Control of Introduced Species
Introduction
One of the most serious consequences of human colonization of previously undisturbed habitats is the host of exotic species which usually accompanies settlement. In the West Indies, introduced exotic mammals include the black or roof rat (Rattus rattus), the Norway rat (Rattus norvegicus), the Indian mongoose (Herpestes javanicus [=auropunctatus]), the feral house cat (Felis catus), the domestic dog (Canis familiaris), the European pig (Sus scrofa), and the feral goat (Capra hircus). Each of these animals directly threatens wild iguanas at some stage of the life cycle, and some constitute a significant threat at every stage.
It must be realized that there is a wide gap between short-term control and complete eradication of exotic species from an area. In order for reduction of populations of exotics to have a positive long-term effect on iguana populations, either complete eradication or perpetual control programs are necessary. Eradication programs are most likely to be successful on small islands (<20ha), where some other stressor, such as lack of standing water, is also present. Control programs can thus be initiated when population numbers are at a natural low. For example, lower levels of food availability during the dry season increase the chances of an exotic consuming a poisoned bait.
Many managers of island ecosystems have had to deal with the problems of introduced predators. The most successful outcomes have been the result of a fully integrated predator management strategy which utilized a variety of control options. These are usually a combination of four basic methods: shooting, poisoning, trapping, and disease. There are always some animals in a target population which will be gun-shy, trap-shy, bait-shy, or poison and disease resistant. Thus, managers must use every means at their disposal to reduce or eliminate the target species. The efficacy of each of these methods varies with the target species and the site at which control is being attempted.
A common fault of unsuccessful exotic animal control programs is the failure to realize the extent of commitment, both in manpower and financial resources, necessary to eliminate a target species. Removal of any exotic from an ecosystem is a difficult and time-consuming task, requiring relentless attention to detail. In a typical shooting or trapping effort, the last few animals require the most effort to remove, as contact decreases in proportion to population size. In addition, the remaining animals may have developed an aversion to any type of human activity, making shooting, trapping, and poisoning much more difficult than they were initially.
It is also an inescapable fact that any exotic animal control program will face stiff opposition from some elements of society, and managers may find that themselves confronted with a well-organized, well-funded adversary that is inalterably opposed to any killing of any animal species for any reason, even if the failure to eradicate that species means the death of an entire ecosystem. The best basis for defense of any control program for exotics is a well-defined plan that absolutely minimizes any danger to humans or non-target species.
Advantages and disadvantages of various control options
Trapping
Trapping does not require constant effort, but may require considerable expense in setting up the trapline due to the need to clear vegetation for trails, transects, or grids. Live traps are often heavy and bulky, and thus are difficult to transport to interior areas where roads or trails are not available. Trapping has the major disadvantage of putting non-target species in peril. Trapping in an area populated by iguanas would have to be performed at night, with sets prepared just before dark, and traps disabled before daylight. Trapping under these conditions would be ineffective for diurnal predators such as dogs and mongooses, but could be very effective for cats. Attempts to trap larger animals such as feral pigs and goats are generally not cost-effective.
Shooting
Shooting can be a very effective form of control when the target species is large and there is minimal vegetative cover. It is most useful when there has been no previous history of hunting in the target area, and when there is no human habitation of the target area. It is generally low-cost in terms of equipment expenditures, but requires a very heavy manpower commitment. It is less effective at night.
Poisoning
Poisoning can be an effective form of control for certain exotic predators, especially as an adjunct to trapping. However, poisons are not free of disadvantages. Many poisons are toxic to fish, birds, other wildlife, and domestic animals, and pose a secondary hazard to birds of prey and carnivorous mammals. Carelessly placed baits could potentially be consumed by iguanas. Accidental poisonings can be avoided by used of bait boxes with small entry holes that exclude iguanas. Because of their hazardous nature, some poisons may be controlled by federal or local use restrictions, and may require permits for use.
Disease
Introduction of pathogens or parasites can be useful in reducing host fecundity and lowering population density of exotic species. However, such programs by themselves are unlikely to completely eradicate host populations. The major concern with this type of control program is that populations of endemic species may become infected. To ensure that this does not occur, it is important to choose a highly specific pathogen (Dobson 1988).
Black and Norway rats
The black rat is one of the most destructive introduced predators in the West Indies. Its invasion of the Greater Antilles probably began with its introduction to Hispaniola following the wreck of the Santa Maria at La Navidad in 1493, and it has continued to spread since then (Allen 1911; Atkinson 1985). Atkinson (1985) documented the invasion of western Europe by the Norway rat in the early 1700s. As a result, the Norway rat became the dominant rat in European ports, and thus the most common ship rat on sailing ships of that era. Between 1700 and 1830, Norway rats successfully invaded many islands.
Rats are extremely adaptable in their food preferences, can exist for long periods without water, and have an exceptionally high reproductive rate. Although approximately 95% of the diet consists of plant material, they are known to prey on invertebrates (Strecker et al. 1962), amphibians and reptiles (Whitaker 1978), and birds (Austin 1948; Atkinson 1985; Johnstone 1985). Rats pose an extreme hazard to many endangered species in the West Indies. Because Norway rats are significantly larger and more terrestrial than black rats, they potentially pose a more serious threat to iguana nests. However, Norway rats are far more common in urban and suburban areas, whereas outlying cays are almost exclusively populated by black rats. D. Nellis (personal communication) has evidence that the mongoose, a terrestrial predator, may effectively exclude Norway rats from natural areas in the Virgin Islands. Although many natural predators such as raptors and the West Indian boas prey on rats, the rate of reproduction for rats is so high that population numbers easily keep pace with any losses to predation. This has led to several past attempts to control rats, many of which have failed because of high rat immigration rates and lack of adequate resources and commitment.
The most effective poisons currently used today to control rats are the anticoagulant compounds such as brodifacoum and bromadiolone. The active ingredients are imbedded in a paraffin matrix which is relatively impervious to weather. Ten gram blocks of poison are easily carried and distributed, remain palatable for long periods, and do not cause bait-shyness. Rats may consume a lethal dose in one feeding. The most successful rat poisoning attempts are those carried out during the dry season, when foods such as fruits, buds, and tender leaves are at a minimum. For complete eradication, three applications of poison of 5-10 blocks per interstice on a 10m x 10m grid spaced six months apart are recommended. Heavier applications can be made in areas which show the greatest amount of rat activity, such as rock piles and tree-root warrens. Grids need to be examined every day for three to seven days and poison replaced as it disappears. Use of bait boxes with small entry holes are important to reduce the risk to non-target species. One of the greatest problems of application is consumption of bait by insects and other invertebrates, particularly hermit crabs. There seems to be no problem with toxicity to these species, but they can rapidly deplete a bait placement. Consumption of bait by invertebrates can be reduced by anchoring the bait to trees approximately one meter above the ground.
Trapping is also often used to reduce rat populations, but usually in conjunction with poisons if complete eradication is the goal. Certain individuals are completely trap-shy and will not be captured using trapping methods. In addition, a significant number of rats escape from traps and in all probability will not be trapped again. More effective trapping can often be accomplished by minimizing human scent around the trap site through application of urine-soaked domestic rat bedding and use of rubber gloves when setting traps.
Indian mongooses
Urich (1914) reports that most of the mongooses in the New World are descended from 5.4 animals imported from Calcutta to Jamaica by Espuet in 1872 to combat black rats. From there, mongooses were released onto the remaining Greater Antilles. In less than 30 years, all West Indian islands supporting a sugar industry had introduced mongoose populations derived from these original sources (Nellis and Everard 1983).
Despite initial effectiveness in rat control, depredations of mongooses soon expanded to include several species of ground nesting birds, endemic snakes, and terrestrial lizards, including iguanas. Because of their small size, mongooses are not considered a major threat to adult iguanas. However, mongooses certainly have the ability to kill and eat juveniles, and they may enter nest emergence holes and eat full-term embryos in the egg. In Jamaica, 12 to 17 years after the mongoose was introduced to the Goat Islands, there were only five iguanas left (estimated decrease of 29-38% per year). The efficiency of the mongoose as a diurnal, terrestrial predator in the West Indies is unsurpassed.
Mongooses are readily trapped in box traps. Nellis and Everard (1983) used Tomahawk live traps baited with pork liver to capture mongooses on St. Croix, U.S. Virgin Islands. An eradication program initiated on nearby Buck Island by the National Park Service used live traps baited with chicken meat to eliminate the mongoose population. The entire island was grided with transect lines and traps were placed every 50m on the grid. Although mongooses are susceptible to poisoning with diphinacinone, bait delivery would have to be accomplished with an exclusionary device such that iguanas would not be at risk. Shooting is ineffective due to the small size and speed of mongooses. Mongooses are apparently sensitive to canine distemper, but the extent to which this would represent a viable control option is unknown.
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Setting traps for the Indian mongoose control program in the Hellshire Hills, Jamaica. |
Feral house cats
House cats were probably introduced to the West Indies shortly after discovery by Europeans. As human habitation of an area increases, greater numbers of cats arrive, mostly brought in initially as house pets. Because they can be extremely efficient diurnal predators, house cats pose a severe threat to hatchling iguanas. The following program serves as an example of how difficult cats can be to eradicate. A four-year eradication project for cats was carried out from 1977 to 1980 on Little Barrier Island, New Zealand. The project involved 128 people, and over 950 leg-hold traps and 27,000 poisoned baits were used. One hundred sixty one cats were known to have been killed (Rauzon 1985; Veitch 1985).
Although cats may be poisoned using a compound such as sodium fluoroacetate (1080), the extreme danger of this compound, particularly the risk of secondary poisoning, is so great that its use is restricted to professional operators, and it poses a clear risk to iguanas. Trapping appears to be a better alternative, and leg-hold trapping in particular is very effective for cats (Veitch 1985). Because they are padded, the Victor Soft Catch coil spring traps are more humane than conventional leg-hold traps. Baited Havahart or Tomahawk box traps may also be used, but are inferior to leg-hold traps. If box traps must be used, double-door cage traps with two open ends are preferred, as cats are normally reluctant to enter closed spaces. Trapping success will be enhanced if every attempt is made to minimize human scent at the trap site. Shooting is another potential control option for feral cats. At, night a good light source will illuminate the tapetum and create an eyeshine that will aid in location of cats (Rauzon 1985). Finally, feline panleucopaenia virus (FPL) has been used effectively in controlling (not eradicating) cat populations on several sub-Antarctic islands such as Marion Island (South Africa), Macquarie Island (New Zealand), and Jarvis Island in the Pacific (Johnstone 1985; Rauzon 1985; Rensberg et al. 1987).
Domestic dogs
Feral dogs represent one of the earliest invaders of the West Indies, as virtually all Spanish expeditionary forces had them (Las Casas 1552). Dogs pose the most severe threat to adult iguanas, particularly nesting females. Iverson (1978) reported reduction of a population of Turks and Caicos iguanas on Pine Cay from 5,500 to a very few individuals after the introduction of a small number of dogs and cats to the island by a hotel construction crew.
Control of domestic dogs may best be accomplished by preventing owners from bringing their dogs into areas inhabited by iguanas. Personal contact between researchers, managers, and local forest users is essential in this regard. Shooting, which has been extensively used in coyote control programs in the western United States, may be the most effective option for feral dogs. Trapping of dogs with leg-hold traps is also very effective. However, because the diurnal activity of dogs corresponds to that of iguanas, traps would pose a considerable risk of injury to iguanas. As with feral cats, the risk to non-target animals precludes poisoning as a viable control option.
European pigs
Pigs were often liberated on small islands throughout the West Indies to serve as a food source for shipwrecked sailors. Pigs are primarily a threat to iguana nests. Pigs locate nests by smell, making nests most vulnerable in the three to four days following egg deposition by females. After this period, the iguana scent surrounding the nest dissipates, and pigs are much less likely to detect nests. Because pigs forage in groups, discovery of a densely-utilized communal nesting area could result in the destruction of a significant number of iguana nests.
Pig predation can be effectively controlled by construction of exclosure fencing around nesting areas, as has been done on Mona Island, Puerto Rico. Density of pigs can also be reduced by shooting, but this can be dangerous, as wounded animals will often turn and attack. Pigs are most easily hunted with dogs, but if this strategy is adopted only neutered males and spayed females should be used to prevent establishment of feral dog packs.
Feral goats
Goats were introduced to island ecosystems by European man, probably early in the 16th century. As with pigs, they were often liberated on small islands of the West Indies to serve as a food source for shipwrecked sailors. Feral goats are notorious for habitat destruction and extirpation of endemic plants on oceanic islands (Coblentz 1978; McFarland 1991). Forest destruction by goats has been implicated in the disappearance of many species, from Hawaiian honeycreepers (Baker and Reeser 1972) to snakes (Coblentz 1978). Wiewandt (1977) considered goats to be a major competitor of Mona Island iguanas, removing high quality herbaceous iguana forage by overbrowsing. Goats have been shown to increase avian predation on juvenile Galapagos land iguanas (genus Conolophus) by overbrowsing and the subsequent loss of vegetative cover (Dowling 1964).
Although expensive, goat browsing can be controlled by construction of exclosure fencing. Such fencing must be at least 2m in height. Shooting of goats is probably the most effective means of control. Goats tend to congregate in sleeping areas at night where they can be ambushed. Use of bells on trapped and released "Judas goats" may be useful in locating the herd (Keegan et al. 1994). Finally, the protozoan Trichomonas foetus has been suggested as a potentially useful control agent for feral goats (Dobson 1988). Because this is a sexually transmitted disease, the risk that endemic species will accidentally become infected is greatly reduced.
