The Scientific Bases for Preservation of the Mariana Crow (1997) / Chapter Skim
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3 Brown Tree Snake
3 Brown Tree Snake
Pages 39-52
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From page 39... ...
It grows up to 3.1 m long on Guam (2.3 m in many parts of its native range) , but the total length of most specimens encountered on Guam is less than 1.2 m.
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From page 40... ...
Active at night, it seeks refuge from high temperatures and bright light during daylight hours in the forest canopy, in dense vegetation, and even on the forest floor under logs and other structures, in limestone crevices, and in a multitude of sites made by humans, including buildings, vehicles, and shipping containers. The use of the latter daytime refuges places the brown tree snake in contact with cargo moving from Guam to other geographic areas.
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From page 41... ...
Whether snakes are able to assess prey availability on the basis of chemical cues or are merely foraging in habitats on the basis of proximal capture success and search images is unknown. Field studies in the brown tree snake's native range strongly suggest that hatchling and juvenile survivorship is low because of the lack of prey small enough for it (Rodda and others in press c)
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From page 42... ...
Within a decade after the species' arrival on Guam, large brown tree snakes were increasingly encountered by residents near the harbor and central Guam; by the 1960s, bird populations in the central part of the island began to disappear (Rodda and Fritts 1992; Savidge 1987)
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From page 43... ...
includes geographic patterns of bird and lizard losses concordant with population eruptions of the snake; predation of snakes on the vertebrates that declined, including lizards, birds, and mammals; lack of indications of substantial habitat loss, disease, pesticide contamination, or other causal factors; vulnerability of, and recorded impact on, both native and introduced species; unprecedentedly high populations of the snake coinciding with the disappearance of the avifauna; reductions in snake populations on northern Guam after the birds disappeared; and lack of comparable extirpations on nearby islands and islets where the snake is not present or conspicuous. Estimates of snake density suggest unprecedented numbers of snakes on Guam (Rodda and others 1992, in press a)
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From page 44... ...
An incident in which a relatively small snake was discovered leaving an aga nest with an egg in its mouth suggests that three-quarters of male and more than half of female snakes are large enough to consume crow eggs and that the predatory pressure is high on nests in situations where snake densities are 30–50 snakes/ha. The density of snakes might have fallen more in natural habitats than in disturbed ones close to development: It is these disturbed habitats that produce snakes most likely to enter the transportation network, interrupt electric transmissions, and attack humans.
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From page 45... ...
A few aga are also in captivity. Four bird taxa are presumed to have disappeared before the arrival of the brown tree snake; at least, their absence cannot be directly linked to snake predation (Engbring and Fritts 1988)
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From page 46... ...
Snake predation has probably reduced populations of at least half the introduced species; grassland and urban species remain more abundant than forest dwellers. Of the introduced birds, only the Philippine turtle dove, black francolin, and Eurasian tree sparrow can be judged common.
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From page 47... ...
and pruning canopies of adjacent trees and vegetation to prevent snakes from crossing into the nest trees via canopies. Initiating control only after identification of an active nesting attempt can lead to human disturbance of nest-building, egg-laying, and incubation; however, DAWR staff report that aga have renested in trees that were protected by barriers the previous breeding season suggesting that the human disturbance of emplacing the barriers might not cause abandonment of the nest.
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From page 48... ...
Source: Aguon and others, in press. maintenance of effectively snake-free plots; about 5% of the number of snakes recorded in comparable habitats without controls were recorded in plots during the experiment, and a substantial increase in lizard abundance was evident in snake exclosures relative to that in plots with snakes.
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From page 49... ...
probed the critical elements of controlling snakes in small plots. Recent results suggest that snake exclosures (1–50 ha)
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From page 50... ...
THREAT OF SPREAD OF THE BROWN TREE SNAKE TO OTHER ISLANDS In addition to Guam, the brown tree snake has been reported on 10 other islands -- including islands in the Northern Marianas, the Federated States of Micronesia, the Ryukyu Islands, Diego Garcia Atoll, the state of Hawaii, and Wake Island -- as a result of dispersal from Guam (Fritts and others in press)
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From page 51... ...
The incidence of brown tree snakes in cargo from Guam is also a result of the extremely high snake populations on Guam and the snakes' tolerance for disturbed habitats, including commercial, residential, and industrial sites from which cargo is shipped. The brown tree snake avoids high temperatures and bright sunlight by seeking daytime refugia.
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From page 52... ...
Snake control focused on transportation from Guam is not complete: problems include limited ADC personnel on Guam, relying on voluntary compliance with ADC snake searches, periodic ship departures with no advance notice, and the complex dynamics of moving commercial, private, and military cargo in a heavily used port and airport system.
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