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Non-Indigenous Species: Government Response to the Brown Tree Snake and Issues for Congress

M. Lynne Corn
Specialist in Natural Resources Policy
Environment and Natural Resources Policy Division

Update May 2 1997

97-507 ENR


Species found abundantly outside of their normal range can represent an ecological and economic hazard as great or greater than the most far-reaching efforts to protect an endangered species. One current example is the brown tree snake (Boiga irregularis). This species invaded Guam from its native habitat in New Guinea and Australia in the 1950s. Its effects include not only major disruptions of electric power transmission, telephone service, military operations, computers, and tourism, but also devastation of the island's bird life. While only mildly poisonous, the snakes have bitten over 200 sleeping humans, including infants. Their secretive behavior predisposes these aggressive nocturnal snakes, which may reach 10 feet long, to stow away in ships and airplanes. Hawaii and the Commonwealth of the Northern Mariana Islands are at particular risk of invasion by this species, but Gulf Coast states, southern California, and Puerto Rico also face some risk. The issue for Congress is oversight of coordination among federal agencies and other institutions, and funding levels for research, control, and prevention of the spread of this species.



Economic Damage of Introduced Species
Federal Framework for Non-Indigenous Species
The Brown Tree Snake Example

NANPCA and the Brown Tree Snake
Arrival in Hawaii and Other Destinations?
Preventing the Spread of Brown Tree Snakes

Public Education

A Tangle of Appropriations

Current Controversies
Appendix: Basic Methods of Pest Control for the Brown Tree Snake 11

Baits and Attractants
Fumigants, Repellents, and Barriers
Biological Controls
Summary of Control Methods

Economic Damage of Introduced Species

Concerns over biodiversity have focused on the loss of species, but an abundant species where it does not belong can be an even more severe problem, not only biologically but also economically. Some plants, for example, have been notorious for years for causing both economic and ecological damage; kudzu, melaleuca, tamarisk, purple loosestripe, spotted knapweed, and Russian thistle are just a few examples of unwanted plants now creating ecological and economic havoc in large areas of the United States. Their damage includes lowering water tables, competing with other plant species, poisoning livestock, and increasing pest control costs. Introduced invertebrate pests are even more obvious: gypsy moths, Japanese potato beetles, fire ants, Africanized honeybees, and zebra mussels are among the most well-known. Introductions of vertebrate pests (e.g., walking catfish, cane toad, monk parakeet, starlings, and feral goats) are comparatively rare, aside from fishes.

In some of these cases (e.g., kudzu, melaleuca, gypsy moths, Africanized bees, zebra mussels, and starlings), the source of the introduction is either known or strongly suspected. Governments at all levels might have worked to prevent these introductions at the time, had they foreseen the damage these species would later cause.

For any introduced species, the range of control actions falls into six basic categories: (1) baits and attractants; (2) fumigants, repellents, and barriers; (3) traps; (4) poisons; (5) biological controls; and (6) bounties and commercial exploitation. The entire arsenal is unlikely to be used on any given species for a variety of reasons: lack of information needed to implement the approach; probable effects on non-target species; expense; and risk of providing economic incentives to spread the pest species to other locations, to name a few.

Federal Framework for Non-Indigenous Species

According to a study on control of non-native species by the Office of Technology Assessment (OTA), "(t)he current Federal framework is a largely uncoordinated patchwork of laws, regulations, policies, and programs. Some focus on narrowly drawn problems.... In general, present Federal efforts only partially match the problems at hand."1 One impediment, according to the study, is the federal focus on preventing the entry of specific agricultural pests rather than on species harmful to other sectors of the economy. (The brown tree snake (BTS-Boiga irregularis) does represent a threat to agriculture, but only indirectly: it feeds on birds, many of which may feed on insect pests.)

A second legal impediment to federal control of non-indigenous species, according to the OTA study, is that entry of harmful species may be prohibited under such laws as the Lacey Act and the Federal Noxious Weed Act only after the species has become established or caused damage within the United States. Paradoxically, the species covered by these laws can be legally imported until it is established, or at least classified as injurious, and added to a list, after which it may not be imported. Additions to the controlled list can be costly and time-consuming. Agencies fear lawsuits from interested groups such as agriculture and the pet, aquarium, and horticulture trades if they add a species to a controlled list without proof that it is harmful.2

The OTA study reviewed the responses of numerous federal agencies to the problem of unwanted introductions. These agencies included the Animal and Plant Health Inspection Service (and specifically its Animal Damage Control program) in the Department of Agriculture, the Defense Department, and the Fish and Wildlife Service (FWS) in the Department of the Interior. The study noted that "current shortcomings of the FWS law enforcement division might compromise expanded efforts" to play a greater role in regulating import of species. (The Biological Resources Division of the U.S. Geological Survey did not exist at the time of the OTA study, but its research efforts on the biology of some non-indigenous species are significant, and could lead to control techniques that might be implemented by other agencies.) Interestingly, the brief OTA discussion of the Department of Transportation did not mention the Federal Aviation Administration, whose role in regulating and managing commercial air traffic and airports might be critical for some pests.

Partly as a result of gaps in current law, but particularly because of the introduction of the zebra mussel into the Great Lakes and Mississippi River Basins, Congress passed the Nonindigenous Aquatic Nuisance Prevention and Control Act of 1990 (NANPCA; P.L. 101-646). The stimulus was the huge economic impact on utilities from the prolific zebra mussel due to the clogging of pipes, drains, etc. The impact of the zebra mussel on imperiled native clams and mussels was a relatively minor consideration.

The Brown Tree Snake Example

The brown tree snake (BTS) is native to New Guinea, various nearby Pacific Islands, and northeastern Australia. It lives in trees, where its agility helps it to find its chief prey of birds and small mammals.3 Its mild venom is used to immobilize its prey. The propensity of this nocturnal snake to seek daytime shelter from heat and sun in confined and hidden spaces, as well as its very high population densities on some islands, make it a pre-adapted stowaway. The species was introduced to Guam, an important Navy and Air Force base, in the 1950s. Most likely, it arrived as a passive stowaway in one of many military cargo ships moving material in the aftermath of World War II.4 By the late 1960s it had spread throughout the island. The BTS has wandered into, among other places, the wheel wells and cargo holds of large aircraft. And a cargo ship moving from Guam to Diego Garcia in the Indian Ocean was the most likely source for a brown tree snake seen at the military base there.

With an abundant prey base and lack of natural predators in Guam, the species has built populations as high as 12,000 per square mile. More than 200 people have been treated for bites, with attacks occurring primarily on people asleep in their beds. Occasionally, babies have been attacked, and numerous small household pets have been eaten. By crawling on electrical lines, the snakes cause frequent power outages, sometimes affecting the entire island, both military and civilian facilities.

Why Islands are Vulnerable to Biological Invasions

In general, island species are more vulnerable than mainland species when non-native species are introduced, due to the island species' evolution in relative isolation from mainland predators, diseases, and/or competitors. The more remote the islands, the more isolated and therefore vulnerable they are. Birds, for example, that have lived for millennia on a snakeless island, usually lose their mainland relatives' defenses, such as hidden or inaccessible nests, fear of snakes, attacking snakes near their nests, etc. Perhaps no islands are more vulnerable to introduced species than Hawaii, the planet's most isolated archipelago. Hawaiian fauna is under siege from introduced species of birds, mosquitoes transmitting avian malaria (formerly absent from the islands), rats, and other non-natives.

On average, these outages occur once every four days; in 1996, the snakes caused over 170 outages on the island. Results include spoiled food, computer failures, increased business losses, etc. The decline of the island's bird fauna seems likely to increase crop loss to insects and reliance on more costly chemical controls. Once established, the BTS has proven impossible to eradicate and extremely difficult to control.

The snakes first came to the attention of mainland biologists after puzzling reports of the crash of the island's bird populations. Eventually tracing these crashes to the BTS, biologists now lay the loss of at least 9 of the island's 11 native land bird species (including some found nowhere else) on the snake.

In a comprehensive 1988 report on the BTS, the FWS considered the six basic methods of pest control for their applicability to this species.5 (See Appendix, p.11, for a discussion of these methods as applied to the BTS.) None is considered fully effective at control, much less eradication. The FWS report holds out far more hope of preventing dispersal to other islands of the Pacific than that the snake can be eliminated where it has established a foothold. An appendix in the FWS report outlined a plan for control and eradication of the BTS with detailed research proposals and sketchy plans for actual control of the snake--reflecting the low level of basic knowledge about this pest. To date, all approaches have been deemed to suffer from lack of funding.

NANPCA and the Brown Tree Snake

Though the BTS is not aquatic, one provision (16 U.S.C. 4728) of NANPCA gave authority to a Task Force to "undertake a comprehensive, environmentally sound program in coordination with regional, territorial, State and local entities to control the brown tree snake (Boiga irregularis) in Guam and other areas where the species is established outside of its historic range.' It is interesting to note that, taken at face value, this wording conveys no authority to prevent the introduction of the BTS, an endeavor which seems far more likely to succeed than efforts to control the species after it is established. More general provisions for the Task Force arguably allow attention to problems of preventing the further dispersal of the snake. Certainly the final report on a BTS control plan emphasizes prevention.

The resulting Task Force, chaired by FWS 6, has federal representatives from the Geological Survey, the Department of Defense (DOD), and APHIS. The Guam Department of Agriculture, the Hawaii Department of Agriculture, the Department of Land and Natural Resources of the Commonwealth of the Northern Mariana Islands, and the University of Guam Marine Laboratory are also members. Initial implementation of a number of BTS provisions of NANPCA by the Task Force was sketchy or delayed due to inadequate funding, failure to allocate sufficient staff, and legal conflicts in chartering the Task Force (e.g., compliance with the Federal Advisory Committee Act). But the Control Plan (cited above) has been completed, and agencies are generally following its outline.

Arrival in Hawaii and other Destinations?

Hawaii, especially Oahu, is very much at risk for the next invasion of the BTS, as is the Commonwealth of the Northern Mariana Islands (CNMI--a U.S. territory), according to the Control Plan, as well as most experts. If the snake were to become established on Oahu, its spread to the other islands of the state would seem inevitable, given the snake's behavior and the heavy traffic among the islands. If successful in its invasion, the snake's effects on the islands' power grids, computer systems, tourism, etc., while difficult to estimate, would certainly be substantial, based on the Guam experience.

Both military and civilian flights between Hawaii and Guam occur daily, providing potential for snake stowaways. Despite a prevention program, BTSs have been found on at least seven occasions in Hawaii (at Honolulu International Airport, Barbers Point Naval Air Station, and Hickham Air Force Base), on or near aircraft arriving from Guam. To date, their dispersal has been prevented.

Military and civilian cargo ships are also potential sources of transport of the BTS, not only to Hawaii and the CNMI, but also to the mainland. One snake was found in a cargo of military household effects arriving in Texas. Potential mainland habitat includes portions of southern California and Gulf Coast states, as well as much of Central and South America. (See figure 1.) Mainland states, with well-established populations of snake predators (e.g., certain bird species, and perhaps some weasels) might be less likely to see the huge snake population densities now seen in Guam, or potentially threatening Hawaii and the CNMI, but this possibility is not proven. Specific areas at risk on the mainland are those warm habitats containing trees and shelter near airports, and dock facilities that receive foreign passengers and cargo from affected areas.

Preventing the Spread of Brown Tree Snakes

The U.S. Customs Office and the Animal and Plant Health Inspection Service (APHIS) together inspect about 15% of international air carrier baggage entering or passing through Honolulu. In contrast, APHIS x-rays 100% of the baggage bound from Honolulu to the mainland. This reflects a greater concern for Hawaii as a source of bio-contamination, than as a recipient of it.7

Figure 1. States with some potential habitat for brown tree snake.

(source: The Nature Conservancy. America's Least Wanted. 1996.p.21.) Puerto Rico (not shown) would also be a potential BTS habitat.

(See box.) This asymmetry of protection efforts is a source of concern to Hawaiian citizens.8 Military and civilian maritime cargo to Hawaii is irregular, and shipments undergo irregular APHIS examination.

Trained detection dogs are important in screening incoming planes. In June 1996 in Hawaii, there were two such fully-trained dog (handler teams and two other teams in training. The number of teams is not adequate to provide full inspection of all incoming military and commercial aircraft and cargo, nor regular inspection of maritime transport. Five to eight dog/handler teams are working in Guam, but are not sufficient to provide full coverage for all outgoing flights and shipments. The effectiveness of these dogs either on Guam or Hawaii has not been established. According to FWS employees, the dogs sometimes miss snakes later found by humans with flashlights, and sometimes give false alert signals for non-target species, such as rats.


1 U.S. Congress, Office of Technology Assessment. Harmful Non-indigenous Species in the United States. OTA-F-565. Washington, DC: U.S. Government Printing Office, September 1993. p.163. (Hereafter referred to as the "OTA study.")

2 Examples of laws which rely on listing noxious species as a prelude to exclusion or regulation are the Federal Noxious Weed Act (7 U.S.C. 2809, administered by APHIS), and the Lacey Act (16 U.S.C. 3371, administered by FWS).

3 For a discussion on BTS biology and effects, see report of The Brown Tree Snake Control Plan, report of the Brown Tree Snake Control Committee, Aquatic Nuisance Species Task Force. June 1996. 55 p. For photographs and maps, see (no period at end of web address).

4 U.S. Dept. of the Interior, Fish and Wildlife service. The Brown Tree Snake, Boiga irregularis, A Threat to Pacific Islands. Biological Report 88(31). Washington, DC: September 1988. p.5. (Hereafter referred to as BTS Threat.)

5 BTS Threat. 36p.

6 Perhaps due to the heritage of authorization in a law intended primarily to prevent aquatic nuisance species, the BTS program in FWS is under the Assistant Director for Fisheries.

7 OTA study, p. 250.

8 For example, see remarks of Sen. Daniel Akaka, Congressiona1 Record, April 8, 1992, p.S5085-S5092.