Boiga irregularis is distributed across many locales including Australia, Guam, Indonesia, the Northern Mariana Islands, Papua New Guinea, and the Solomon Islands (Rodda et al. 1999, Rodda and Savidge 2007, Rodder and Lotters 2010). The brown tree snake is regarded as a habitat generalist and is known to occupy natural forests, grasslands, agricultural areas, and urban areas (Rodder and Lotters 2010).
In Australia, Boiga irregularis does not inhabit the interior of the continent even though temperatures are similar to the coastal ranges it occupies. Evidence indicates that a lack of prey, relative absence of trees, or desiccation may explain why it is not present in this area (Rodda and Savidge 2007). Instead, the brown tree snake inhabits humid areas close to the sea (Rodder and Lotters 2010). Seasonal activity is limited in the summer months with the most precipitation (Shine 1991). Also, relative humidity levels lower than 60% prevent the brown tree snake from shedding properly (Rodda et al. 1999). Precipitation and humidity are thus good predictors for distribution (Rodda and Savidge 2007, Rodder and Lotters 2010).Temperature may be a limiting factor for Boiga irregularis as higher temperatures affect reproductive success (Mathies and Miller 2002). Low temperatures corresponding to hard frosts also indicate a distribution predictor with the brown tree snake’s southern limit being the northern limit of hard frosts in Sydney, Australia (Rodda and Savidge 2007). Similarly, the upper elevation limit in New Guinea is the altitudinal frost line (Greer 1997).
The brown tree snake now occupies several Pacific islands due to being accidentally transported on naval vessels. Presently, similar means of anthropogenic dispersal are increasing the snake’s distribution via stowaways on cargo ships and aircraft (Rodder and Lotters 2010, Buden et al. 2014). Boiga irregularis has been detected but not yet established on the Cocos Islands, Diego Garcia, the Hawaiian Islands, Indonesia, Japan, New Zealand, the Saipan, Tinian, Malaysia, the Marshall Islands, the Federated States of Micronesia, the Ryukyu Islands, Spain, Taiwan, Texas and Wake Island (Mito and Uesugi 2004). Rodder and Lotters (2010) created a Climate Envelope Model which identifies the Northern Mariana Islands, Hawaiian Islands, Madagascar, New Caledonia, and the Fiji Islands as potential habitats the brown tree snake may thrive in.
The brown tree snake is an arboreal species though still uses the ground as a means of predating (Rodda and Fritts 1992). The amount of available moonlight influences the choice of microhabitat the snake uses. As moonlight levels increase, the canopy is used more for hunting while the ground is used more when moonlight levels are lower despite prey availability (Campbell et al. 2008). When traversing along the ground, Boiga irregularis crosses roads at a low rate. Instead, the brown tree snake tends to move parallel to roads, potentially indicating an aversion to roads or a high prey density next to roads (Siers et al. 2014).
The negative ecological impact of Boiga irregularis on Guam’s avifauna, reptiles, and small mammals was made clear by Savidge (1987). She demonstrated that the brown tree snake was responsible for avian range contraction and the disappearance of several vertebrate species. Since that time, further evidence of the snake’s impact on Guam’s biodiversity has been produced and several management plans have arisen.
Goals:
Shared goals among management plans are: 1) to prevent the spread of brown tree snakes on Guam and their dispersal to other islands 2) brown tree snake population control 3) and ecosystem restoration and protection of wildlife from brown tree snake predation (BTSCC 1996, Colvin et al. 2005, USDD 2008). Brown tree snake interdiction is identified in several recovery plans as necessary to prevent further decline in species population numbers such as the Rota bridled white-eye. If the snake is introduced to the island of Rota, the recovery of the bridled white-eye may be impossible as the remaining populations would be at risk of extinction (Amidon et al. 2006). Other management goals include creating more effective and environmentally safe control methods, the reduction of brown tree snake disruptions of electrical equipment, and a decrease in human-snake encounters (BTSCC 1996). Additionally, increasing public education and awareness to reduce further risk of ecological and economic damages as a result of the brown tree snake is another goal (BTSCC 1996, USDD 2008).
Management Techniques (Implemented and Proposed):
Instances of brown tree snakes boarding cargo ships as stowaways departing Guam to other islands has been documented (Buden et al. 2014). The most likely stowaway destinations of caught brown tree snakes are the Hawaii and Micronesian Islands (Engeman et al. 1998). The consequences of dispersal to other Pacific islands like Tinian, which have declining bird populations, would be devastating to the island’s ecosystem (Camp et al. 2012). To prevent Boiga irregularis from spreading to other islands, management techniques such as the use of canine detection have been implemented. The use of detector dogs have proved to be highly efficient at locating and preventing snakes from leaving ship ports (Engeman et al. 2002). Other techniques implemented include hand capture and trapping around key dispersal zones. Perimeter trapping around forest perimeters was found to be effective and may prevent snake dispersal from Guam (Engeman et al. 2000). A potential, though unexplored, technique could be using artificial moonlight around critical areas like airports and shipping ports as brown tree snakes tend to stay in tree canopies as moonlight increases (Campbell et al. 2008).
To control Boiga irregularis populations in Guam, several techniques have been suggested including the introduction of parasites and diseases, reproductive inhibition, toxicants and attractants, and manipulation of non-native prey species (BTSCC 1996). Of these, toxicants have been a major focus for population control. Acetaminophen, which is deadly to brown tree snakes, was found to quickly reduce brown tree snake numbers on Guam (Savarie et al. 2001). Aerial delivery of baits that were designed to entangle in the forest canopy resulted in high uptake, making bait delivery more efficient (Shivik et al. 2002). Furthermore, acetaminophen baits were found to not pose a risk to non-target populations, making this method more favorable (Johnston et al. 2002). Lastly, juvenile brown tree snakes are highly active predators and move large distances, which may make them vulnerable to snake traps or toxic bait (Lardner et al. 2014).
In an effort to protect endangered species from brown tree snake predation, techniques such as erecting electrical barriers around Mariana crow nests have been undertaken (Aguon et al. 2002). To increase public awareness of the negative impact of Boiga irregularis, an increase in the production and dissemination of public educational materials is needed (BTSCC 1996).
