Photo of Brown Tree Snake photo courtesy of USGSPhoto of Brown Tree Snake courtesy of USGS

Biology and Control of Brown Tree Snakes

Principal Investigator: Julie Savidge (CSU Fishery and Wildlife Biology)

Project Officer: Gordon Rodda

Graduate Student: Valerie Boyarski

Project Start Date: 08/21/00

Expected Completion Date: 10/01/05

Funding Agency: U.S. Geological Survey (RWO 63)

Introduction: The brown treesnake, Boiga irregularis, a native of Indonesia, New Guinea, the Solomon Islands, and Australia, was accidentally introduced to Guam following World War II. Since that time, it has spread throughout the island and has extirpated most of Guam’s native terrestrial vertebrates, including bats, lizards, and virtually all of the island’s forest birds. In addition, brown treesnakes in Guam have caused more than a thousand power outages, damaged agricultural interests by preying on poultry, killed many pets, and bitten numerous children. Brown treesnakes have been discovered on Hawaii, the Northern Mariana Islands, various islands “freely associated” with the US, and in the US mainland. The danger of invasion on other islands and subsequent impacts similar to those observed on Guam is high. Results of this research will be used for controlling brown treesnakes on Guam and for prohibiting their spread to other suitable locations.

Objectives: (1) Quantify the structure and dynamics of brown treesnake populations, (2) evaluate features of the snake’s biology and reproductive ecology that provide opportunities for control of the population, and (3) determine the physiological limits of brown treesnake temperature tolerance and assess the attributes of the snake’s thermal biology that may be used to limit spread of the snake or control populations.

Progress: An Environmental Assessment was prepared for the closed-population project and approved by the Air Force. The site was identified, surveyed and the 5-ha double-sided snake exclosure/enclosure was constructed through an independent contractor, and completed on 7 May 2004. It is being monitored using a series of visual transects and a trapping array, as well as a combination of the two. As of 4 March 2005, there is confidence that very few undetected individuals exist within the population. This has provided the project with an early idea of a size distribution for the entire population within the enclosure.

The graduate student studying detection of brown tree snakes (BTS) at low densities completed his Masters Degree in 2004 for work done on this study. Rodenticide was used to manipulate the density of exotic small mammals on two 4-ha plots. BTS movements and capturability were compared with two adjacent but unmanipulated 4-ha plots. Results indicate a sharp increase in capture success. Additionally, a study of daytime refugia was conducted in relation to this study. In savanna habitat, snakes appear to be using burrows of exotic small mammals. A second M.S. student, evaluating BTS activity patterns in response to abundances of an introduced skink, successfully completed her second field season.

Results/Products:
Boyarski, V., G.Rodda, and J.Savidge. 2004. Evaluation of harmonic radar for monitoring movements of brown treesnakes, Boiga irregularis, on Guam. Herpetological Review. Submitted.

Gragg*, J., J. Savidge, and G. Rodda. 2003. An investigation of rodenticide use for management of brown treesnakes. Western Section Wildlife Society, Invasive Species Symposium, Sacramento, CA. (Abstr.)

Gragg, J., J. Savidge, and G. Rodda. 2004. Prey-base reduction for management of Brown Treesnakes. Brown Treesnake Technical Committee Meeting, Honolulu, HI (Abstr)

Gragg, J., G. Rodda, J. Savidge, G. White, K. Dean-Bradley, A.Ellingson. 2004. Site fidelity and capture probability of Brown Treesnakes (Boiga irregularis) subjected to rodent reduction. Journal of Herpetology. Submitted.

Gragg, J. 2004. Rodent reduction for enhanced control of brown treesnakes (Boiga irregularis). Master’s Thesis. Submitted.

Tyrrell, C.L., M.T. Christy, D.L. Vice, G.H. Rodda and J.A. Savidge. 2004. Introduced frog species on Guam: potential impacts on brown treesnakes, Brown Treesnake Technical Committee Meeting, Honolulu, HA (Abstr).