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The Disappearance of 's Wildlife

Gordon H. Rodda; Thomas H. Fritts; David Chiszar

BioScience, Vol. 47, No. 9. (Oct., 1997), pp. 565-574.

Stable URL: http://links.jstor.org/sici?sici=0006-3568%28199710%2947%3A9%3C565%3ATDOGW%3E2.0.CO%3B2-V

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http://www.jstor.org Thu Feb 21 10:55:57 2008 The Disappearance of Guam's Wildlife New insights for herpetology, evolutionary ecology, and conservation

Gordon H. Rodda, Thomas H. Fritts, and David Chiszar

uam is an American island - - some forests were leveled by artillery near the middle of Micro- The Guam experience fire and more than 80% of the island's Gnesia, an archipelago of "mi- structures were destroyed (Morison cro" islands in the middle of the showed ecologists that 1953). After the war, the island was western Pacific . The largest extensively reseeded with an exotic island of , Guam covers snakes can attain legume, Leucaena leucocephala, only 541 km2. It is shaped like an which permanently replaced native elongated peanut, 4 km across the densities that are trees over vast areas (Craig 1994). narrow waist and 45 km long. The As far as is known, no native bird closest larger island is Manus, which sufficient to suppress s~eciesor other vertebrates were lost is 1740 km to the south, across the as a result of this ecological upheaval equator and north of . prey populations (Baker 1946).The ecological effects Few species are found on small, re- of the war may have been somewhat mote islands such as Guam. To reach delicate flyers, such as mosquitoes, analogous to the that natu- a land mass with levels of fail to make the journey without rally strike Guam every few years. In comparable to what is found on con- human assistance. Indeed. Micro- 1992, for example, Guam was hit by tinents, one must travel over 2000 nesia was free of flies and mosqui- six typhoons, three of which were km south to New Guinea, west to the toes until Spanish "super-typhoons" with sustained , or north to (Fig- brought these insects to Guam winds in excess of 240 kdh. The ure 1).Neither water currents nor (Carano and Sanchez 1964). fierce storms of the western Pacific wind brings animals from those di- It is difficult to know how many denude forests and thereby select for rections, so Guam's native verte- species of animals were found on species that can tolerate severe natu- brates are limited to those species Guam before the arrival of humans, ral habitat modifications. that can fly (e.g., birds and bats), or but from archeological excavations Major changes in Guam's verte- whose eggs can ride for many weeks on the nearby island of , scien- brate fauna became evident in the on small clumps of floating vegeta- tists know that the original human 1960s, when wildlife authorities no- tion (e.g., small lizards). Large colonists-or the domestic animals ticed that birds were entirelv absent nonvolant vertebrates and even small that the colonists brought with from the southern one-third of the them-extirpated many species thou- island and that the boundary of Gordon H. Rodda is a zoologist, and sands of years ago. On Rota, as on birdlessness seemed to be creeping Thomas H. Fritts is chief of the Biological , approximately half of the steadily northward. By the end of the Survey Program, at the Patuxent Wildlife native birds were exterminated by , birds were missing from the Research Center, Biological Resources Di- prehistoric humans (Steadman southern two-thirds of the island vision, Geological Survey, 1995);presumably, Guam lost a simi- (Engbring 1983, Jenkins 1983). By National Museum of Natural History, lar number. But following that spate 1985, most of the bird species were , DC 20560. They study tropi- of prehistoric extinctions, the native either isolated in small pockets at the cal reptiles and the effects of species trans- wildlife community remained rela- northern tip of the island or were locations. David Chiszar is a professor in the Department of Psychology, tively stable until the 1960s. Surpris- completely gone (Figure 2). of , Boulder, CO 80309-0345. ingly, the island's fauna was little What was killing the birds? Two His interests revolve around the behavior disrupted by the savage fighting of theories generated particular inter- of reptiles, especially the feeding adapta- World War I1 (Engbring and Pratt est. One was that pesticides, which tions of snakes. O 1997 American Insti- 1985), which subjected Guam to had been used in large amounts after tute of Biological Sciences. naval bombardment so severe that World War I1 to control mosquitoes,

October 1997 565 The snake turned out to be respon- sible for not only the extinctions of the birds, but also the decimation of the island's lizards (Figure 4; Rodda and Fritts 1992b), mammals (Wiles 1987), and domestic animals (Fritts and McCoid 1991). When Savidge reported her discovery at a meeting of the American Ornithologists' Union in 1983, she met with skepti- cism from some members of the au- - GUAM .-. . dience. Marshall (1985) noted that "Few could believe that a mere snake was that efficient a predator and could build up the numbers com- mensurate with such devastation" (p. 260). One commentator responded to Savidge's conclusion by devoting a column to promoting the pesticide . hypothesis (Diamond 1984). .. The fact that Savidge's conclusion I was initially rejected but is now \ ** widely accepted, reflects the growth of ecology. How do we know that the snake caused the extirpations? Six lines of evidence point to the as the primary cause of Guam's avifauna extinctions CJ (Savidge 1987): the geographic pat- B tern of bird losses mirrored the si- multaneous population expansion of Figure 1. Guam is the largest of the more than 2100 tiny islands of Micronesia (most the snake (that is, the snake spread are too small to be seen in this view). Lying north of the equator near the middle of the northward across Guam on approxi- western , Guam is roughly equidistant (over 2000 km) from Japan, the mately the same schedule as bird Philippines, and New Guinea. distributions retreated to Guam's north end); the snake is an efficient had poisoned the birds, as DDT had cally through the bird populations, predator of the species that declined; done to some birds in mainland North perhaps carried by the introduced there is little or no evidence for alter- America. The second, and leading, mosquitoes. An avian pathologist, Julie nate causes of declines, such as pes- hypothesis was that an introduced bird Savidge, was hired by Guam's Divi- ticides, habitat destruction, diseases, disease, such as had ravaged the birds sion of Aquatic and Wildlife Resources or environmental contaminants; all of Hawaii (van Riper et al. 1986, War- to find the disease that was killing the bird species were affected, including ner 1968), had spread catastrophi- birds of Guam. In collaboration with both native and introduced species other pathologists (thus, the natives did not retreat in and pesticide spe- response to expansion of introduced cialists, she scoured species); the brown tree snake is un- the forests and birds expectedly common on Guam; and of Guam for evi- no comparable bird extirpations were dence of this dis- observed on similar nearby islands ease, but she came that lacked the snake. up empty-handed For many of the skeptics, the evi- (Grue 1985, Sav- dence that the brown tree snake is idge 1987, Savidge unexpectedly common on Guam was et al. 1992). She the clincher. Counting snakes is concluded instead tough. They are notoriously difficult that the acciden- to spot (Rodda 1993),and the brown tally introduced tree snake is particularly hard to find brown tree snake, because it is nocturnal and arboreal. irregularis, It moves slowly through the foliage Figure 2. These fairy terns are among the 12 bird species (both Was responsible at night, looking and often acting land and sea birds) that have disappeared from Guam in the for the loss of the like a drab vine. Few visitors to Guam wake of the introduction of the brown tree snake. birds (Figure 3). ever see a snake. However, our mark-

BioScience Vol. 47 No. 9 recapture and trapping By October 1996, only 3 studies suggest that the of Guam's 12 native forest snake achieved peak densi- bird species still survived in ties on Guam of approxi- the wild. Of these, the Mari- mately 1001ha (Rodda et al. ana crow, Corvus kubaryi, 1992a). By contrast, large whose population is declin- snakes away from water or ing rapidly, appears doomed dens have maximum densi- (the 1996 count was five ties of 1-lOlha (Parker and individuals); the island Plummer 1987), and such swiftlet, Aerodramus vani- snakes rarely attain densities korensis, is relatively safe in excess of a few individuals (several hundred individu- per hectare. For example, als are in one unstable bullsnakes (Pituophis mela- colony); and the Micrones- noleucus; Parker and Brown ian starling, Aplonis opaca, 19801, rat snakes (Eluphe to New Guinea and nearby areas; it was accidentally intro- is precarious (one probably obsoleta; Stickel et duced to Guam after World War 11, with catastrophic viablepopulationof~O-lOO 1980), and rattlesnakes results for native wildlife. individuals is in one urban ( Crotalus horridus; Fitch ). It is, perhaps, telling 1982) all have densities of that in Guam a metapopula- less than llha. Small snakes, tion of several hundred birds such as subterranean worm- is considered "safe." Unlike eating snakes, can reach very Hawaii, where low-eleva- high densities (e.g., Carpho- tion forests are densely phis amoenus, worm snake, populated with non-native has been recorded at densi- bird species, most forests of ties of up to 729ha; Clark Guam are now empty of 1970), but at 0.3 m, this avian life. The silence is con- snake is much smaller than spicuous even to a casual the brown tree snake, whose observer (Jaffe 1994). maximum total length is ap- Although Guam's bird proximately 3.1 m. Small extirpations have received aquatic snakes, such as the most attention, many Regina alleni, the striped Iother species are important crayfish snake, whose maxi- Figure 4. The spotted-belly gecko (Perochirusateles) is found components of the brown tree mum length is 0.6 m, can only in Micronesia and on Japan's tiny Marcus Island (Minami- Tori-shima). It has disappeared from Guam, where it was snake's diet. Juvenile snakes reach of 12901ha common in forests before the arrival of the brown tree snake. eat lizards primarily, and (Godley 1980), but only in adult snakes eagerly ingest small water bodies. Snakes small mammals (Greene also reach high concentrations approximately 261ha. Thus, at the 1989, Savidge 1988, Shine 1991). Al- around dens, but on a tropical is- crest of the initial snake population though the disappearance of silent land, such as Guam, no wintering irruption, the predator would have nocturnal mammals and minute liz- behavior is seen. Thus, in relation to outnumbered potential avian prey ards was not as obvious as the disap- the densities expected of a compa- by approximately 4:1, and the peak pearance of noisy, colorful birds, the rable snake, the peak densities of avian biomass of approximately 0.8 loss of biodiversity was nearly as brown tree snakes on Guam were kgtha would have been only 15%- complete; only three of Guam's liz- unprecedented. 25% of the peak predator biomass, a ard and mammal species have stable The density of this predator was precarious predator-to-prey ratio. populations. Moreover, two of the also excessive in relation to the den- For comparison, a garter snake three native bat species vanished in sity of the prey. Prey densities on predator-prey system in was recent decades, leaving only the Guam prior to the arrival of the found to have a peak biomass ratio Marianas fruit bat, marian- snake were not known with any ac- of 1:67, much lower than the 1:4 nus. Compared with the bat popula- curacy, but an estimate of the upper ratio that is theoretically sustainable tions on nearby islands that do not limit for numbers of bird individuals (Reichenbach and Dalrymple 1986). have snake populations, nonvolant can be calculated by adding the maxi- With the even higher 4:l predator- juvenile bats on Guam suffer near mum densities known for each of the prey ratio found temporarily in 100% mortality at the age when they species (Engbring and Ramsey 1984). Guam, it is not surprising that such are first cached by their foraging This calculation makes the unrealis- predation pressure caused bird abun- mothers (Wiles 1987).An unattended tic assumption that all species might dances to plummet. The Guam expe- nonvolant bat would be highly vul- have been at their maximum density rience showed ecologists that snakes nerable to brown tree snake preda- in the same place, but even so, the can attain densities that are suffi- tion, and the elevated mortality of aggregate total bird density is only cient to suppress prey populations. nonvolant young is presumed to be

October 1997 567 the cause of its continued example, one does not ex- population decline. For the pect to hear of a sudden other two bat species, no surplus of garter snakes population data exist to in- causing a localized short- dicate the cause of their de- age of frogs. The increase clines on Guam. in snake numbers, if it hap- Of Guam's 12historically pens at all in response to an native lizards, only one spe- irruption of prey, is likely to cies appears to be as dense be long term and demo- on Guam as on nearby snake- graphically minor. A good free islands; six have been year for rattlers is generally extirpated from Guam (Fig- the result of a bumper crop ure 5), three are rare and of rodents, rather than the localized (Figure 6), and two cause of a shortage of ro- are common but reduced in dents. Accordingly, snake numbers are typically regu- abundance (Rodda and Frim (an American chain of islands within 1992b). The interpretation of Micronesia). Guam was the largest of the islands inhabited by lated by "bottom-up" rather these population changes is this lizard, but it is no longer found there, possibly due to F~~~ trO~hic in- predation by the introduced brown tree snake. interactions. troductions of lizard and By contrast, extinction mammalian predators and of prey populations by a competitors, but it is likely predator is a decidedly - that the snake caused the de- down proposition. What clines of several lizard species enabled the brown tree (Rodda and Fritts 1992b).All snake to have such an im- native lizard species persist pact on so many of Guam's on Guam's tiny offshore is- native vertebrates? On one lets, which the snake has level, it is not necessary for not yet reached. a snake to be an exceptional The details of each extinc- predator to exert top-down tion may be open to debate, pressure, it is only neces- but the aggregate impact is sary that the snake be an unquestionably an astonish- exception to the generality ing loss of biodiversity. Of that snakes are not abun- the native vertebrates, only dant. On Guam, the brown one bird and three lizard tree snake was abundant. species retain long-term vi- Figure 6. The oceanic gecko (Gehyra oceanica) is one of Before returning to the ques- ability on Guam. several large geckos that vanished from Guam forests follow- tion of why the brown tree The profound effects of ing colonization and irruption of the brown tree snake. This snake became so numerous, Guam's snakes stand in gecko is still occasionally found on ornamental plants in we describe how the snake's stark contrast to the earlier Cham's urban areas. Apparently, the arboreal snake rarely high population density generalization that snake visits these isolated plants. turned it from an annoy- populations would be of ance into a significant prob- little ecological consequence to prey be small. Indeed, there is no evidence lem for the human population. populations. That generalization had that snakes routinely reduce prey been based on the relative rarity of populations except on small islands The strange case of Guam's most snake species and on the fact such as Guam. baby bites that their feeding is seasonal and The fact that snakes normally have opportunistic. That is, snakes can little effect on prey populations may The brown tree snake is a member of eat when prey are abundant and fast be related to another characteristic the snake family Colubridae, most when prey are scarce (Pough 1980). that most snakes share: low repro- of which lack the sophisticated This feeding strategy crops the ductive potential. Snakes have far venom apparatus and highly toxic "doomed surplus" (Errington 1956), fewer offspring than rodents, frogs, venom of the "truly" venomous co- rather than controlling or depress- or other common prey. Snake popu- bras and vipers. Many of the natricine ing baseline prey populations. To lations may increase following an colubrids (thegarter snakes and their discourage the wanton killing of irruption of prey, but the subsequent allies) have saliva that can be irritat- snakes, conservationists often advise buildup of the predator is limited ing (McKinstry 1983, Minton 1979), farmers to allow snakes to live for and slow. In response to a prey popu- but the irritation is usually mild. No the purpose of rodent control. But if lation increase, snake populations herpetologist would hesitate to snake populations are incapable of rarely increase so much that they handle a garter snake. Thus, little controlling the abundances of their overtax the food supply and ulti- credibility was accorded to the early prey, their benefit to the farmer will mately depress prey abundances. For reports of serious snakebites on

568 BioScience Vol. 47 No. 9 Guam. Still, the reports kept com- strong visual stimulus, but a con- ing. There have been more than 160 cealed bird egg offers few visual cues; such cases, or approximately 1 in thus, olfaction probably plays a ma- 1200 emergency room visits on jor role in helping brown tree snakes Guam, including nine cases of in- to locate eggs. fants who received ventilation or Similarly, actively foraging garter intubation to assist breathing (Fritts snakes, which often search for im- et al. 1990, 1994). mobile or slow-moving prey, rely What was shocking about these heavilv on chemosensation to iden- reports was not the seriousness of tify prey. However, the brown tree the bites but rather the circumstances snake's foraging decisions appear to of the encounters. Whereas the modal be more sophisticated. Unlike garter snakebite victim in is snakes. which will often attack and a young adult suffused with a dan- attemdt to swallow an inappropriate gerous combination of testosterone object (e.g., a cotton swab) that has and alcohol (Minton and Minton been soaked with an appropriate 1980), the typical snakebite victim odor, brown tree snakes may ignore in Guam was a sleeping infant of less food cues that appear out of context. than six months of age. In a few For example, Chiszar et al. (1988b) cases, the snake appeared to select found that brown tree snakes ignore the young infant in preference to isolated odors of mice but will attack larger children: in 2 of 11 medically olfactory cues in more realistic set- serious bites, the victim was an in- tings (Chiszar et al. 1992,1993). In fant sleeping between its parents or Figure 7. Although most of Guam's brown some situations, brown tree snakes older siblings who were not bitten. tree snakes are only approximately 1m in will attack mice that they see but do Even if one includes teenage and length, the species reaches a maximum not smell (i.e., mice temporarily adult snakebite victims. 80% of all size of more than 3 m, enabling it to kill or placed in airtight but transparent reported bite victims were sleeping seriously harm medium-sized birds and boxes). They will also investigate an at home, not active or sleeping out- mammals, such as chickens, puppies, and odor-infused opaque , but they side of their homes. In a small per- small children. will not expend comparable effort centage of bites to sleeping persons on a similar but transparent box (7%), the victim was being con- Foraging by brown tree snakes (Lankford 1989).Apparently, the vi- stricted by the snake when discov- sually detectable absence of prey is ered. ~lthou~hconstriction may not Guam's infant envenomations and sufficient to redirect prey-seeking be- have changed the medical conse- wildlife extinctions have, not sur- havior. Similarly, in some captive quences of any bite cases, the occur- prisingly, evoked countermeasures studies (Chiszar et al. 1988a), air- rence of constriction is important by wildlife managers (Fritts 1988). borne olfactory cues have been in- because it suggests snake feeding One of the most successful measures sufficient to elicit predation, but sub- behavior rather than self-defense. has been to trap the snakes (Fritts et strate-borne chemical cues have Many bite victims also exhibited al. 1989, Rodda and Fritts 1992a). induced brown tree snakes to follow multiple bites, as if the snake were Perhaps due to their broad diet, odor trails to hidden prey. By con- repeatedly regripping the victim in brown tree snakes readily enter traps trast, Fritts et al. (1989) and Rodda an attempt to ingest prey that is far that are baited with prey items, typi- et al. (1992b)found that wild brown too large. Although inferring moti- cally laboratory mice (the mice are tree snakes were attracted to free- vation is always risky, this pattern of protected from the snakes and are hanging traps that were baited with snakebites more closely resembles not harmed by the experience). chicken litter but that adding olfac- that of predatory strikes than that In the process of perfecting the tory trails leading to the trap bait did associated with defensive behavior. trap design, many experiments have not enhance trap capture success. Apparently, the brown tree snake been performed to elucidate the Thus, brown tree snakes appear to stumbles into the corridors of homes searching algorithm that is used by be facultative in their use of chemi- at night, willing-although presum- foraging brown tree snakes (Rodda cal, visual, and other types of infor- ably not seeking-to bite exposed et al. 1992b), which turns out to be mation. The factors that determine infants (Figure 7). This phenomenon, different from that used by most the snakes' reliance on their various even if it involves only a less venom- other snakes. For instance, whereas sensory modalities are still not un- ous rear-fanged snake, has added an most rattlesnakes hunt by ambush derstood. entirely new perspective to snake- and garter snakes typically forage Wild brown tree snakes also do bite. Moreover, this snake's appar- actively, brown tree snakes do both not respond to pungent mammal ent willingness to enter occupied (Rodda 1992). Consequently, the baits, even though in the laboratory buildings may help explain how it brown tree snake is likely to use they respond to the same cues with reached Guam; it could have acci- different sensory modalities for cap- high rates of tongue flicking. Brown dentally been transported with turing active and inactive prey. For tree snakes seem to be adept at dis- shipped goods. example, a fleeing lizard presents a tinguishing between live prey and all

October 1997 569 artificial prey; we have yet to find an maggot-infested rabbits, paper tow- terials and loaded them on barges artificial cue that, in nature, elicits els, spareribs, rotting lizards, orna- for recycling or disposal on Ameri- more than approximately 5% of the mental betel nuts, larger conspecif- can soil (i.e., Guam). Once on Guam, captures obtained with real prey, ics, dog placentas, and soiled the material would have been un- although in laboratory tests some feminine hygiene products. Curi- loaded during the day, and the snake prey extracts are indistinguishable ously, many of these items do not would, naturally, have remained con- from live prey. Consequently, dis- have the visual appearance of a tra- cealed until nightfall, when its dis- embodied odors are of minimal use ditional prey item. Moreover, some persal into the jungle would have for trapping brown tree snakes, even of these items, such as betel nuts, do gone undetected. though it is clear from laboratory not have the odor, color, tempera- Since reaching Guam, the brown work that the snakes are aware of ture, vibration, or behavior of a tra- tree snake has gained access to other these odors and investigate them. ditional live food item (although the previously snake-free islands. In the Our working hypothesis is that nuts do resemble eggs in shape). Per- last six vears. more than 40 snakes brown tree snakes use multiple cues haps the snake's habit of switching have been spotted on the previously and avoid traps that do not provide between active and passive foraging snake-free island of , approxi- multi-modality cues that confirm the modes has preadapted it to a wider, matelv 175 km north of Guam. We presence of living prey. more sophisticated definition of suit- know of seven occasions in which Given this sophisticated algorithm able prey or to facultative reliance the brown tree snake has been acci- for identifying prey, it seems remark- on sensory systems that are more or dentally transported the 6100 km able that brown tree snakes would less obligate in other, less flexible from Guam to Hawaii. Other indi- mistake hopelessly large sleeping in- species. Undoubtedly, the brown tree viduals have been reported from sites fants for potential prey items. How- snake's liberal attitudes about pro- such as Atoll (Indian ever, in laboratory trials, brown tree spective food items has allowed it to Ocean); Corpus Christi, ; and snakes routinely kill and attempt to successfully colonize new habitats, . eat rodents that are well beyond their including Guam. Thus, brown tree snakes seem to gape limits. It is also possible that experience no difficulty in reaching when searching visually, brown tree Snakes as colonists new locations. But a single stow- snakes fail to comprehend that a away snake is unlikely to lead to a portion of the infant is concealed by Brown tree snakes occur naturallv in new brown tree snake population, bedding. For whatever reason, brown eastern , New Guinea, the unless it happens to be a gravid fe- tree snake herpetoculturalists have , and the north and male. One feature promoting suc- consistently found that this species east coasts of . As soon as cessful colonization in many snake has poor judgment with regard to sailing ships began to ply the seven species is the ability of females to the size of potential prey. Brown tree seas, rats (first Rattus rattus and store sperm. Although the brown snakes will attack prey that are too later Rattus norvegicus) began ap- tree snake has never been tested for large for them to swallow; perhaps pearing throughout the world, on this ability, other species of the same some of their attacks on children virtually every island contacted by genus are capable of storing sperm reflect this error. the ships (Atkinson 1985). By con- for at least two years (Groves 1973), Nevertheless, brown tree snakes trast, snakes are not generally con- and several closelv related snakes are capable of prodigious meals sidered to be good colonists. With store sperm for at least six years (Chiszar 1990).A snake's maximum the exception of the widespread (Haines 1940). Thus, it is theoreti- meal size depends on its taxon: Vi- parthenogenetic blindsnake Ram- cally possible for a single snake to pers and other heavy-bodied venom- photyphlops braminus (every indi- start a population, even if she was ous and nonvenomous snakes are in vidual is a female, and each is ca- not gravid at the time of accidental one class, whereas the more slender pable of starting a population), few transport. Scientists do not know species, including the brown tree snakes have colonized remote islands. how many females were responsible snake, are in another (Pough and Are brown tree snakes uniquely ca- for the Guam population, but it was Groves 1983).However, brown tree pable colonists, or is it a myth that probably a small number because snakes stand apart from their class in snakes are poor colonists? most stowaways probably died in the size of meals ingested; we have Brown tree snakes are believed to transit or failed to find mates in the found brown tree snakes in the wild have been transported accidentally new environment. with prey equal to more than 70% of to Guam through the postwar sal- If the initial colonizing popula- their mass. This is without precedent vage of derelict vehicles and equip- tion was small, then presumably there in the nonviper group. ment that were deposited in the New were special circumstances on Guam Another unexpected feature of Guinea area during World War I1 that made it possible for a small brown tree snake foraging is that the (Rodda et al. 1992a). Manv of the population to irrupt into an unprec- snakes readily consume carrion and salvaged items are likely ;o have edented infestation. Many explana- organic matter, which are not con- housed snakes, including brown tree tions have been suggested for the sidered typical snake food. For ex- snakes. Being nocturnal; brown tree extraordinary irruption, and the con- ample, brown tree snakes have been snakes would be quiescent and un- sequent exceptional impact, of brown found eating or having eaten dog detected during the day, when sal- tree snakes on Guam (Pimm 1987, food, chicken bones, raw hamburger, vagers would have collected the ma- Savidge 1987). Of the reasons that

BioScience Vol. 47 No. 9 have been suggested, we are most sampling, snakes had been eliminated Is the brown tree impressed by the importance of co- from two of the sites, and lizard snake unique? evolution between predator and prey. densities in these sites averaged The overwhelming predominance of 19,6501ha (52.3 kglha). In the two Our understanding of possible snake islands in the record of anthropo- areas still occupied by snakes, the behaviors and ecological interactions genic extinctions (Brockie et al. 1988) average lizard density was lower has been broadened by the study of is consistent with a heightened like- (13,2901ha; 33.7 kglha) but still the brown tree snake. However, it is lihood of predator irruption and prey higher than in comparable mainland important not to generalize too much extinction when predator and prey areas. For example, Duellman (1987) from a single example. Is this species lack a shared evolutionary history. found an average of 57 lizardslha exceptional? Or has our understand- An anecdote from Guam illustrates (1 kglha) in the lowland tropical ing simply been limited by the pau- this point. In the process of search- forests of Amazonico, Peru. city of opportunities to study tropi- ing for the hypothesized disease that Comparable data for the density of cal snakes, non-North American was eradicating Guam's birds, either rats or lizards are not avail- snakes. or nocturnal arboreal snakes? Savidge (1987) housed a flock of able for the brown tree snake's na- Does the brown tree snake have at- bridled white-eyes (Zosterops c. tive range, but our relative counts tributes that make it different from conspicillatus) in a laboratory avi- indicate that nocturnal lizards are most other snakes? ary. While sleeping, these birds roost approximately five times as abun- Typical pest species often have in aggregation. One night, Savidge dant, and diurnal lizards approxi- high reproductive rates; however, the discovered that a brown tree snake mately four times as abundant, on brown tree snake does not. In recent had found a way into the aviary and, Guam as in the brown tree snake's vears. the modal size of detected by the time it was discovered, had native range. The abundance of liz- Llutches on Guam has been 3-4 eggs. consumed three of six white-eyes ards on Guam is not unique to Guam Thus, it is not surprising that it took sleeping side by side on a branch. but has been reported for many is- manv decades for brown tree snakes The surviving three remained in place lands. Thus, the success of the brown to build dense populations through- on the branch near the snout of the tree snake on Guam may be due as out the island of Guam. Unlike the advancing snake. Unlike birds in much to the unique characteristics of irruption of the zebra mussel, which other locations, bridled white-eyes island environments as to the unique s~readover most of the North Ameri- on Guam appear not to have evolved attributes of the snake. can in a few years (Benson the behavior of waking or flying when One unique feature of all modern and Boydstun 1995),the biodiversity a neighboring bird is eaten. Had environments is the un~recedented crisis on Guam moved relativelv Savidge not intervened, the birds' level of human commerce. Guam slowly. However, as is evident from lack of coevolutionary experience imports virtually all its food, build- the history of the human species, with this predator would likely have ing materials, and other goods. Al- even taxa with low reproductive rates cost all six their lives. most all of this material comes from can eventuallv overshoot local car- With their generalist feeding hab- localities with snakes and other po- rying capacities, causing the extinc- its, brown tree snakes were pre- tentially damaging exotic species. For tions of vulnerable prey. adapted to find suitable forage on example, a number of snakes, prob- The above observations suggest Guam, where prey density was ex- ably from mainland United States, re- that in comparison to all other po- traordinarily high. Although prey cently arrived on Guam in a shipment tential pests, brown tree snakes have densitv on Guam was not measured of Christmas trees. Similar introduc- relatively low fecundity, but how do at the'time when the snake arrived tions are apparent on other islands. they compare with other snake spe- (i.e., 1950 or so),measurements from Okinawa, for example, has recently cies? Although natricines and 1993 to 1995 indicate that Guam been colonized by cobras that have crotalines (rattlesnakes) have some- continues to have higher mammal escaped from roadside attractions, and what larger average litters than and lizard densities than are found Hawaiian customs authorities have brown tree snakes (Seigel and Ford on comparable tropical mainland ar- intercepted an inbound snake once 1987), neither these taxa nor any eas. For example, we recently re- every two weeks, on average, in recent other snakes can be described as moved (in a span of days) an average years.' Island economies are unusu- highly fecund. Yet the brown tree of 55 ratslha from a forested area of ally dependent on imports, but most snake case illustrates that an organ- northern Guam. Comparable main- industrial communities also obtain the ism need not be highly fecund to be land forests have population densi- majority of their goods from elsewhere. a successful or potential pest. ties in the range of 1.5-19lha (0.8-6 Thus, although snakes may not be Thus, in terms 'of reproductive out- kglha) for all rodent species com- particularly good colonists under put, the brown tree snake is not bined (Fleming 1975). natural conditions, present condi- unique; many other species of snakes To measure the absolute densities tions provide an extraordinary num- are more fecund and therefore have of Guam's lizards, we placed lizard- ber of opportunities for accidental the reproductive potential to become proof fencing around four 10 x 10 m translocation and colonization. colonists or pests under appropriate patches of forest and counted all (i.e., undesirable) circumstances. lizards that we encountered as the 'L. Nakahara, 1992, personal communica- The brown tree snake is well vegetation within each patch was tion. Hawaii Department of Agriculture, known for its willingness to eat a removed. A year prior to the lizard , HI. diversity of foods. Are other snakes

October 1997 571 Figure 8. This snake snakes, herpetologists should strive to was attempting to eat study a wider diversity of species and a baby pigeon out of a clades, particularly tropical species. nest on a power pole For conservation biologists, the in Guam in 1988 when the weight of inference is that snakes can cause the struggling bird biodiversity crises in a wide variety caused the snake and of contexts. Prey species on islands its meal to sag enough seem to be especially vulnerable, but to contact another many prey species on also electrical conductor. lack coevolutionary experience with The resulting surge of nocturnal arboreal snake predators. electricity killed the If not B. irregularis, the culprit could snake and bird in- 1 be Boiga trigonata (the gamma cat stantly. It also caused snake, a native of ), Tri- in an islandwide power outage that deprived 125,000 Guamanians of electricity for morphodon biscutatus (the lyre approximately 8 hours. Most of the 1500 power outages caused by the snakes have affected smaller portions of the island. snake, a native of North America), or the deadly Trimeresurus flavo- viridis (the habu, a native of Japan). precluded from becoming pestifer- brown tree snake's willingness to These three snakes are, like the brown ous by the specificity of their dietary live alongside people (Fritts 1993). tree snake, venomous, nocturnal, and requirements? Greene (1989) noted The brown tree snake's tolerance of at least partially arboreal-but so explicitly that the broad diet of the the ecological disturbance and hu- are hundreds of other species. Would brown tree snake is widely shared by man environments on Manus after the invasion of the Galipagos Islands its approximately 30 congeners. World War I1 contributed to its ar- by a generalized predatory snake that Many other snakes, including many rival to Guam as a stowaway in mili- threatened the unique radiation of crotalines, are also similar to the tary traffic. Darwin's finches differ from what oc- brown tree snake in exhibiting an Successful human-aided coloniza- Q curred on Guam? More important, ontogenetic shift from ectothermic tion reauires not onlv a likelihood of could it happen? As the world be- to endothermic prey. Moreover, the being piaced aboard'a ship, but also comes more tightly united through ingestion of carrion is unusual but the capability to survive during the commerce, the probability of global not unprecedented among snakes. sea voyage. Colonization is undoubt- fauna homogenization and cata- Crotalines, in particular, will eat non- edly (aczitated in species that can strophic snake introductionswill grow. living, even putrefying, prey (Gilling- fast during dispersal through inhos- ham and Baker 1981). Brown tree pitable habitat (e.g., on ships or air- snakes are exceptionally good climb- planes), and all snakes appear to Acknowledgments ers (Chiszar 1990),enabling them to have an exceptional ability to fast Our research has been made possible gain access to food sources that are between meals (Greene 1983, Pough primarily through the support of the denied to more terrestrial snakes (Fig- and Groves 1983). A brown tree US Departments of the Interior and ure 8). But hundreds of arboreal snake can conceal itself in amazingly of Defense. We thank the Guam Di- snakes have similar capabilities small spaces, but this advantage of vision of Aquatic and Wildlife Re- (Lillywhite and Henderson 1993), supple vermiform morphology is not sources for hospitality and assistance. and Shine (1983)concluded that food unique to snakes, much less to brown We greatly appreciate the skill and habits of arboreal snakes are similar tree snakes. Thus, the brown tree persistence of the many people who worldwide. Even the bizarre willing- snake is not unique in either its ecol- have assisted us in the field. We wish ness of brown tree snakes to attack ogy or its behavior. to thank Marie Timmerman, Kathy sleeping humans is found in South If the brown tree snake is not Dean-Bradley, and RenCe Rondeau, Asian snakes of the genus Bungarus unique, we are led to two key conclu- who suggested improvements to this (De Silva 1992, Hati et al. 1988) sions, one applying to herpetology article. and, indeed, other Boiga species. and the other to conservation biol- Thus, the dietary habits of the brown ogy. The herpetological conclusion tree snake are not unique. is that the insights gained through References cited Other asDects of the brown tree the studv of the brown tree snake Atkinson IAE. 1985. The spread of commen- snake's hist;ry on Guam are also not could h&e been gained through the sal species of Rattus to oceanic islands unique. The introduction of the study of any number of other snakes. and their effects on island avifaunas. Pages brown tree snake to Guam was de- A cursory review of the snake ecol- 35-81 in Moors PJ, ed. Conservation of island birds. Cambridge (UK): Interna- vendent on humans. Human trans- ogy appearing in the Journal tional Council for Bird Preservation. Tech- port requires of a snake a willingness of Herpetology from 1985 to 1995 nical Publication nr 3. to be around people and a propen- indicates that approximately half of Baker RH. 1946. Some effects of the war on sity for entering artificial objects. the research in this area is devoted to the wildlife of Micronesia. Transactions of the North American Wildlife Confer- Many other tropical colubrids, espe- just rattlesnakes and natricines, es- ence 11: 207-213. cially several species that have colo- pecially garter snakes. Given that Benson AJ, Boydstun CP. 1995. Invasion of nized tropical islands, share the there exist over 2600 species of the Zebra Mussel in the United States.

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Comparative irregularis) 1. Effects of visual cues aris- Fritts TH, Scott NJ Jr., Smith BE. 1989. ecology of two colubrid snakes, Masti- ing from prey. The Snake 20: 151-155. Trapping Boiga irregularis on Guam us- cophis t. taeniatus and Pituophis melano- Chiszar D, Fox K, Smith HM. 1992. Stimulus ing bird odors. Journal of Herpetology leucus deserticola, in northern . Mil- control of predatory behavior in the brown 23: 189-192. waukee Public Museum Publications in tree snake (Boiga irregularis) IV: effect of Fritts TH, McCoid MJ, Haddock RL. 1990. Biology and Geology 7: 1-104. mammalian blood. Behavioral and Neu- Risks to infants on Guam from bites of the Parker WS, Plummer MV. 1987. Population ral Biology 57: 167-169. brown tree snake (Boiga irregularis). ecology. Pages 253-301 in Siege1 RA, Chiszar D, Dunn TM, Smith HM. 1993. American Journal of Tropical Medicine Collins JT, Novak SS, eds. 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October 1997 Ecology 68: 660-668. ,1988.Food habits of Boiga irregularis, an introduced predator on Guam. Journal of Herpetology 22: 275-282. Harvard University Savidge JA, Sileo L, Siegfried LM. 1992. Was disease involved in the decimation of I~ullardFellowships in ore st Research Guam's avifauna? Journal of Wildlife Dis- eases 28: 206-214. II Seigel RA, Ford NB. 1987. Reproductive ecol- ach year, Harvard University awards a limited number of ogy. Pages 210-252 in Seigel R, Collins J, EBullard Fellowships to individuals in biological, social, phys- Novak S, eds. Snakes: ecology and evolu- tionary biology. New York: Macmillan. ical, and political sciences to promote advanced study, research, Shine R. 1983. Arboreality in snakes: ecology or integration of subjects pertaining to forested ecosystems. The of the Australian elapid genus Hoplo- Fellowships, which include stipends up to $30,000, are intended cephalus. Copeia 1983: 198-205. . 1991. Strangers in a strange land: to provide individuals in mid-career with an opportunity to ecology of Australian colubrid snakes. utilize the resources and to interact with personnel in any Copeia 1991: 120-131. department within Harvard University in order to develop their Steadman DW. 1995. Prehistoric extinctions of Pacific Island birds: biodiversity meets own scientific and professional growth. In recent years, Bullard zooarchaeology. Science 267:1123-1131. Fellows have been associated with the Harvard Forest, Depart- Stickel LF, Stickel William H, Schmid FC. ment of Organismic and Evolutionary Biology, and the J. F. 1980. Ecology of a population of black rat snakes (Elaphe o. obsoleta). Kennedy School of Government and have worked in areas of American Midland Naturalist 103: 1-14. ecology, forest management, policy, and conservation. Fellow- van Riper C 111, van Riper SG, Goff ML, ships are available for periods ranging from four months to one Laird M. 1986. The epizootiology and ecological significance of malaria in Ha- year and can begin at any time in the year. Applications from waiian land birds. Ecological Monographs international scientists, women, and minorities are encouraged. 56: 327-344. Fellowships are not intended for graduate students or recent Warner RE. 1968. The role of introduced diseases in the extinction of the endemic postdoctoral candidates. Further information may be obtained Hawaiian avifauna. Condor 70: 101-120. from: Committee on the Charles Bullard Fund for Forest Re- Wiles GJ. 1987. Current research and future search, Harvard University, Harvard Forest, P.O. Box 68, Peter- management of Marianas fruit bats (Chiroptera: Pteropodidae) on Guam. sham, MA 01366. Annual deadline for applications is February 1. Australian Mammalogy 10: 93-95.

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References cited

Factors Limiting Higher Vertebrate Populations Paul L. Errington Science, New Series, Vol. 124, No. 3216. (Aug. 17, 1956), pp. 304-307. Stable URL: http://links.jstor.org/sici?sici=0036-8075%2819560817%293%3A124%3A3216%3C304%3AFLHVP%3E2.0.CO%3B2-I

Trapping Boiga irregularis on Guam Using Bird Odors Thomas H. Fritts; Norman J. Scott, Jr.; Brian E. Smith Journal of Herpetology, Vol. 23, No. 2. (Jun., 1989), pp. 189-192. Stable URL: http://links.jstor.org/sici?sici=0022-1511%28198906%2923%3A2%3C189%3ATBIOGU%3E2.0.CO%3B2-W

Symptoms and Circumstances Associated with Bites by the Brown Tree Snake (Colubridae: Boiga irregularis) on Guam Thomas H. Fritts; Michael J. McCoid; Robert L. Haddock Journal of Herpetology, Vol. 28, No. 1. (Mar., 1994), pp. 27-33. Stable URL: http://links.jstor.org/sici?sici=0022-1511%28199403%2928%3A1%3C27%3ASACAWB%3E2.0.CO%3B2-5

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Energy Use, Life Histories, and the Evaluation of Potential Competition in Two Species of Garter Snake Norman G. Reichenbach; George H. Dalrymple Journal of Herpetology, Vol. 20, No. 2. (Jun., 1986), pp. 133-153. Stable URL: http://links.jstor.org/sici?sici=0022-1511%28198606%2920%3A2%3C133%3AEULHAT%3E2.0.CO%3B2-D

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