Human–Wildlife Interactions 6(2):181–203, Fall 2012 Brown treesnakes: a potential invasive species for the United States SAMANTHA S. KAHL, 274 Ellington Plant Sciences Building, Department of Forestry, Wildlife and Fisheries, Southern Appalachian Research Branch, The University of Tennessee, Knoxville, TN 37996, USA [email protected] SCOTT E. HENKE, 700 University Boulevard, MSC 218, Caesar Kleberg Wildlife Research Institute, Texas A&M University, Kingsville, TX 78363-8202, USA MARC A. HALL, 233 Pangelinan Way, USDA/APHIS/Wildlife Services, Barrigada, Guam 96913, USA DAVID K. BRITTON, University of Texas, UTA Box 19498, U.S. Fish and Wildlife Service, Arlington, TX 76019, USA
Abstract: Brown treesnakes (Boiga irregularis) are mildly venomous, exotic snakes that have the potential to become an invasive species in North America, Hawaii, and the Commonwealth of the Northern Mariana Islands. The snake is native to northern and eastern Australia, New Guinea, and other islands of northern and western Melanesia. The snakes were fi rst found outside their native range on Guam in 1953. The exact date they reached the island is uncertain, but they are believed to have arrived on military cargo transport vessels some time during or just after World War II. During the years that followed, the population of brown treesnakes increased considerably on Guam. The snakes have extirpated or endangered many native animal populations, attacked pets and poultry, bitten humans, and caused power outages resulting in millions of dollars in damage. This snake species has been found on ships and aircraft, which have transported it to other islands in the Indo-Pacifi c, as well as Hawaii and the continental United States (i.e., Texas, Oklahoma, and Alaska) in military cargo. Because the U.S. military is expanding its bases on Guam, resulting in increased shipments and military movements from Guam to the United States, there is an increasing risk for brown treesnake invasion into the United States, as well as other islands in the Pacifi c. Two-thirds of the literature concerning brown treesnakes is in gray area publication outlets that can be diffi cult to ascertain. A literature review is offered to provide a background of past research on brown treesnakes. This review of literature elaborates on the native range, morphology, behavior, biology, ecology, venom, diet, reproduction, habitat, mortality, and control of the brown treesnakes.
Key words: biology, Boiga irregularis, brown treesnake, control, diet, ecology, Guam, human– wildlife confl icts, invasive species, reproduction
Invasive species are a serious threat to more species arriving in new ecosystems is a ecosystems and are rated second aft er habitat signifi cant problem. loss as the greatest threat to endangered Already a devastating invader on the island of species (Wilcove et al. 1998, Simberloff et al. Guam, the brown treesnake is considered by the 2005). The history of invasive species is a long- U.S. government as a potential threat to other recognized, international problem that can be ecosystems, particularly Hawaii (OTA 1993). linked to people’s movement around the globe It is believed that the brown treesnake arrived (Westerkov 1952, Craighead and Dasmann on Guam with returning military equipment 1966). The Invasive Species Specialist Group during or just aft er World War II from the (ISSG; 2001) began the Global Invasive Species Admiralty Islands, north of New Guinea (Rodda Database (GISD) by providing a brief overview 1991, Rodda et al. 1992b, Whitt ier et al. 2000). It of the most damaging invasive species around did not achieve an island-wide distribution on the world, highlighting which species are Guam, whose area is 540 sq km, until the early the most problematic global invaders. Brown 1980s when the species became a widespread treesnakes (Boiga irregularis) were among them. and recognized problem (Savidge 1987a, Rodda The Offi ce of Technology Assessment (OTA) et al. 1992b, Fritt s and Rodda 1998). Savidge report (1993) identifi ed ≥50,000 invasive species (1987a) fi rst identifi ed the brown treesnake as in the United States, and this number may be a major factor in the disappearance of Guam’s underestimated (Pimental et al. 2005). The avifauna. Species that cause the removal or loss rate of invasion is expanding with increased of an entire taxon can have cascading eff ects global trade and tourism (Pimental et al. 2005, throughout an entire community (Simberloff Simberloff et al. 2005), and the potential for 1990). A review of literature pertaining to the 182 Human–Wildlife Interactions 6(2) brown treesnake regularly cites problems typical slender body shape typical of arboreal snakes. of most invasive species: it is a major threat to All members of the Boiginae sub-family are native wildlife; it causes economic damage; it arboreal or semi-arboreal, with 1 exception, poses a threat to human health; and it has the Boiga trigonata (Rodda et al. 1999b). potential to move to new localities and the Brown treesnake coloration varies throughout ecosystems therein, with the help of humans the species’ geographic range. Individual (Elton 1958, Cox 1999). It has also been noted patt erns vary greatly from indistinct markings that damage caused by brown treesnakes on to striking banding patt erns in parts of Australia Guam could occur elsewhere (McCoid 1993). (Shine 1991a, Qualls and Fritt s 2000). Even with We conducted a literature search that yielded this wide range of coloration in the native >300 citations about brown treesnakes. With range, local populations on Guam tend to have this wealth of information, we have provided uniform color morphology (Rodda et al. 1999b). a detailed summary of brown treesnake This uniformity is likely a result of substantial information. We found that nearly half (47%) of changes in the morphological characteristics of the citations were in gray literature, including: the snake between the native and introduced 15% in government bulletins, reports, and populations (Whitt ier et al. 2000). Within the conference proceedings; 13% in foreign-based native range, banded forms of Boiga irregularis journals (i.e., Japan, Philippines, Australia, are commonly referred to as banded cat snakes Micronesia); 15% in books or chapters within or night tigers and may be recognized as books; 3% in theses and dissertations; and 1% subspecies B. irregularis ornate (Whitt ier et al. in state journals. The remaining publications 2000). were located in medical-based journals and All arboreal snakes, including brown internationally distributed journals. Because treesnakes, have distinct morphological much of the literature concerning brown adaptations that enable them to be successful in treesnakes is diffi cult to acquire and because their habitat. These include a slim body (i.e., low so many references exist, we conducted a body mass:body length ratio), which facilitates literature review to assist researchers to gain an movement on thin branches in the forest understanding of this species without having to canopy and enables a snake to bridge wide recreate such an extensive list of references. gaps between branches. The body of the brown treesnake is dorso-ventrally fl att ened, and the Geography and morphology long, slender tail oft en is used as an anchor Brown treesnake are a native to Indonesia (Pough et al. 1998, Rodda et al. 1999b). Brown and northern and eastern Australia (Rodda treesnakes are large for an arboreal species et al. 1999b, Savarie et al. 2001). Analysis (Rodda et al. 1999b). At the peak of population of mitochondrial DNA showed founding expansion, specimens from the native range individuals for Guam’s population came from reach >2 m in snout-to-vent-length (SVL); the Admiralty Islands (Rodda et al. 1992b, specimens on Guam historically att ain similar Rawlings 1995, Whitt ier et al. 2000, Rodda lengths and in rare cases >3 m (Fritt s 1988, and Savidge 2007). They are a member of the Fritt s and McCoid 1991, Rodda et al. 1999b). family Colubridae, sub-family Boiginae whose Whitt ier et al. (2000) att ribute the larger sizes members are found throughout tropical Africa on Guam to ecological release, where wider to eastern Asia (Rodda et al. 1999b). Brown habitat use and freedom from competition treesnakes are one of 9 species of colubrids enable the snakes to develop unhindered. that occur in Australia (the primary location Brown treesnake have the typical narrow neck where specimens have been collected within and wide head of an arboreal species, allowing the native range) and one of only 3 arboreal them to take a wide-ranging size of prey; the colubrids there (Shine 1991a). Members of the throat region is highly elastic to allow passage Boiginae sub-family oft en are referred to as cat- of larger prey items (Rodda et al. 1999a). eyed snakes due to their large protruding eyes and vertical pupils, which dilate in darkness Venom (Reinhard and Vogel 1975). These snakes The venom is produced by Duvernoy’s are nocturnal, oviparous, and have the long, gland, a modifi ed salivary gland located in the Brown treesnakes • Kahl et al. 183 temporal region. The gland has a smaller storage (Savidge 1991, Santana-Bendix 1994, Shivik capacity than the venom glands of viperids and Clark 1999a). Preferred daytime resting (Zalisko and Kardong 1992). Brown treesnakes places seem to be dark, cool, narrow areas that deliver venom to prey in a drip form that runs aff ord protection from the sun and predators. down the channel present in the 2 posterior Brown treesnakes have been observed resting maxillary teeth; venom is not injected via a in a variety of microhabitats during the day, hollow fang as in viperids (Hayes et al. 1993). including treetops, under rocks, and on the Kardong and Lavin-Murcio (1993) described crowns of Pandanus plants (Santana-Bendix the diff erences between the 2 delivery systems 1994, Rodda et al. 1999b, Tobin et al. 1999, as low-pressure (colubrids) and high-pressure Hetherington et al. 2008). Tobin et al. (1999) (viperids). Delivery of venom by a constant drip found juveniles more frequently above ground is ineffi cient in comparison to injection, and in (>2.5 m high) and adults more frequently on brown treesnakes, it can take several minutes to the ground while resting in daytime refugia. deliver approximately 50% of its stored venom The species will readily travel on the ground, to the prey (Hayes et al. 1993, Kardong and particularly if ground cover is dense, and will Lavin-Murcio 1993). This has led to speculation also move across open areas, such as lawns and that the venom serves some other purpose than roads (Savidge 1987b). While brown treesnakes just a killing agent for capturing prey. Vest et al. have prodigious climbing abilities, they have
(1991) reported that a LD50 (dose that kills half diffi culty negotiating the sheer vertical surfaces the victims) of the venom was 80mg/kg body of buildings (Rodda et al. 1999a). They will weight for mice. However, the snake delivers readily enter homes and other urban structures approximately 174 mg/kg of venom over a (Fritt s 1988, Fritt s et al. 1990, Rodda et al. 1997, period of minutes, which is considerably more Campbell et al. 1999). than necessary to kill (Hayes et al. 1993). Foraging and diet. Brown treesnakes use Venom in brown treesnakes aids in prey both sight and odor to detect prey items during capture and quiescence (Lumsden et al. 2004) nocturnal foraging. They may use ambush and promotes digestion (Hayes et al. 1993, Hill tactics with some prey (Shine 1991a, Rodda and Mackessy 2000). Some of the properties 1992, Rodda et al. 1999b). In their search for found within the venom indicate digestive prey, brown treesnakes seemingly will att ack capabilities; Hayes et al. (1993) speculated that movement, which appears to have led to att acks components of the venom may act upon a prey's on infants and pets, although researchers are integument to degenerate it around the puncture unsure whether or not snakes view the object wounds and allow easier entry of the digestive of their att ack as a prey item (Fritt s 1988; enzymes. Weinstein et al. (1991) showed that Fritt s et al. 1990, 1994). Small prey items are the venom is less eff ective on mammalian tissue swallowed whole, and large prey are subdued than on bird tissue. In general, larger brown by constriction (Rochelle and Kardong 1993, treesnakes deliver larger volumes of venom than Kardong 1999). do smaller individuals (Weinstein et al. 1993). Brown treesnakes are considered dietary generalists, eating a wide variety of vertebrate Biology prey, including reptiles, birds, and small Habitat. Brown treesnakes are associated with mammals in both the snakes’ native and humid climates and occupy a variety of habitats introduced ranges (Savidge 1988, Shine 1991a). from sea level to >1000 m, with a preference for They seem to be opportunistic in their diet, dense arboreal foliage (Fritt s 1988, Shine 1991b, and proportions of prey items tend to refl ect Rodda et al. 1999b). Overall, habitat preference local availability (Savidge 1988, Shine 1991a, is for dense arboreal foliage, although the snake Shivik et al. 2000). Following the drastic drop in can be found using most habitat types (Fritt s bird and small mammal abundance on Guam 1988, Shine 1991a, Tobin et al. 1999, Rodda et (Savidge 1987b), lizards have become the main al. 1999b). On Guam, they use all habitat types prey for the snakes on the island (Rodda and available, including native forest, secondary Fritt s 1992b). Savidge (1988) viewed insect growth forests comprised mostly of invasive consumption as incidental, perhaps already plant species, grasslands, and urban areas consumed by the prey item. Brown treesnakes 184 Human–Wildlife Interactions 6(2) have been observed consuming carrion and New Guinea, as supported by the continuous various discarded human food items (Rodda et synthesis and secretory phases of the sexual al. 1997, Rodda et al. 1999b, Shivik 1999, Jojola- segment of the kidney (Aldridge et al. 2011). Elverum et al. 2001). In the southern hemisphere, female brown There do not appear to be any diff erences treesnakes begin ovulation in the summer in diet between sexes, only between age and (November and December) and produce size classes (Savidge 1988, 1991; Shine 1991a; eggs during the next spring (September and Caudell et al. 2002). Brown treesnakes shift October), while males appear to maintain sperm their prey selection and preference as they throughout the year, with peak testicular volume mature (Fritt s 1988; Savidge 1988; Shivik and in April (Shine 1991a, Whitt ier and Limpus 1996, Clark 1999a, b). Juveniles (i.e., SVL <1,020 mm) Bull et al. 1997). Reproductive cycles appear rely exclusively on ectothermic prey (Savidge to be infl uenced by climatic conditions, and, 1991, Linnell et al. 1997, Rodda et al. 1999b). thus, change with latitude. In warmer, wett er Adults (SVL >1,050 mm) rely on endothermic climates, males appear able to inseminate prey because smaller, ectothermic prey (insects receptive females throughout the year (Bull and lizards) cannot satisfy their physiological and Whitt ier 1996, Whitt ier and Limpus 1996, needs. Data collected during the population Mathies et al. 2010). When in captivity, both expansion period in northern Guam showed sexes of wild, brown treesnakes from Guam that medium-sized snakes (SVL 1,020 to 1,050 have exhibited the ability to alternate between mm) had the most varied diet (Savidge 1988, continuous and seasonal reproduction due to 1991; Rodda and Fritt s 1992b). Brown treesnakes changes in temperature (McCoid 1994, Greene collected during that time had been found to and Mason 2000, Moore et al. 2005, Mathies consume prey >30% of their body weight (Fritt s et al. 2010), which potentially increases the 1988, Savidge 1988), and, under laboratory capabilities of brown treesnakes as an invasive conditions, they have been known to consume species (Mathies et al. 2010). prey ≤50% of their body weight (Chiszar et al. Brown treesnakes are an oviparous species, 1991) and in the wild 70% of their body weight, but fecundity is still poorly known. Clutch size which is substantially greater than most non- varies widely within the snake’s native range. viperid species studied (Fritt s et al. 1994, Rodda Shine (1991a) reported a clutch size range of et al. 1997). between 3 and 11 eggs (average 5.5), with larger Reproduction. Whitt ier and Limpus (1996) females laying larger egg masses. Incubation reported that brown treesnakes from Australia periods of brown treesnakes in Australia become sexually mature at about 70 cm SVL. ranged from 76 to 90 days, depending on However, Shine (1991a) showed that the size incubation temperature (Shine 1991a). Mathies of males at maturity within the native range et al. (2004), suggesting that brown treesnake may vary among populations, and Aldridge et females may be induced ovulators, possibly al. (2010) reported that male brown treesnakes requiring coitus for ovulation. Reproductively from the native range reach maturity at smaller active females have been found at all times of sizes than do snakes on Guam. The amount the year (McCoid 1994), though not in large of fat reserves, or coelomic fat mass, does not numbers. There has been no determined clutch appear to aff ect reproductive activity in males size for brown treesnakes on Guam, and there (Mathies et al. 2010). Reproduction in male is speculation that females could lay ≥1 clutch brown treesnakes is seasonally restricted to per year (Fritt s 1988, Whitt ier and Limpus 1996, the warmer, wett er months in Queensland, Savidge et al. 2007). Very few clutches have Australia, but is continuous with seasonal been found in the wild on Guam. Rodda et al. spermatogenesis and a brief stage of testicular (1999b) speculated that gravid females may be regression from January to March in New under-collected (<0.5% of snakes sampled) due Guinea (Bull and Whitt ier 1996, Bull et al. 1997). to their secretive behavior before laying a clutch. Aldridge and Arackal (2005) disputed this The few clutches found on Guam were located fi nding (supported in Mathies et al. 2010), and in varying types of substrate, including a tree deduced that evidence from male specimens hole, a coconut frond, and a solution hole on supported highly continuous reproduction in a limestone cliff face (Rodda et al. 1999b). The Brown treesnakes • Kahl et al. 185 clutch sizes found on Guam typically have been development and shutt ing down reproductive 3 to 4 eggs, with up to 8 eggs per clutch (Fritt s processes, possibly due to stress. 1988, Shine 1991a, Rodda et al. 1997, Rodda et Reproductive data for brown treesnakes in al. 1999b). Incubation periods for the clutches the wild on Guam are scant, but Moore et al. on Guam have been as short as 94 days and as (2005) gave evidence that the population on long as 125 to 126 days (Linnell et al. 1997). Guam is becoming less reproductively active, Behaviors exhibited by nocturnal, tropical as shown by low proportions of reproductively snakes may have very few thermal limitations active adults; they also reported that high (Anderson et al. 2005). Mathies and Miller levels of corticosterone in wild snakes may (2003) found that brown treesnakes respond to suggest that the prey resources on the island short periods of cool temperatures by increasing have been overexploited. Waye and Mason reproductive activity, but potential response to (2008) found lower levels of corticosterone in seasonally cold temperatures is yet unknown. brown treesnake specimens collected in 2003 Sperm storage, a capability shared by many and suggested that the snakes were no longer species of snakes, is speculated to occur in experiencing high levels of stress and that few brown treesnakes, but there is some debate as to mature females appeared to be reproductively how and by which sex sperm is stored (Whitt ier active. However, more recent studies suggested and Limpus 1996, Pough 1998). Bull et al. (1997) that because coelomic fat mass does not found no special structures to support this contribute to or reduce reproductive activity idea and argued that it is the male that stores in male brown treesnakes (Aldridge et al. 2010, sperm for prolonged periods. Savidge et al. Mathies et al. 2010), high levels of corticosterone (2007) concluded that although sperm storage may be due to an increase in male–male is crucial in certain areas of the native range, it encounters and combat (Aldridge et al. 2010). may be uncalled for by either sex on Guam. Population expansion on Guam. Before Typical courtship behavior displayed by male World War II, there were no recorded instances brown treesnakes includes tongue fl icking and of brown treesnakes on Guam. The possible head jerking as the most obvious behaviors. transportation of snakes to the island in Males and females show slightly diff erent military equipment probably introduced them responses to each other’s courtship behavior in extremely low numbers (Savidge 1987a, (Greene and Mason 2000). Methyl ketones, 1987b, 1991). Sometime during the 1950s, which have been identifi ed as sex pheromones in reports of snakes were made (Engebring and Thamnophis sirtalis, have been found in the skin Fritt s 1988, Fritt s 1988), but skepticism and lipids of brown treesnakes (Murata et al. 1991). disbelief of the reports were common until 1955 Brown treesnakes rely on sex pheromones for when specimens were captured (Rodda et al. courtship and combat behaviors (Greene and 1992b). Initial populations appear to have been Mason 2000, Greene et al. 2001). Greene and concentrated around the main port of Apra Mason (2003) found that release of female cloacal Harbor, with island-wide dispersal occurring secretions prior to copulation may actually from this point, fi rst south, then north, with inhibit male courtship in brown treesnakes and rapid range expansion in the 1960s and 1970s act as a defense mechanism for females. Greene (Savidge 1987b, Rodda et al. 1992b). The snakes and Mason (2005) reported that aged cloacal probably spread with the relative availability secretions may elicit defensive behaviors from of prey items, and peak population densities male snakes, which could be the cause of stress were not reached in the north of the island until and high levels of corticosterone. There has the early 1980s (Savidge 1987b), followed by a been limited success in rearing these clutches population decline in the northern end of Guam in captivity, and resultant surviving hatchlings starting in 1985 (Rodda et al. 1992b). Reasons for were smaller than their wild-born counterparts the increased rate of range expansion during the (Rodda et al. 1999b). There are only 2 known 1970s are still unexplained (Rodda et al. 1992b), cases to date of brown treesnakes reproducing though Vice and Engeman (2000) suggested in captivity (Mathies and Miller 2003). Aldridge that military movements, increases in training and Arackal (2005) showed that captivity causes exercises, or response to natural disasters might decreased reproductive activity by preventing have led to increased shipments of goods and materials around the island. 186 Human–Wildlife Interactions 6(2)
In their invaded range, brown treesnakes within the native range of brown treesnakes have unusually high densities for any snake (B. irregularis, Dendrelaphis calligastra, and species (Fritt s 1988). Peak densities of 50 to Dendrelaphis punctulata; Shine 1991a). Shine 100 snakes/ha occurred on Guam in the 1980s, (1991a) found that the diet of brown treesnakes followed by a decline to persistent densities of within Australia is much more fl exible than 30/ha (Fritt s 1988, Rodda et al. 1999b, Rodda et that of the other 2 arboreal snake species. al. 1999c). The highest densities on Guam were Brown treesnakes consumed birds and bird described as irruptions within the population eggs, as well as more mammal species, than (Rodda et al. 1999c). Densities within parts of did D. calligastra or D. punctulata. The diet of the snakes’ native range are considerably less. brown treesnakes in Australia consists of 36% Despite brown treesnakes' depletion of much of bird prey items and 23% mammal prey items, their prey base on Guam, they have proven to whereas these items made up <1% of the diets be persistent, with the population remaining at of Dendrelaphis spp. in the same area (Shine suffi cient densities to warrant a control program 1994). As adults, brown treesnakes also tend to prevent them from colonizing additional to be larger than either of the Dendrelaphis spp. locations. Despite the high densities recorded (Shine 1994), which may have some eff ect on on Guam, brown treesnakes are solitary and the types of prey they can consume. One of the do not generally aggregate, though some small main factors owing to these diff erences in prey groups have been observed in the native range consumption could be that brown treesnakes (Pendelton 1947, Bull and Whitt ier 1996). are nocturnal, while the 2 Dendrelaphis spp. The smallest snakes found on Guam have are diurnal in Austrailia (Shine 1991a). Brown been ≤350 mm SVL and retain an umbilical scar, treesnakes in their native range and on Guam which suggests that they are recent hatchlings seem to lack any signifi cant nocturnal arboreal (Rodda et al. 1999c). Jordan and Rodda (1994) competitor (Shine 1991a). classed brown treesnakes as juveniles (SVL <750 Studies of the Solomon Islands and native mm), sub-adults (SVL ≥750 to SVL < 950) and range in Australia show, with 1 exception, litt le adults (SVL ≥ 950 mm). Savidge (1991) indicated obvious depredation of the snakes (Rodda et that the adult population is skewed toward an al. 1999b, Caudell et al. 2002). The mangrove abundance of males. This diff erence between monitor (Varanus inidicus) does prey on brown the numbers of males and females becomes treesnakes, but not enough to signifi cantly even more apparent when the snakes increase aff ect populations (Rodda et al. 1999b). to SVL >1,200 mm. Jordan and Rodda (1994) There appears to be few limiting factors for support this observation, as well as affi rming the brown treesnake population on Guam. Savidge’s (1991) fi nding that there is a ratio of Brown treesnakes have been observed being 1:1 (M:F) among sub-adults and juveniles. Rural killed and eaten by monitor lizards and feral snakes rarely grow to SVL >1,300 mm, probably pigs (Sus scrofa). It has been speculated that due to a scarcity of endothermic prey in rural rodent and crab species (Birgus latro) on Guam areas (Savidge 1991). Larger snakes historically may att empt to take brown treesnakes as prey have been found in southern Guam, which has (Fritt s 1988, Santana-Bendix 1994). more savannah-like conditions than the rest of Some parasites and fungi have been found to the island, and a relatively abundant rodant contribute to captive brown treesnake mortality. population (Savidge 1987b, 1991). Rodda et Brown treesnakes in Queensland, Australia, al. (1999c) showed that there is limited sexual are highly parasitized by haemogregarine dimorphism, as captive females can grow to parasites (Ewers, 1968, Mackerras 1961, Telford sizes comparable to males and much larger 1999), though Caudell et al. (2002) found no than females in the wild when given a regular evidence to suggest that these parasites were diet. Males were larger than females on Guam regulating the brown treesnake populations. (Jordan 1991, Savidge 1991) and in the native These parasites do not occur on Guam, and range (Shine 1991a, Trembath and Fearn 2008). there is also almost no possibility of brown Limiting factors. There are relatively few treesnakes acquiring parasites from the other studies on brown treesnakes within their native snakes on Guam because the only other species range. There are 3 arboreal colubrid species is fossorial (Telford 1999). Brown treesnakes • Kahl et al. 187
The only factor that appears to have a potential Wiles et al. (2003) found that brown to signifi cantly impact population densities on treesnakes have extirpated or caused the severe Guam is prey availability. With the extirpation declines (≥90%) in the 25 native bird species on of avifauna and a low presence of rodents in the Guam. These declines occurred rapidly, within forested regions of Guam, reptiles are the major an average of <9 years, and species with larger prey items of the current population of brown clutch and body sizes were more persistent treesnakes. Lizards are a typical prey item of (Wiles et al. 2003). Wiles et al. (2003) recorded brown treesnakes, and Guam is now home to the disappearance of 9 forest species in only several introduced species of anurans (Christy 2.1 years at the Pajon Basin; this is the fastest et al. 2007). Brown treesnakes feed frequently bird population decline on record. Wiles and (Jackson and Perry 2000), and the high densities Brooke (2009) observed that medium and of reptiles (mean of 13,290 lizards/ha) have large young bats are rare within native bat helped to sustain the high density of brown populations on Guam; however, the loss of treesnakes, despite the absence of birds and small Emballonura semicaudata on Guam should not mammals (Savidge 1991, Campbell et al. 1996). be att ributed to brown treesnakes. By 1990, the only 3 surviving native vertebrate species in Impacts on Guam forested areas of Guam were lizards (Fritt s and The severity of impact an invasive species has Rodda 1998). Surviving native lizard species on any one resource may diff er with species are small in size and have high reproductive (Offi ce of Technology Assessment [OTA] 1993, rates, thus, strengthening their population Aquatic Invasive Species 2003, Simberloff et persistence (Rodda and Fritt s 1992b). The al. 2005). Guam has suff ered from the same success of introduced species on Guam keeps fate as other islands invaded by other species habitat suitability high for brown treesnakes (Elton 1958). It is diffi cult to estimate the overall (Rodda et al. 1999c). damage to Guam’s ecosystem, as there was no It is highly probable that brown treesnakes ecological monitoring of the environment to caused the fi nal demise of many of the bird provide baseline data prior to the extensive species on the island, however, several other research of the brown treesnake (Fritt s and factors should be considered, as well. When Rodda 1998, Rodda et al. 1999b). Brown research was initiated in the early 1980s, many treesnakes pose the same types of threats as of the bird species were already gone, and the do other invasive species, including impacts to dietary composition of the snakes was mainly ecology, economy, and human health. composed of lizards, making it diffi cult to Ecological. Several endemic species occurred estimate the rate of depredation (Savidge on Guam, and the entire native fauna evolved 1988). To further compound matt ers, much in the absence of major predators. The historic of Guam’s forest consists of second-growth lack of predators has led to a susceptibility invasive species, (e.g., Leucaena leucocephla). to depredation, especially from nocturnal, There have been several other non-native arboreal predators, such as brown treesnakes. species introduced, including feral hogs and Brown treesnakes have been implicated in the brown skinks (Carlia fusca). DDT use was extirpation of native and introduced forest, extensive during post-World War II, and there grassland, pelagic birds, native bat species, is continued fragmentation of the island forest introduced rats, and native lizards on Guam habitat by human development (McCoid 1991, (Savidge 1987a, Rodda and Fritt s 1992a, b; Rodda Rodda and Fritt s 1992b, Fritt s and Rodda 1998, et al. 1999c; Wiles et al. 2003). Savidge (1987a) Wiles et al. 2003). It is reasonable to assume that traced the spread of the snake throughout the these factors played some part in disrupting island during the disappearance of the native the ecosystem, further enabling the snakes to birds. This, along with a lack of convincing extirpate the aff ected bird and mammalian evidence implicating other potential causes of communities, and reptile species. the disappearances, has led to the assessment While the loss of individual species is that brown treesnakes are solely responsible lamentable, it is the likely long-term eff ects for the losses (Engebring and Fritt s 1988, Rodda that brown treesnakes have had as a kestone et al. 1997, Fritt s and Rodda 1998, Rodda et al. predator that should produce the most concern. 1999b). 188 Human–Wildlife Interactions 6(2)
In conjunction with some introduced lizard 3 important economic impacts on Guam: (1) species, the basic assemblage of Guam’s native electrical outages; (2) att acks on domesticated lizards has been altered, and the extinction of animals; and (3) increased budgets for their bird species has removed pollinators and seed control. dispersers that help perpetuate native plant Because brown treesnakes are excellent species (Savidge 1987a, McCoid 1991, Rodda climbers, unprotected power lines are easily and Fritt s 1992b, Mortensen and Dupont 2008). accessible to them. Once a brown treesnake Additionally, brown treesnakes appear to creates a connection between 2 high-voltage have altered the behavior of orb web-spinning lines, a short-circuit results. Over a 7-year period spiders (Argiope appensa) on Guam, where there (1991 to 1997) there were 934 electrical outages are reduced number of webs with stabilimenta, on Guam (Fritt s 2002). These outages were of a silk web structure that may function to defend varying lengths and have been estimated to the spiders or to att ract prey (Kerr 1993). Brown cost approximately $375,000 per hour during treesnakes have had eff ects on Guam that daytime on Guam, not including the cost of cascade throughout the island’s ecosystem necessary power line and transformer repairs, (Mortensen and Dupont 2008). which, together, extrapolates into losses that Health. Brown treesnakes are a threat to equal millions of dollars (Fritt s 1988, 2002). human health on Guam primarily because of Pimentel et al. (2005) conservatively estimated their propensity to bite sleeping victims, usually costs for power outages associated with brown small children <5 years of age, and infants (Fritt s treesnakes to be $1 million per year. This et al. 1994, Fritt s and McCoid 1999). There is a cost does not include all of the possible costs signifi cant increase in bite frequency during the att ributed to each outage due to loss or disruption wet season in Guam (August to October), which of business in stores and online, as well as is probably related to the snakes’ increased disruption of regular community processes, levels of foraging activity (Fritt s 1988, Fritt s et such as the traffi c system (Rodda and Savidge al. 1990, 1994, Rodda et al. 1999b). 2007). The frequency of outages varies according Only speculation exists as to why these snakes to the season (wet versus dry); also, wet years att ack infants and small children, especially as produce more snake activity, correlating with they have no hope of swallowing a child. As increased number of power outages (Fritt s et al. many of the bites occur on the extremities (e.g., 1987, Fritt s 2002). Brown treesnakes on Guam hands, fi ngers), it is possible that the snakes are responsible for approximately 1 outage cannot recognize the overall size of their prey every other day, typically aff ecting only 1 small choice, and so the extremities appear to be a area at a time (Rodda and Savidge 2007), but viable prey item (Fritt s et al. 1994). Additional the costs add up quickly. evidence that indicates snakes perceive children Fritt s and McCoid (1991) found that 80% as a prey item is that those snakes found of respondents who raised chickens suff ered att acking were also constricting their intended from some form of depredation (dogs, monitor target (Fritt s et al. 1990). So far, there have been lizards, brown treesnakes), with 45% of their no fatalities associated with bites from brown losses att ributed to brown treesnakes. The treesnakes. However, bites have induced a overall economic impacts from these losses variety of reactions in humans, including have generally meant that most of their poultry swelling, blistering, respiratory distress, and products have had to be imported, thus, raising lethargy (Fritt s et al. 1990, 1994). These were the overall cost of the product to the residents of the most serious symptoms, and they occurred Guam (Fritt s et al. 1987). Losses of pet puppies, only aft er severe envenomation. In general, it kitt ens, and birds due to brown treesnakes have appears that the threat of human mortality from not been monetarily estimated. brown treesnakes is inconsequential. However, Management for brown treesnakes across large snakes carry the greatest potential for all participating agencies for 2004 cost causing a human death, as they these snakes approximately $10 million, with funding barely carry the most toxic venom (Weinstein et al. meeting costs (Brown Treesnake Working 199l, 1993). Group 2004). This estimate does not include Economic. Brown treesnakes have produced any potential increase in shipping costs for Brown treesnakes • Kahl et al. 189
Guam, or cost of decreases in tourism of up to habitat types, and their nocturnal movements $1.5 billion annually that may have resulted in search of prey oft en lead them into cargo and from the snakes (Rodda and Savidge 2007). aircraft areas (Fritt s 1988, Aquatic Nuisance Species Task Force 1996b). Cargo and aircraft Purpose of management already have been sources of off -island Because brown treesnakes are successful transportation for brown treesnakes and are invaders and are capable of colonizing a still considered the main vectors of dispersal, variety of habitat types, it is critical that they be primarily because these modes of transport can controlled in some manner (Fritt s 1988, Aquatic provide suitable daytime refugia for snakes Nuisance Species Task Force 1996b). Should (Engeman and Linnell 1998, Rodda et a1. 1998, brown treesnakes succeed in establishing in Fritt s et al. 1999, Engeman and Vice 2002, other areas, they will likely have some eff ect Perry 2002). In rural areas, the snakes’ ability on susceptible species in the region where they to mimic vines and their lack of refl ective eye colonize; whether there will be the same level of shine make them diffi cult to locate or observe devastation are found on Guam remains to be during nocturnal searches (Rodda et al. 1998). determined. Early studies on brown treesnakes With their cryptic nature and ability to adapt focused on understanding their ecology but to diff erent environments, developing a single have now expanded to include control eff orts eff ective management technique is a daunting and public awareness campaigns on the threat proposition. of the snakes (Fritt s 1988, Aquatic Nuisance Species Task Force 1996a, Rodda et al. 1998, Management techniques Campbell et al. 1999, Burnett et al. 2008). The The various management techniques currently main goals of brown treesnake management being used by the agencies can be categorized are to keep the snakes from adversely aff ecting into 4 major types of control: biological, native wildlife restoration programs on Guam ecological, chemical, and mechanical. Each and to prevent their spread to other areas of these methods has been att empted in some (Rodda et al. 1998). Rodda et al. (1998) cite 4 way to control the current population of brown objectives, in declining order of importance treesnakes on Guam, but each is still under for controlling the brown treesnake problem constant research to determine more eff ective on Guam: (1) eradicate brown treesnakes; (2) methods of control. All techniques being greatly reduce snake populations permanently; developed in the control of brown treesnakes (3) control snake populations over areas large currently are being applied with varying enough for endangered species restoration; degrees of success, each depending on goals and and (4) control snakes in small areas (i.e., cargo size of the area being treated (Aquatic Nuisance areas, transportation craft , etc.) to prevent Species Task Force 1996b, Engeman and Linnell further spread. 1998). There are many large-scale control eff orts An organized management plan for the around the world for various invasive species, control of brown treesnakes is a relatively but there is only 1 other large-scale snake control recent development of the 1990s (Rodda program currently in use. This program is in et al. 1998, Campbell et al. 1999). Research Japan to control for the highly venomous habu into a variety of control techniques has been (Trimeresurus fl avoviridis; Offi ce of Technology occurring for a slightly longer period, but Assessment 1993, Rodda et al. 1999b). However, there was no widespread, unifi ed eff ort the techniques applied to the management of against the snake. With the inclusion of brown the habu cannot readily be transferred to brown treesnakes in the 1990 Nonindigenous Aquatic treesnakes, as the 2 snakes have very diff erent Nuisance Prevention Act, the development of ecologies (Campbell et al. 1999). The control a cooperative program has progressed to the eff orts surrounding the Burmese pythons in point where the control program is a multi- south Florida are not as large scale as those for agency eff ort involving federal, state, and local brown treesnakes and the habu (Willson et al. wildlife agencies across islands in the Pacifi c. 2011). There are several factors that make the On Guam, the high densities of brown control of brown treesnakes diffi cult. Their treesnakes, their willingness to utilize various ability to climb means that they can scale most 190 Human–Wildlife Interactions 6(2) barriers with ease. Their secretiveness makes successfully introducing a dominant predator them diffi cult to track and capture, and their is unlikely because most known predators high mobility makes them diffi cult to eliminate are not generally considered eff ective at on a large scale. Each of the following techniques reducing brown treesnake populations, and has the ability to control numbers with small- the introduction of another potential predator scale application, chemical control having the (e.g., mongoose [Herpestes spp.]) could have greatest promise on a larger scale of application. undesired eff ects on nontarget species or on general ecosystem function (Fritt s 1988, Rodda Biological et al. 1998, Engeman and Vice 2002). Saunders et Biological control can be defi ned as the al. (2010) propose that biocontrol for vertebrate management of a pest through the intentional pests is never the fi nal solution to the problem, use of living organisms (Lazarovits et al. 2007). and long-term mitigation will remain an To date, 2 techniques have been implemented essential component of management. or considered for biological control of brown treesnakes: pathogens (i.e., parasites and Ecological disease) and the introduction of a predator Ecological control can be either a large- or (Hoddle 1999, Nichols et al. 1999, Engeman small-scale endeavor, usually involving the and Vice 2002). Most biological controls that alteration of habitat. However, ecological are successfully used for invasive species are control is potentially limited to restricted areas, generally used for insect control, and the overall because brown treesnakes show litt le habitat effi cacy is dependent on the characteristics preference and are found throughout Guam of the pathogen (Dobson 1988). For brown (Rodda et al. 1992b). Such methods would treesnakes, research continues to look at the include clearing vegetation either mechanically potential of introducing a biocontrol agent, or with herbicides (Fritt s and Rodda 1999). but the potential of unintended consequences This has the advantage of reducing the amount (i.e., infection of nontarget species) must be of habitat that snakes can utilize for cover. considered (Dobson 1988, Rodda et al. 1998, Additionally, the openness of the ground will Engeman and Vice 2002). increase wariness and perhaps discourage Although limited knowledge and research snake movement (Rodda 1991). Limited areas of on specifi c parasites associated with brown control would include the immediate vicinities treesnakes is available, we do know that they around cargo facilities and airports (Fritt s carry blood parasites (Hoddle 1999, Telford 1988, Rodda et al. 1998). Ecological control 1999, Caudell et al. 2002). Additionally, techniques can be applied to urban areas, as Nichols et al. (1999) found that captive brown these have surfaces and a vegetation structures treesnakes were fatally susceptible to a dermal that prove inhospitable to brown treesnakes fungus, although, its use as a form of control (Rodda 1991). The use and placement of bright is unknown. The diffi culties associated with lights in snake capture has been considered, as the use of parasites and disease stem from the brown treesnakes are nocturnal and may well overall lack of knowledge about brown treesnake avoid brightly lit areas, thus, detering their epidemiology and the fact that there has been movements away from such areas (Campbell limited success in controlling vertebrates with et al. 1999). Campbell et al. (2008) found in a introduced pathogens (Caughly and Sinclair laboratory-based study that moonlight aff ects 1994, Rodda et al. 1998). Successful biological microhabitat use by brown treesnakes, which control has been exhibited in agriculture (van use open ground more as light decreases and Lenteren 2007, Wiedenmann et al. 2007), but use the canopy for cover as the light increases. only 3 successful forms of vertebrate pest control Manipulation of the prey base of brown have been recorded: myxoma virus, rabbit treesnakes has been suggested, although, hemorrhagic disease for rabbit control (Fenner diffi culties arise when considering secondary and Ratcliff e 1965, Cook and Fenner 2002), and impacts. However, the general belief is that the feline panleucopaenia virus for control of a removal of a prey item from an area will make small population of feral cats (van Rensburg et that area less att ractive to brown treesnakes al. 1987, Saunders et al. 2010). The potential for (Engeman and Vice 2002). Additionally, there Brown treesnakes • Kahl et al. 191 are the problems of cost eff ectiveness and the delivery and human health issues (Chiszar et al. unknown short- and long-term results of such a 1997, Clark 1997, Engeman and Vice 2002). In method (Campbell et al. 1999, Fritt s and Rodda contrast, Torr and Richards (1996) reported that 1999). As with other control methods, ecological brown treesnakes eat carrion. Shivik and Clark control requires serious consideration as to the (1997) found that carrion cues att ract brown potential to adversely aff ect nontarget species. treesnakes to traps. One issue associated with both att ractants and repellents (primarily to Chemical reduce costs and labor) is longevity (Engeman Snakes, including brown treesnakes, generally and Vice 2002). Clark and Shivik (2002) use odor cues to track their prey (Rodda et al. examined the eff ects of natural compounds 1999b, Shivik et al. 2000). With this knowledge, and aerosolized oils as chemical irritants for researchers have worked to develop att ractants brown treesnakes as remedial snake repellents (for traps), repellents, and lethal toxicants. and found several that could be used instead of Research on att ractants has focused on fi nding harmful organochlorines. Repellents used for the best odors that att ract snakes into traps. brown treesnake control would potentially be In general, snakes have shown preference for successful in small, restricted areas, but would whole blood and carcasses of preferred prey not be eff ective in completely eradicating the (birds and rodents) over synthetic odors (Shivik snake from a more widespread location, such and Clark 1997, Shivik 1998, Shivik et al. 2000, as Guam. Chiszar et a1. 2001, Stark et al. 2002). However, There are numerous literature sources from despite some success in the laboratory, more the 1950s, 1960s, and 1970s detailing that research is necessary for application in the fi eld organochlorine chemicals (e.g., DDT) are viable (Chiszar et al. 1997, Shivik and Clark 1999a, repellents for snakes (Savarie and Bruggers Shivik et al. 2000). 1999). Such chemicals acted as repellents, in Shivik et al. (2002) examined the aerial delivery addition to being lethal to the target species, of toxicants by implanting acetaminophen into and they can be applied orally (i.e., bait) dead mice and dropping them into the canopy. or dermally (i.e., spraying). However, the Radiotransmitt ers were implanted in the bait for diffi culty associated with these methods is data collection on movement, and the authors fi nding the appropriate delivery system (Rodda reported that snakes moved from 1 to 70 m et al. 1998, Engeman and Vice 2002, Shivik et within 5 to 11 days aft er bait consumption, and al. 2002). The search for toxic chemicals for snakes that consumed baits were comparable eliminating brown treesnakes has found several in size and body condition to other snakes potential lethal toxicants, including rotenone, captured in the fi eld. This technique is believed propoxur, and pyrethrins (Johnston et al. to be the best technique for depopulating brown 2001, Brooks et al. 1998, Savarie and Bruggers treesnakes on Guam (Shivik et al. 2002). Savarie 1999). Diff erent levels of lethality occur when et al. (2001) conducted a promising study in the these chemicals are applied via dermal or oral use of acetaminophen as a toxicant, which has doses, independent of application type (Brooks the advantage of being eff ective, inexpensive et al. 1998, Savarie et al. 2000, Johnston et al. and easily accessible. Savarie and Tope (2004) 2002, Shivik et al. 2002). Maudlin et al. (2000) found that the best method for aerial delivery is found a simple analytical method using high- to drop bait in paper food cups. In comparison performance liquid chromatography to identify to 4 other types of fl otation materials, paper the residual capacity of rotenone, and possibly food cups were easiest for researchers to put other toxicants, in brown treesnakes. together and deploy into the canopy. Several factors must be considered before In the search for repellents, research fi ndings there can be widespread use of toxicants. Any diff er in regard to the reactions of brown application must consider the potential impact treesnakes to carrion. Some experiments with on nontarget species and the environment a synthetic pheromone from monkeys and (Rodda et al. 1998, Savarie and Bruggers components of carrion odors elicited avoidance 1999, Savarie et al. 2001, Johnston et al. 2002). behavior in snakes used for the experiments, but Commercially available toxicants, such as Dr. extensive fi eld research is required to overcome T’s Snake-A-Way, that have been advertised 192 Human–Wildlife Interactions 6(2) on Guam as a product for control of brown reduce the dependence on live mouse lures, a treesnakes, but are ineff ective (McCoid et al. logistically intensive operation (Rodda et al. 1993). There have been no studies to examine 1998, Vice 1999, Lindberg et al. 2000). Mechanical the residual eff ects of this type of commercially mouse lures have been experimented with, but available toxicant on nontarget species. a life-like representation is still under long-term With the many economic and ecological development (Lindberg et al. 2000). disadvantages to using chemicals for species The most eff ective form of trap placement has eradication, it is important to consider other been on a perimeter around forest edges; this options. Thermal fumigation also has been placement covers a greater area and reduces the examined as a potential alternative to chemical amount of trap maintenance time (Engeman fumigation of cargo containers leaving Guam. and Linnell 1998, Engeman and Vice 2002). Perry and Vice (2007) found that passive Problems do arise, however, when considering thermal fumigation used along with snake how large an area to cover with traps. Traps barriers may be an economically advantageous are very eff ective in small discreet areas. tool for areas in the Commonwealth of the Increasing the distance between traps both Northern Mariana Islands receiving shipments reduces the chance of a snake encountering a from Guam. Passive thermal fumigation uses trap and increases labor time. Capture-rates sunlight and ambient temperatures to warm are highly dependent on the local density of cargo boxes beyond the point of snake survival. snakes, as well as the abundance of the prey Inconsistencies of daylight and heat patt erns, as base (Rodda et al. 1992a, Engeman and Linnell well as diff erences in cargo, make this technique 1998, Rodda et al. 1999a, Engeman and Vice unpredictable (Perry and Vice 2007). 2002). Engeman et al. (2003) analyzed models of capture rates for management purposes of Mechanical and physical brown treesnakes to predict capture rates over This kind of control involves traps or some time, based on the number of snakes captured form of barrier. Traps (with lures) have been per trap-night. Gragg et al. (2007) found that used eff ectively since the beginning of brown capture rates of brown treesnakes were higher treesnake research, and trap designs continue to in areas with lower rodent abundance, such as be evaluated for improvement (Savidge 1987b, Guam. Brown treesnakes on Guam enter traps Fritt s et al. 1989, Rodda et al. 1992a, Linnell et al. more readily in search of prey than do brown 1998, Engeman and Vice 2002). Engeman and treesnakes in locations with higher mammalian Vice (2002) cite trapping as central to brown prey abundance. This implies that the lowered treesnake control activities on Guam. Funnel rodent abundance may actually enhance brown traps have 1-way doors, with a secondary treesnake control (Gragg et al. 2007). Rodda et chamber located inside the trap that contains a al. (2007a) and Tyrell et al. (2009) found traps to 1ive mouse as a lure (Fritt s 1988, Linnell et al. be more successful in capturing larger snakes 1998, Engeman and Vice 2002). Early traps may SVL > 900 mm than smaller snakes SVL <700 have allowed 80% of snakes to escape under mm, and the authors suggest that trapping can certain conditions, but newer designs are more be used to capture sexually mature adults if eff ective at retaining captured snakes (Rodda et used as a continuous management tool. al. 1992a, Linnell et al. 1998, Rodda et al. 1999a, Because brown treesnakes are adept climbers, Engeman and Vice 2002) and immigration of it has been diffi cult to develop an eff ective snakes or inadequate trapping eff ort are likely barrier for this species, but there are some in use. the cause of persistence of snakes in long-term Rodda et al. (1998) found barriers to be the best trapping areas (Rodda et al. 2007a). Traps can tools for mid-scale control of brown treesnakes be valuable tools because they can be placed on Guam. Barriers are typically placed in and in areas that cannot adequately be patrolled around urban areas, government facilities, and by either people or dogs (Fritt s 1988, Rodda areas that are being used to reintroduce native et al. 1992a, Vice 1999, Engeman and Vice wildlife (Rodda 1991; Aguon et al. 1999, 2002). 2002). Because brown treesnakes respond to Problems with barriers are the costs (dependent polymodal stimuli, experiments have focused on materials used), the amount of area they can on how to make traps more att ractive and cover, and potential damage from typhoons Brown treesnakes • Kahl et al. 193
(Rodda et al. 1998, Campbell 1999, Engeman and Vice 2002). The success of a barrier is dependent on the material used and the size of any particular snake (Campbell 1999, Engeman and Vice 2002, Rodda et al. 2007b). Rodda (1991) tested the validity of chain-link fences as a collection point for the snakes (Figure 1). The fence helped to facilitate their capture due to brown treesnakes’ willingness to climb the barrier and the ease with which they could be spott ed and collected. Perry et al. (1998a) found permanent barriers to be more cost-eff ective than temporary barriers and metal mesh barriers and vinyl seawall barriers to be highly eff ective as permanent barriers against brown treesnakes.
Detection Humans and dogs are used in inspection processes, but mainly they are the last line of defense and are useful in limited areas only (Orcutt 1997, Engeman and Vice 2002). A rapid- Figure 1. Chain-link fences can be used as collec- tion points for brown treesnakes. response team developed in 2002 performs searches to detect and capture snakes following have to be maintained under security for these credible sightings (Stanford and Rodda 2007). training purposes, and this is unacceptable Human searchers are used in cargo areas, on all snake-free islands (Imamura 1999). airports, along fence lines, and transects in forest Costs, time, physical limitations, and protocol areas to locate brown treesnakes (Rodda and associated with dog-team training and searches Fritt s 1992a, Rodda et al. 1992a). Spotlighting can lead to policy and management issues perimeter fences at night to fi nd snakes for (Imamura 1999, Engeman and Vice 2002). hand-capture has become a generally effi cient Canine detection for brown treesnakes does technique, as well (Vice and Pitzler 2000). work on Guam, although a 100% inspection While hand-capture is eff ective, there are goal for cargo is not realistic due to personnel problems involving time constraints, fatigue, and scheduling limitations and overall volume observation capabilities, and also problems of of inspections (Imamura 1999). overcoming a natural fear of snakes (Rodda and Fritt s 1992a, Rodda et al. 1998, Campbell et al. Education and awareness 1999). Dogs (primarily Jack Russell terriers) are Gett ing the public involved in brown trained to detect snakes by odor and are used treesnake control and prevention could be one by the U.S. Department of Agriculture (USDA) of the most eff ective forms of management. to inspect outbound cargo and aircraft wheel Biocontrol studies have had success when wells. Detection of snakes by dogs may diff er by involving public education systems in pest the form of dog training, such as whether dogs management through teacher training, are trained using pre-handled snakes, which student research, and outdoor application may aff ect the odor of the snake that the dog (Wiedenmann et al. 2007). Public and military is trained to detect and may diff er from snakes support and education at transport locations is in the wild (Imamura 1999). This presents the critical for brown treesnake prevention and may diffi culty with having trained brown treesnake increase detection (Engeman and Vice 2002). detector dogs in currently uninhabited locations Educational materials, such as fl yers, posters, for detection and rapid response. In order for rapid response cards, and general information the dogs to continue maintenance training with for the public are available from diff erent the brown treesnake odor, a population would sources (U.S. Department of Agriculture 1997, 194 Human–Wildlife Interactions 6(2)
North American Brown Treesnake Control those areas to increase detection probabilities Team [NABTSCT] 2008, U.S. Geological Survey and jumpstart management to prevent brown [USGS] 2007). Furthering public education and treesnake introduction. Further research is awareness could also benefi t brown treesnake needed on introduction and transport pathways control through increases in funding support and cost-eff ective control of those pathways, from individuals and groups. including ways to enhance technique, speed, The catastrophic results from a long list of scheduling and management of inspections in invasive species and the well-documented order to increase productivity. Future eff orts results from the study of the brown treesnake should also include the latest research for colonization prompted the development of the management, control, and eradication. Several Brown Treesnake Working Group (BTSWG) studies have been published that have yet to and the NABTSCT. The latt er is a multi-agency be widely implemented, such as the use of eff ort to develop a model for prevention of acetaminophen-baited mice over large areas brown treesnake invasion into continental North (Savarie et al. 2001, Shivik et al. 2002, Savarie America through education and awareness, as and Tope 2004), to kill brown treesnakes. well as rapid response assessments of potential There is, of course, no certainty that sightings (Henke 2002). The stakeholders brown treesnakes will be as devastating on a involved include government offi cials from the continental scale as it has been on the island U. S. Fish and Wildlife Service, U.S. Geological of Guam. However, on a local scale, new Survey, USDA/APHIS/Wildlife Services, and invasions could be as devastating. Certainly, various groups from the public, including, brown treesnakes pose a threat to Hawaii but not limited to, herpetological groups, the and Florida, 2 geographic areas that are very Audubon Society, and the pet industry. These hospitable to many other invasive species groups develop a public awareness plan to (Offi ce of Technology Assessment 1993, Kraus alert people to the dangers associated with the and Cravalbo 2001). By 2008 there had been 14 brown treesnake, something that is missing for credible brown treesnake sightings in Hawaii a great many invasive species. By developing a and 3 confi rmed sightings in the mainland preemptive plan and refi ning control methods United States (Perry and Vice 2008), including on Guam, the NABTSCT hopes to prevent one in Texas in May 1993 (McCoid et al. 1994), another invasive species from entering the one in Anchorage, Alaska (Stanford and Rodda continental United States (Acquatic Nuisance 2007), and one in McAlester, Oklahoma, in Species Task Force 1996a). 2005 (S. H. Henke, NABTSCT, unpublished report). The most recent credible sighting on Future plans the mainland was in August, 2011, near San Complete eradication of brown treesnakes Antonio, Texas, but the snake was not captured from Guam is probably unrealistic (Rodda (S. H. Henke, North America Brown Treesnake et al. 1998). Thus, preventing further spread, Control Team, personal communication). which is still feasible, should be considered The increase in dispersal events has raised successful management. Therefore, increased the interest of military authorities on Guam research opportunities to refi ne and implement (McCoid et al. 1994). Management for brown prevention techniques are necessary. Continued treesnakes in Hawaii and on other at-risk islands research is needed, not only on Guam, but also in the Commonwealth of the Northern Mariana in regions considered under threat. Because Islands is diffi cult due to several uncertainties, the invasion success of brown treesnakes are including knowledge of the number of snakes dependent on ecological preadaptation (Shine currently located there and the minimum viable 1991a), it may be possible to further refi ne population size for brown treesnakes (Burnett defi nitions of areas that are most at risk. This 2007). Without proper management, islands in can be accomplished by focusing research on the Pacifi c are at high risk for receiving brown pathway analysis, potential species competition, treesnakes due to military restructuring and and resource-use in areas receiving shipments increased military movements in the region as from Guam, thus, focusing management, Guam increases in strategic importance (Pitt et rapid response, and education programs in al. 2010). Brown treesnakes • Kahl et al. 195
Pathway risk assessments and ecological Literature cited modeling are the cutt ing edge in invasive Aguon C. F., R. E. Beck Jr., and M. W. Ritter. 1999. species prevention (Hulme 2009). Pathway A method for protecting nests of the Mariana analysis allows researchers to defi ne the most crow from brown treesnake predation. Pages likely transport pathways to the most at-risk 460–468 in G. H. Rodda, Y. Sawai, D. Chiszar, locations, providing a focus for management of and H. Tanaka, editors. Problem snake man- those pathways on Guam and giving priority agement: the habu and brown treesnake. Cor- to inspections of those pathways, thus, making nell University Press, Ithaca, New York, USA. inspections more cost-eff ective. Climatically Aguon, C. F., E. W. Campbell III, and J. M. Mor- suitable areas for brown treesnakes within ton. 2002. Effi cacy of electrical barriers used the U.S. mainland are located in California, to protect mariana crow nests. Wildlife Society the Southwest, and the Southern Coastal Plain Bulletin 30:703–708. where temperature and precipitation most Aldridge, R. D., and A. A. Arackal. 2005. Repro- closely match those of the tropics (Rodda et al. ductive biology and stress of captivity in male 2007c, Wisniewski 2010). San Diego, California, brown treesnakes (Boiga irregularis) on Guam. receives more shipments from Guam than Australian Journal of Zoology 53:249–256. any other location in the continental United Aldridge, R. D., B. C. Jellen, D. S. Siegel, and S. States, and is, therefore, at relatively high risk S. Wisniewski. 2011. The sexual segment of for the potential transfer of brown treesnakes, the kidney. Pages 477–509 in R. D. Aldridge especially with the expected increase in and D. M. Sever, editors. Reproductive biology shipments due to military restructuring on and phylogeny of snakes. Science, Enfi eld, Guam (Wisniewski 2010). New Hampshire, USA. Spread of brown treesnakes from Australia Aldridge, R. D., D. S. Siegel, A. P. Bufalino, S. S. and New Guinea is much less likely due to Wisniewski, and B. C. Jellen. 2010. A multiyear lower densities there (Rodda and Savidge 2007). comparison of the male reproductive biology of Rödder and Lött ers (2010) projected climatic the brown treesnake (Boiga irregularis) from suitability for brown treesnakes in the Pacifi c Guam and the native range. Australian Journal and found that the highest suitability is located of Zoology 58:24–32. in the Commonwealth of the Northern Mariana Anderson, N. L., T. E. Hetherington, B. Coupe, G. Islands, Hawaiian Islands, Madagascar, New Perry, J. B. Williams, and J. Lehman. 2005. Caledonia, and the Fij i Islands. The island of Thermoregulation in a nocturnal, tropical, arbo- Oahu holds approximately 150,000 ha of brown real snake. Journal of Herpetology 39:82–90. treesnake habitat (Burnett et al. 2008). Burnett Aquatic Invasive Species. 2003. Aquatic invasive et al. (2008) found that it is economically species management plan, unpublished report. advantageous to actively search for a potential State of Hawaii, Honolulu, Hawaii, USA. population of snakes in Hawaii, rather than to Aquatic Nuisance Species Task Force (ANS). wait for discovery. Alien species invasions are 1996a. Risk Assessment and Management one of the most serious problems hindering Committee: generic nonindigenous aquatic or- conservation programs, and, due to their likely ganisms risk analysis review process. Aquatic irreversibility, have the potential to undo other Nuisance Species Task Force, Washington, conservation programs (Howarth 1999). D.C., USA. Aquatic Nuisance Species Task Force. 1996b. Acknowledgments The brown treesnake control committee. We thank G. Perry (Texas Tech University), Brown treesnake control plan. Washington, L. Brennan (Caesar Kleberg Wildlife Research D.C., USA. Institute), and M. Tewes (Caesar Kleberg Wildlife Brooks, J. E., P. J. Savarie, and J. J. Johnston. Research Institute) for their professional review 1998. The oral and dermal toxicity of selected and helpful comments on this manuscript prior chemicals to brown treesnakes. Wildlife Re- to submission. We thank the U.S. Fish and search 25:427–435. Wildlife Service for funding. This is manuscript Brown Treesnake Working Group. 2004. Allocation number 11-111 of the Caesar Kleberg Wildlife and spending overview. U.S. Fish and Wildlife Research Institute. Service, Houston, Texas, USA. 196 Human–Wildlife Interactions 6(2)
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ces: temporal variation in body condition and (Python molurus) in southern Florida. Biologi- corticosterone in brown treesnakes (Boiga ir- cal Invasions 13:1493–1504. regularis). General and Comparative Endocri- Wisniewski, S. S. 2010. Preventing brown tree- nology 155:607–612. snake introduction to the continental United Weinstein, S. A., D. Chiszar, R. C. Bell, and L. States through education, awareness, and A. Smith. 1991. Lethal potency and fraction- risk assessment modeling. Dissertation, Texas ation of Duvernoy’s secretion from the brown A&M University, Kingsville, Texas, USA. treesnake, Boiga irregularis. Toxicon 29:401– Zalisko, E. J., and K. V. Kardong. 1992. Histology 407. and histochemistry of the Duvernoy’s gland of Weinstein, S. A., B. G. Stiles, M. J. McCoid, L. the brown treesnake Boiga irregularis (Colubri- A. Smith, and K. V. Kardong. 1993. Variation dae). Copeia 1992:791–799. of lethal potencies and acetylcholine receptor binding activity of Duvernoy’s secretions from the brown treesnake Boiga irregularis Merrem. SAMANTHA S. KAHL currently is a post-doc- Journal of Natural Toxins 2:187–198. toral research associate at the University of Tennes- Westerskov, K. 1952. Acclimatization of new game see in Knoxville. She earned her B.A. degree from Black- species. New Zealand Fishing and Shooting burn College in 2005, her M.S. 19:4–8. degree from Saint Louis Uni- Whittier, J. M., and D. Limpus. 1996. Reproductive versity in 2007, and her Ph.D. degree in wildlife science from patterns of a biologically invasive species: the Texas A&M University–Kings- brown treesnake (Boiga irregularis) in eastern ville in 2010. Some of her in- terests include ecological risk Australia. Journal of Zoology 238:591–597. assessment, amphibian and Whittier, J. M., C. Macrokanis, and R. T. Mason. reptile conservation and ecol- 2000. Morphology of the brown treesnake, ogy, invasive species management, and St. Louis Cardinal’s baseball. Boiga irregularis, with a comparison of native and extralimital populations. Australian Journal SCOTT E. HENKE (photo unavailable) is a re- of Zoology 48:357–367. gents professor and chair of the Animal, Rangeland and Wildlife Sciences Department at Texas A&M Uni- Wiedenmann, R. N., S. L. Post, M. R. Jeffords, and versity–Kingsville and a research scientist with the D. J. Voegtlin. 2007. Biocontrol for everyman: Caesar Kleberg Wildlife Research Institute. His re- public participation in a weed project. Pages search interests include wildlife disease and human– wildlife confl icts. 92–104 in C. Vincent, M. S. Goettel, and G. Lazarovits, editors. Biological control a global perspective. CAB International/AAFC 2007. MARC A. HALL (photo unavailable) is a su- Cambridge, Massachusetts, USA. pervisory wildlife biologist with USDA/APHIS/Wildlife Services on Andersen Air Force Base on Guam. Wilcove, D. S., D. Rothstein, J. Dubow, A. Phil- lips, and E. Losos. 1998. Quantifying threats to DAVID K. BRITTON is the southwest regional imperiled species in the United States. Biosci- aquatic nuisance species coordinator for the U.S. ence 48:607–615. Fish and Wildlife Service. He helps protect our na- Wiles, G. J., J. Bart, R. E. Beck Jr., and C. F. tion’s fi sheries and con- Aguon. 2003. Impacts of the brown treesnake: serve aquatic resources by patterns of decline and species persistence coordinating invasive-spe- cies prevention and control in Guam’s avifauna. Conservation Biology strategies across state and 17:1350–1360. regional boundaries. He has been a participating Wiles, G. J., and A. P. Brooke. 2009. Conservation member of the North Amer- treats to bats in the tropical Pacifi c islands and ica Brown Tree Snake Control Team since 2007. He earned his Ph.D. degree in quantitative biology from insular Southeast Asia. Pages 405–459 in T. H. the University of Texas–Arlington in 2005. Fleming and P. A. Racey, editors. Island bats: evolution, ecology, and conservation. Univer- sity of Chicago Press. Chicago, Illinois, USA. Willson, J. D., M. E. Dorcas, and R. W. Snow. 2011. Identifying plausible scenarios for the establishment of invasive Burmese pythons