its numbers have been drastically reduced. Considered a delicacy by the native Chamorro residents, these fruit bats were regularly hunted until the 1980s, and a few may continue to disappear at the hands of poachers. Tightened regulations have helped to curb poaching but can do nothing to stop the brown Treesnakes, which have been found to be effective predators particularly against juvenile bats.
It is estimated that there were several thousand fruit bats on Guam in 1958. This number dropped to about 50 individuals by 1978, rebounded to almost 1,000 by 1982, but has continued to fall since that time. The main problem, it seems, is that the young bats are unable to reach adulthood due to encounters with brown Treesnakes. Because there was a relative abundance of adult and baby bats in the remaining colony but a noticeable absence of juveniles, investigators wondered what was happening. The most likely explanation is that bats in the early juvenile stage are preyed on when they are dropped off in treetops close to the feeding sites. Left unprotected even for a short period of time, these young bats are easy prey for brown Treesnakes. Evidence of this, as cited by G. Wiles (see article) "includes one report of a snake discovered with three small fruit bats in its stomach and a baby bat found dead with possible snake saliva on its head."
P. mariannus continues to decline. As the adults in the colony age without successful recruitment of young, its future on Guam without advances in brown Treesnake control is questionable at best, despite periodic arrivals of bats from nearby Rota during or following major storms. Like the avian species, fruit bats are pollinators of some forest plants and natural dispersers of seeds, helping to maintain forest diversity and contribute to recovery after typhoons and other catastrophic events. It remains to be seen what the overall effect of this disappearance may be on Guam's forest and plant life.
The third species on Guam was the Pacific sheath-tailed bat, Embollonura semicaudata. This diminutive insectivorous bat was widespread in the central pacific region (Guam, Rota, and Aguijan with unverified sightings on Saipan, Anatahan, and Maug). It frequently roosted in large colonies in caves, abandoned buildings, and hollow cavities of trees and was active in twilight and nighttime hours. It was last recorded on Guam in the northern area in 1972 and is thought to be extirpated from Guam at present. Although its numbers may have been reduced due to damage to caves during World War II battles and as a result of heavy use of pesticides in post war years, its ultimate demise on Guam is likely to be related to the influence of the snake. As a species with individuals clustered in colonies, it may have been easy prey for snakes that could climb on the walls and ceilings of caves where the bats were resting. In its absence, and as other insectivorous birds and lizards disappear, insects on Guam are able to proliferate unchecked causing agricultural problems and allowing for outbreaks of insect-carried disease, such as dengue fever, which is carried by mosquitoes.