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Control and Containment Strategies on Guam: Physical Barriers
Canine Detection Teams |
Habitat Modification |
Physical Barriers |
Trapping |
Visual Searches |
Adapted from Department of the Interior "Integrated Pest Management Approaches to Preventing the Dispersal of the Brown Treesnake and Controlling Snakes in Other Situations" 1999.
Barriers to contain or prohibit the movement of brown Treesnakes are significant devices for ongoing use in control and interdiction efforts. A variety of materials and barrier styles have been tested for durability and practicality in different situations. Recently tested designs include permanent barriers constructed from vinyl or concrete panels that are resistant to physical deterioration and damage by rats, machinery, vandals, and high winds. An example of this technology is being applied on Tinian where a concrete snake enclosure barrier was constructed in association with a recent construction project by Voice of America contractors.
A less permanent design for barriers has also been tested using hardware cloth instead of concrete panels. This type of barrier is less expensive and can be fitted onto existing chain-link fences, but is also less durable in the face of high winds and oxidation, hence not likely to last more than 3-5 years. A hardware cloth enclosure was constructed in the seaport of Rota to reduce risks to that island from snakes arriving in ship cargo. A similar barrier was constructed on Guam around the released rail habitat to prevent snakes from re-invading the 22.5-ha area after a thorough effort was completed to remove snakes. In practice, the reduction in snake numbers was only possible after installation of the barrier.
Temporary barriers have been used in association with large military training operations on Guam and in the Commonwealth of the Northern Marianas Islands. While a variety of strategies for use of barriers are possible and will surely find wider application in the near future, research is focused in four main classifications of barrier possibilities including exclosures, enclosures, linear barriers, and temporary barriers.
Exclosures
As a complement to visual and canine searches of cargo, exclosures bounded by snake barriers can be used to prevent the risk of snake contamination prior to material being shipped off Guam. By maintaining snake-free areas and storing material known to be free of snakes in such areas, the problems of timing canine or visual searches can be reduced, allowing inspections at optimal times for dog performance, dispatch schedules, or other logistic factors. Storage of materials in exclosures would reduce the necessity of re-inspection in the event of delayed shipments, etc., or eliminate the need for inspections for material transiting through Guam from other snake-free areas. Exclosures also offer considerable promise for the protection of wildlife and the habitats in which endangered species occur.
Enclosures
The use of snake barriers to reduce the likelihood of snakes escaping
from cargo into vegetated habitats on otherwise snake-free islands could
be an important addition to interdiction efforts. By coordinating
placement of high-risk cargo in snake enclosure areas until it can be
adequately disassembled, visually inspected, or fumigated, any snakes
that may exit will be prevented from escaping outside the containment
area. An enclosing barrier increases the probability of capture
using visual searches, traps, or features lethal to snakes included in
the barrier design. This strategy allows searches and trapping efforts
to be concentrated in a relatively small area rather than being spread
throughout the island or large areas in the vicinity of the port or transportation
facilities. Although control of snakes inside enclosures using visual
searches, trapping, and detector dogs would be possible, further protection
could be incorporated by electrifying the barriers. This would deliver
electrical shocks to snakes attempting to climb on the barrier without
adding risk to humans or other vertebrates. While a lethal electrical
barrier might not be desirable on Guam where large numbers of snakes could
foul and therefore incapacitate an electrical control device, the lower
numbers of snakes likely to contact such a barrier on other islands would
allow its use and thereby reduce reliance on trapping and hand capture
efforts. Yet, even with the best enclosure, it can only benefit
control programs if sufficient efforts are expended inside the barrier
to interdict snakes temporarily confined within it. These enclosures
also must be protected and prevented from being degraded through physical
damage by machinery, becoming fouled by vegetative growth, or from falling
into disuse because of failure to overcome space or logistical problems.
Properly used and maintained, enclosure barriers would greatly increase
the chances of intercepting arriving snakes, while additionally reducing
costs in terms of funds and manpower needed for inspections when barriers
are not deployed. By allowing inspections and control efforts to
be partially unlinked from transportation schedules, personnel and detector
dog teams can be used more effectively with decreased risk of escapes.
Linear Barriers
The potential exists for barriers
to reduce the movements of snakes from high-risk situations where abundances
may be high and accessibility to control personnel low (limestone cliff-lines,
karst substrates, hazardous waste areas, or landfills). These non-enclosing
barriers would be positioned between potential sources of snakes and transportation
facilities to reduce the numbers of snakes that would need to be trapped,
detected, and captured using other techniques. Location of traps,
perpendicular wings, and other devices at the ends of linear barriers
could serve to interdict any snakes moving along the barriers and reduce
the risks posed by a linear rather than enclosing design.
Temporary Barriers
These sloping barriers were originally designed to allow rapid deployment over sizable areas in response to needs for large cargo dispatch or staging areas during military training exercises, but they could also function in emergency control operations (e.g., snake sightings on islands on which the snake is not yet established). By encircling the area most likely to contain one or more snakes sighted, the emergency response could be better focused and sustained with a reduced chance that the snake or snakes will move out of the area without being intercepted. Although detector dog teams are not known to function in natural areas, intensive searches in areas prescribed by barriers might increase the potential for successful detections.
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