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Non-Indigenous Species: Government Response
CONTENTS FOR THIS SECTION
Control of the BTS and prevention of its spread are unlikely to show much success without public awareness of the problem. The military in Guam has instituted BTS awareness programs even among personnel without direct responsibility for snake control and eradication. A video showing the general problems of non-native species introduction has been instituted on certain flights to Hawaii from the mainland; these are not shown on flights from Guam. The film is said to be quite friendly, and may play an important role in preventing innocent introductions by the unsuspecting; but some suggest that a film with more emphasis on penalties may be required to stop hobbyists and professionals travelling with illegal seeds, ornamental or edible plants, aquarium fish, exotic pets, etc. These shipments are potentially harmful in themselves, and might inadvertently conceal young BTSs.
Federal funding for the Brown Tree Snake Control Task Force (CTF) and the control plan it produced is complicated due to transfers of funds among various federal agencies. Some order is given to this confusion by the control and prevention plans in the report. One participant in the CTF said that as long as funds are spent according to the plan, then the goals of the plan should be achieved, regardless of where the appropriations are initially allocated. For example, DOD (with less expertise on pest control) has transferred some of its funding to APHIS to cany out control efforts on military bases. Table 1 shows recent funding levels for various federal agencies involved in control and prevention of BTSs.
Lack of funding, especially at early phases of a threatened introduction, is a problem in preventing the spread of nonindigenous species.9 Moreover, the support of some agencies for funding for their control has been lukewarm. For the BTS, the number of dog'handler teams available for inspecting cargo, aircraft, and ships is insufficient to inspect more than about half of the shipments at risk, and the effectiveness and training of these teams has not been evaluated thoroughly.
The location of airports receiving shipments from Guam and other places where the BTS is established clearly affects the risk of the spread of this species. An escaped BTS in Anchorage, Alaska, is unlikely to survive, but an escapee at Homestead Air Force Base outside Miami, or at the commercial airport in San Juan, Puerto Rico, well might. Less recognized is that habitat in the area immediately surrounding an airport may encourage or inhibit the spread of the species: surrounding acres of parking lot may aid detection, while nearby tropical or subtropical forest may increase the chance that the escapee may survive. A proposed expansion of a commercial airport on Maui to permit the arrival of long-range flights could be near suitable snake habitat and therefore present a greater risk of introducing this or other non-native species to the state. Ironically, if the expansion airport resulted in establishment of a BTS population on Maui or other islands, the expansion could severely harm the Hawaiian tourism industry it was intended to support. An Environmental Assessment of the expansion is currently underway and is expected to evaluate pest problems. Congress may wish to consider oversight on the location and design of civilian and military airports and docking facilities at risk of receiving brown tree snakes in cargo and baggage.
A larger issue is the ad hoc federal management of introduced species. Fragmented authorities and a tendency to respond seriously at a point well after prevention -- when measures are most likely to be biologically and economically efficient -- are quite clear in the various agencies' response to this species over the last 40 years. The fairly strong federal, state, and local response shown for the BTS (with both breadth and significant financial commitment) in the last 5 years or so was preceded by decades of inability of field experts to gain the attention of higher authorities. If the efforts currently shown in preventing BTS introduction in Hawaii had been made 30-40 years ago in Guam, the effects on Guam's economy might have been much reduced, and the snake might have been prevented from becoming established. The BTS's spread to the CNMI could, according to some observers, be at a similar stage. If the lessons of Guam have been learned, then control methods will be brought to bear very quickly on the very small BTS population--literally, an overkill, at least for the time being.
The OTA study cited above suggests that a response after the most economically efficient time is the rule rather than the exception. The general problem (of delayed responses) could appear on the congressional oversight agenda. In the BTS case, problems of federal response remain, since I?at current levels [research] will not be sufficient to develop the new techniques that will be required to meet brown tree snake control objectives."10 Some techniques likely to offer higher chances of eradication (e.g., biological controls) require especially high research commitments.
Federal appropriations (actual and proposed)
(Source: Robert Peoples, Nonindigenous Species Coordinator, Fish and Wildlife Service. FY1998 numbers are estimates.)
To some extent, the OTA study noted a larger problem: the tension between eliminating the entrance of harmful species, and allowing the entrance of useful or desirable ones. At present, for intentional introductions, species are permitted to enter the country unless they have been shown to be harmful (particularly as agricultural pests), and placed on a list for regulation. The burden of proof is on those who would prevent introduction to show that a species is harmful. One federal regulator claimed that his agency feared being sued unless it could show quite clearly that a species posed a threat. Where harm is less clear, it seems likely that species will be allowed to enter and at least some of these will ultimately cause harm.
For unintentional introductions, certain paths and avenues can be identified, and opposition consists only of those who might be harmed by effects like delay or paperwork (rather than the purposes) of control measures. For one of these avenues, aquatic nuisances arriving via shipping, NANPCA is Congress' response. Other predictable avenues--air cargo, air passenger traffic and baggage, and commercial shipments of legal organisms--are less directly addressed, unless the importation clearly risks fairly direct harm to agriculture or human health. Where the threat is to a region's power grid, communications system, or other industries, preventive measures are minimal for many potential avenues of pest transport.
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 control; and (6) bounties and commercial exploitation. To apply any of these basic strategies to the control of the BTS, substantial knowledge of the snakes' behavior, biochemistry, prey preferences, diseases, or other aspects of its total biology may be essential. The pros and cons of these six strategies and their information gaps are described below.11
Baits and attractants may be used to draw an unsuspecting snake toward a potential food source or mates; once there, they can be counted, killed, or studied. Dead meat items, even dog food and pork spare ribs, have been consumed by free-ranging BTSs. Traps using caged live mice as bait are currently used to protect the perimeters of military bases in Guam. In some snake species, it is known that chemicals given off by snakes allow males and females to find each other; these chemical communication signals are called "pheromones." While no such chemicals are known for BTSs, their discovery could be a powerful species-specific control method. Difficulties with baits and attractants commonly include the need to check the traps and the need to prevent non-target species from being harmed by or interfering with the bait. Baits and attractants seem most promising when the area needing protection has a clear boundary or well-defined area, and a significant density of snakes. Sex pheromones hold the added possibility of functioning even when snake densities are low.
Substances might be found which kill BTSs or drive them from an area. Other snake species are known to try to evade certain substances including tear gas and gasoline. Obviously, these substances can be used only to a limited extent, and probably not over large areas. Fumigants seem especially promising in driving snakes from confined areas such as cargo containers and the like, provided that these containers do not require frequent human access. Recently, the Environmental Protection Agency has approved methyl bromide as a fumigant for this species. Light is also known to repel this nocturnal snake. Physical barriers are used to prevent the snakes from climbing onto telephone and electrical wires, among other things.
The use of traps is limited primarily by cost, time to service the traps, and inability of traps to control the snakes over a very large area. Various designs are used, and most are used in combination with some sort of bait. One type of trap consists of two chambers. The inner chamber holds a mouse that cannot be reached by a snake. The outer chamber has a funnel opening through which the hungry snake may enter but not exit. For confined areas such as cargo holds, buildings, etc., these traps have had notable successes. On the other hand, they have obvious drawbacks in an open situation with either abundant alternative prey or very low snake densities. Sticky traps used in rodent control have also been used on BTSs. Trapping methods are comparatively safe for humans, though they require some care when the snake in the trap is killed and removed.
There are no poisons registered specifically for snake control. Broadly toxic substances would risk harming other species. A poison would most likely have to be used in conjunction with baits to reduce the risk to pets, children, and other non-target organisms.
A biological control preys on, parasites, or causes disease in a targeted pest species. Ideally, it attacks that species and no others. Considerable knowledge of the BTS's basic ecology would be necessary to select a suitable control. Mongooses are often mentioned as potential snake predators. However, none of the known species of mongoose prey selectively on snakes. Where they have been introduced, they quickly turn to feeding on other species, often leading to further endangerment of native fauna. In the case of the BTS, mongooses are particularly unsuitable, since they are diurnal: nocturnal BTSs are likely to be hidden and inactive when the mongooses are hunting. The introduction of the king cobra, one of the very few truly selective snake predators, seems bound to cause objections.
A disease or a selective parasite seem like a particularly attractive option for the BTS, since in many instances, few or no other snakes live in the areas at risk of invasion 12, thereby decreasing the chance that the disease or parasite will attack a non-target species. Unfortunately, little or nothing is known about any diseases or parasites to which this species may be susceptible. Field work in the species' native habitat would be necessary to find them. Though the requisite research might be expensive, biological control holds out great hope for long term control.
Under a bounty system, someone is paid to catch the target species. (Commercial exploitation serves the same purpose.) However, the BTS is an aggressive, nocturnal, secretive, and somewhat venomous species. Hand-trapping would require training in the habits of the species. Capture at night would present serious logistic difficulties, especially if the snakes dropped below abundant levels. According to most sources, people generally find these snakes repellent, and high bounties would probably have to be paid to have a substantial effect. If BTS population levels dropped substantially, bounties sufficient to stimulate some people to continue to search for fewer and fewer snakes might encourage others to raise snakes in captivity to reap the bounty. In a worst case scenario, citizens of other islands with incomes below those on Guam might consider importing BTSs in order to reap the bounty fee.
Control of the BTS is really two related problems: eradication where that is possible, and reduction to bearable levels where eradication is not possible. No single method of snake control would be a panacea. None so far promises eradication under any conditions in which the snake is well-established, but several in combination and used indefinitely might reduce the Guamanian BTS population to tolerable levels. On the other hand, if many are used intensively and in combination they might succeed in eradicating the very small population on Saipan (CNMI) that is threatening to become established--if it is not already.
9 America's Least Wanted: Alien Species Invasions of U.S. Ecosystems. Bruce A.Stein and Stephanie R. Flack, eds. The Nature Conservancy. Arlington, Virginia, p.9.
10 BTS control plan, p.4.
11The discussion below draws heavily on p.18-20 in U.S. Dept. of the Interior. Fish and Wildlife Service. The Brown Tree Snake, Roiga irregularis, A Threat to Pacific Islands. Biological Report 88(31). Washington, DC: September 1988. The BTS Control Plan, cited above, was also used extensively.
12 One native snake, a burrowing blind snake, lives on Guam. One might speculate that its habitat is so different from that of the arboreal BTS that it might still be safe from the disease or parasite, even if it could theoretically be infected. Research would be necessary to determine the risk.
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