Beyond the Brown Tree Snake: Other Species Pose Similar Threats

posted in: March 1998 | 0

Hawai`i and virtually all other oceanic Pacific islands lack native terrestrial snakes. Consequently, the native birds lack adaptive behaviors to deal with these predators. The apparently inadvertent introduction of the brown tree snake (Boiga irregularis) into Guam well illustrates the effects alien snakes may have on native island ecosystems. Within 40 years of introduction, the brown tree snake had exterminated nine of Guam’s 12 native forest birds and approximately half the native lizard fauna and left the three surviving forest bird species and remaining fruit bat highly endangered. Huge reductions have also been observed in the populations of introduced birds, mammals, and lizards.

It is sometimes claimed or implied that the brown tree snake is somehow unique in its ability to wreak ecological devastation on island communities. But this argument derives from ignorance of snake ecology and the fact that the brown tree snake invasion of Guam is the only snake invasion to be well studied to date. In fact, several snake species have invaded other islands (or, in the case of peninsular Florida, areas ecologically similar to islands), and damage to native biotas has been documented or inferred in some instances.

There is every reason to be concerned with snake invasions; the brown tree snake may be only the vanguard of a potentially great ecological problem. There is nothing especially remarkable about the ecology of the brown tree snake. Its clutch size of 4-12 eggs is unexceptional and lower than that of many snakes. It apparently produces at most a single clutch per year in its native range, but may produce two per year in Guam. Hence, its intrinsic rate of increase is probably fairly low. It is not adapted to extremes of either temperature or humidity.

The most noteworthy features of the ecology of the species are its catholic diet of vertebrates, its arboreal proclivities, which allow it greater access to forest birds than most snakes would have, and its nocturnal habits. But these features are by no means unusual for snakes: many snakes are general vertebrate predators, many are arboreal, and many are nocturnal, especially in the tropics and subtropics.

Many arboreal snakes specialize on birds or feed on them opportunistically and could be expected to devastate Pacific avifaunas if they were to become established. Lastly, any snakes to become established on oceanic islands would be in environments largely free of predators and disease organisms, just as is the brown tree snake in Guam (the widely touted, terrestrial and diurnal mongoose would have no effect on nocturnal or arboreal snakes, nor on pit vipers, whose strike is faster than that of the mongoose). Hence, introduced snakes on most oceanic islands could be expected to lack significant sources of premature mortality.

A reasonable estimate is that several hundred of the world’s approximately 3,000 snake species could prove damaging to island avifaunas previously unexposed to snakes, although the major effects of many of these would be primarily on ground-dwelling birds. Several potentially invasive snake species are dangerously venomous and could be expected to have negative consequences for humans, too.

Snakes are likely to be introduced to islands in two ways. The first is by hitchhiking in cargo or on vessels used for transportation. This is how the brown tree snake is thought to have arrived on Guam (and other islands). The second is by deliberate introduction as pets followed by escape or intentional release. Most of the free-roaming snakes captured in Hawai’i each year are clearly in the latter category.

The number of snake species that would prove adept at hitchhiking is unknown but probably fairly small. Secretive and nocturnal species having high densities and with facultative or obligate parthenogenesis are likely to make the most successful hitchhikers. While the group of snakes meeting these specifications is relatively small, it has nevertheless furnished the most accomplished ophidian agents of ecological destruction so far.

In Hawai`i, the past three decades have seen a dramatic increase in the rate of pet reptile introduction, release, and establishment. Given the burgeoning number of species bred and available within the mainland pet trade, Hawai’i and other Pacific islands remain highly vulnerable to further introductions. Many snake species introduced for the purpose of furnishing pets may well prove just as great a threat to native avifaunas as has the brown tree snake, judging from their ecological attributes.

King snakes (Lampropeltis) are veritable generalists. Many boas, pythons, pit vipers, and rat snakes are arboreal and feed primarily or to a large extent on avian prey. All these taxa have clutch sizes of the same magnitude as brown tree snakes, or, in the case of the commonly kept boa (anaconda) and python species, are much larger (from 30 to more than 100 eggs). Several of these species can potentially produce two or more clutches per year when food is freely available, as it is in Hawai`i, where the environment is artificially enriched with an abundance of alien rodents, lizards, and birds.

Furthermore, some species are suspected to be facultatively parthenogenic [i.e., not requiring mates to reproduce], an attribute whose significance for colonizing oceanic islands should be obvious.

The only ecological parameter for which some of these common pet species cannot match brown tree snakes is elevational range. In its native New Guinea, brown tree snakes can live at elevations from sea level to 1,400 meters. Most commonly kept pythons and boas probably cannot live at such high altitudes, although many Elaphe, Lampropeltis, Pituophis, and pit vipers would have no trouble doing so, judging from their native latitudinal and elevational ranges. The significance of these considerations for Hawai`i, and perhaps other islands, is that most of the snakes captured and identified in Hawai`i are in the genera Boa, Python, Elaphe, and Pituophis. That these snakes have not elicited the same level of concern in Hawai`i that brown tree snakes have is remarkable and probable attributable to the general ignorance of snakes and their biology that prevails at any location in which they are naturally absent.

– Fred Kraus is a herpetologist with the Hawai`i Department of Land and Natural Resources. This is an excerpt (without citations) from Loope, L.L., F.G. Howarth, F. Kraus, and T.K. Pratt. “Newly emergent and future threats of alien species to Pacific landbirds and ecosystems,” draft submitted for publication in J.M. Scott et al. (eds.), Proceedings of Symposium on Threats to Pacific Landbirds and Endangered Ecosystems, Hilo, Hawai`i, May 1-2, 1997, Cooper Ornithological Society, Studies in Avian Biology. We thank the authors for permission to reprint portions of the article.

— Fred Kraus

Volume 8, Number 9 March 1998

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