A Summary of the Current Demography of Guam's Native Lizards

While the extirpation of Guam's forest birds has been a prominent and highly publicized concern with the introduction of the brown Treesnake, the plight of Guam's native lizard species has been equally dramatic but less visible. Nevertheless, these species have suffered in the face of introduced predators including the brown Treesnake, the curious brown skink, and the musk shrew, as well as from competition with other introduced lizard species. The brown Treesnake in particular seems to have had a major impact, especially on the native geckos. Of the species that have suffered declines, the snake probably had a major role in the apparent extirpation of Perochirus ateles, the virtual disappearance of Gehyra oceanica, and the substantial reduction in the numbers of Gehyra mutilata. The snake may also have had a role in the disappearance of Cryptoblepharus poecilopleurus, Emoia slevini, and Nactus pelagicus, but little data exists to conclusively evaluate these losses.

Recent studies show the dramatic changes in the composition of lizard specimens found on Guam. This graph shows categorically how the numbers have been influenced.

To draw some possible areas of causation, these two graphs show: (1) how the Blue-tailed skink, E. caeruleocauda, has declined in collection while the Curious brown skink, C. fusca, has increased, and (2) how the overall composition of the gecko fauna has changed due to extreme predation by the brown Treesnake.

Changes in the proportion of skink specimens of the two common skinks on Guam (below).

Changes in the proportion of gecko specimens of the five common geckos on Guam (below).

A number of conclusions are possible after years of monitoring these lizard species. Overall, the impact of the brown Treesnake on Guam's lizards has varied. The native geckos have all declined relative to their populations on nearby snake-free islands and their former relation on Guam. Current surveys indicate that the one introduced gecko Hemidactylus frenatus has increased relative to other nocturnal lizards, but this does not mean that H. frenatus is more common. This species is less abundant than on the adjacent snake-free island of Rota, where it was also introduced. While evidence does not demonstrate that all native geckos on Guam have declined as a result of the snake's irruption, there does not seem to be any gecko that is more abundant on Guam as a result of the snake.

The impact of the snake on the diurnal lizards is more contradictory. Several skink species are no longer found on Guam, despite the difference between Guam and the adjacent snake-free islands in overall skink density and the relative increase in the proportion of skinks in collections. The recent changes in Guam's lizard-eating guild include the extirpation of the Micronesian kingfisher, which has probably reduced predation on all diurnal lizards, but may have intensified competition among skinks as a result (McLaughlin and Roughgarden, 1989). The following paragraphs evaluate the evidence of the brown Treesnake's effect on individual species.

Anolis carolinensis is a preferred food item for the brown Treesnake. The anole's habit of sleeping soundly in exposed places in the foliage predisposes it to predation by brown Treesnakes. While it has disappeared from many non-urban areas, it is common in urban settings. The anole will most likely persist on Guam in isolated patches and urban areas.

Carlia fusca has done very well on Guam. It is more conspicuous than Emoia caeruleocauda in open areas and may have had an adverse effect on E. cyanura. Higher densities of Carlia are difficult to conceive, but the brown Treesnake does prey heavily on the species based on the large number found in snake stomachs.

Cryptoblepharus poecilopleurus has never been recorded in substantial numbers on Guam, so it is not possible to make inferences about its population trends. The absence of any co-occurrences with brown Treesnakes in 68 islands of sympatry suggests that the snake may have had an effect on this species. The habitat preferences of the snake and lizard overlap; thus the absence of co-occurrences appears to not be due to their having divergent habitat preferences. Heatwole (1975) observed an apparent instance of competitive displacement of C. poecilopleurus by Emoia jakati on small islands. It is possible that this or similar interactions are partially or fully responsible for the absence of C. poecilopleurus from some islands inhabited by Boiga irregularis.

Emoia caeruleocauda is less common on Guam than on the nearby snake-free island of Rota, but this scarcity is probably due primarily to displacement by Carlia fusca. No definitive evidence exists to suggest that this species has declined as a result of the brown Treesnake, even though it is eaten by snakes.

Insufficient data exists to evaluate the population status of Emoia cyanura and Emoia slevini.

Gehyra mutilata is still locally common but has declined over large areas of the island. The only adverse predation change has been the irruption of the snake, which is the most likely cause of the decline of this gecko. The expansion of Hemidactylus frenatus may have also contributed to problems for G. mutilata.

Gehyra oceanica disappeared from Guam during the time when the brown Treesnake was irrupting. The snake is the most likely cause of its disappearance. This species' tendency to remain motionless when a predator approaches probably helped facilitate its demise on Guam. It is of a size suitable for food for almost all snakes and may have been an especially desirable meal for young adult snakes, which are probably too small to capture rats, Rattus, (the most abundant rodents on Guam).

Hemidactylus frenatus is the only gecko species that has increased in abundance on Guam since the arrival of the brown Treesnake. This lizard has two characteristics that may reduce its vulnerability to predation by brown Treesnakes. First, the gecko is normally found in houses on walls that are inaccessible to the snake. Second, this gecko is unusually adept at avoiding capture. They escape by sprinting downward or jumping off a branch when it vibrates. This is probably an effective tactic for avoiding the nocturnal movements of a brown Treesnake climbing through foliage. The recent spread of Hemidactylus to forest areas on Guam suggests that its urban distribution on other islands is influenced by competition or predation from other forest species. The lizard species most likely to be responsible is the larger carnivorous gecko Gehyra oceanica. Thus, the snake's apparent elimination of Gehyra oceanica may have offset the direct predation of the snake on Hemidactylus.

Lepidodactylus lugubris seems to have declined on Guam compared to the snake-free island of Rota, probably as a result of snake predation. However, it continues to thrive in houses, and its proportion of the total gecko fauna is unchanged.

Nactus pelagicus may have suffered from the irruptions of both the brown Treesnake and the musk shrew. The fact that Nactus is still relatively abundant on the shrew-free island of Rota and relatively rare or absent from the shrew-infested but snake-free islands of Tinian and Saipan suggests that the shrew may have had a greater impact on this gecko than the snake.

Perochirus ateles appears to have been extirpated from Guam, probably as a result of the brown Treesnake. This gecko is larger than all other species except G.oceanica and would be appropriately sized for most brown Treesnakes.

Varanus indicus (monitor lizards) has declined on Guam, though the explanation is not obvious. Anecdotal reports from many places in Australia (Covacevich and Archer, 1975) and Pacific Islands (M. McCoy, pers. comm.) associate the decline of Varanus with poisonings by the introduced toad, Bufo marinus. Although the monitor is eaten by brown Treesnakes, only a few have been found in necropsied brown Treesnakes, and biologists on Guam have observed predation by monitor lizards on the snake. It is also possible that the snakes prey on the eggs of Varanus and decrease the number of hatchlings.

A Summary of the Current Demography of Guam's Native Lizards

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Mammals

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