BIOLOGICAL CONTROL Of & By VERTEBRATES
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Biological Control by Vertebrates (Mammals, Birds & Fish). Van Driesche & Bellows (1996) noted that birds and predacious small mammals for many years were believed by some to be important forces suppressing populations of pests insects, especially in forests (Burns1960). However, there are few experimental demonstrations of the effectiveness of terrestrial vertebrate predators for the control of specific pests (Bellows et al. 1982a; Campbell and Torgersen 1983; Torgersen et al. 1984; Atlegrim 1989). Terrestrial predators of importance have included a wide variety of insectivorous birds and small mammals such as mice and shrews. The wide dietary range of such vertebrates and the flexibility of their food-collecting behaviors make the introduction of vertebrate predators to new regions potentially more dangerous than the introduction of other taxa of biological control agents (Legner 1986; Harris 1990). The principal method in which birds and mammals are used in biological control is then the conservation and enhancement of existing native species, rather than the introduction of new species. Zhi-Qiang Zhang (1992) summarized the literature on birds as pest control agents in China.
Several species of fish have been used to control insects that breed in water, such as mosquitoes and chironomids. The species most widely used for control of mosquitoes have been two species of top-feeding minnows in the family Poecilidae, the mosquito fish (Gambusia affinis Baird and Girard) and the common guppy (Poecilia reticulata Peters) (Bay et al. 1976). Use of these species has been successful in many cases (Legner et al. 1974, 2000 ; Bay et al. 1976) but ineffective in others (see Blaustein 1992). Some species of cichlid fish (Tilapia, Oreochromis, etc.) have also been used to suppress mosquitoes by reducing proctive plant biomass, thus rendering the habitat less favorable (Legner 1986). Introductions of poecilids and other fish species may affect native fish through competition or hybridization and this possibility should be carefully considered before releasing an adventive fish into a new area (Arthington and Lloyd 1989; Courtenay and Meffe 1989). Use of native fish should be considered as an alternative to such introductions. A rating system describing species characteristics that influence the potential of fish as mosquito and weed control agents exists that can guide the selection of species (Ahmed et al. 1988). Native status should be assessed for regions, not countries, because moving fish between distinct zones within a country may have effects similar to those from introducing species from other countries (Van Driesche & Bellows 1996).
Biological Control of Vertebrates. Van Driesche & Bellows (1996) reported that vertebrates have been the targets of biological control efforts in relatively few cases. In those efforts that have been made, pathogens have frequently been chosen. Compared with other taxa, relatively few cases of biological control of vertebrates have been attempted. Several important projects involved viruses. The European rabbit, Oryctolagus cuniculus (L.), was controlled in Australia through the introduction of a myxoma virus from Sylvilagus rabbit species from South America (Fenner & Marshall 1957, Ross & Tittensor 1986). Populations of domestic cats, Felis cattus L., preying on seabird colonies on oceanic islands have been reduced through the introduction of feline panleucopaenia virus (van Rensburg et al. 1987). A liver nematode, Capillaria hepatica (Bancroft) is currently being considered as a means to counter house mice (Mus domesticus L.) in Australia (Singleton and Redhead 1990). A venereal disease of feral domestic goats (Capra bercus Linnaeus), caused by the protozoan Tticbomonas foetus Donné, may offer a tlielns to alleviate the destruction of native vegetation of uninhabited oceanic islands caused by by introduced goats (Dobson 1988).
Non-native vertebrates, including feral populations of domestic animals, have caused signifiant damage to indigenous species (especially plants and ground nesting birds) in many locations, and their suppression on many oceanic islands is an environmental priority (Chapuis et al. 1994). Vertebratess that prey on other vertebrates generally are inappropriate for introduction outside of their historical range as the specificity of this class of agents is usually not sufficient to limit their effects to the target pest, and such agents may pose dangers to other indigenous vertebrate species. Habitat modification that favors the action of native vertebrate predators, nevertheless, may be effective in some instances and prove safe to native species. The abundances of predatory birds such as owls and hawks and mammalian species such as foxes (Pseudalopex spp.) may be enhanced, for example, though provision of nesting structures (for birds) and vegetation modification to increase predation rates (by clearing strips to increase prey visibility) on prey such as rabbits and rodents (Muñoz and Murça 1990).
Pathogens have in a few cases been used successfully in the biological control of vertebrates. A well-known example is the introduction of the myxoma virus of rabbits to Australia and, later, to Europe for control of the European rabbit, Oryctolagus cuniculus, with dramatic results (Ross & Tittensor 1986). Rabbit haemorrhagic disease. Another rabbit athogen, was introduced to Macquaries Island between Tasmania and Antarctica (van Driesche & Bellows 1996).
Another example of successful use of biological control against a pest vertebrate is the introduction of the feline panleucopaenia virus of domestic cats (Felis cattus) into a population of feral cats on Marion Island in South Africa. This was done to reduce the killing by cats of up to 450,000 seabirds per year in a nesting colony on the island (van Rensburg et al 1987). Initial results were successful, with a reduction from an estimated 3409 cats to 615 in the first five years.
Clearing oceanic islands of introduced herbivorous mammals such as goats is criticall for the regeneration of indigenous plant communities (North et al. 1994). Dobson (1988) suggested that opportunities may exist to use pathogens against such pest vertebrates on oceanic islands where the vertebrates are particularly destructive to native ecosystems and rare species. Dobson noted that island populations of many feral mammals have fewer species of parasites and pathogens attacking them than mainland populations. Some of these, such as the sexually transmitted protozoan Trichomonas foetusr that affects goats, may be sufficiently specific that they could safely be used to reduce reproduction or survival rates rates. This avenue would be especially valuable because attempts to eradicate such pests through hunting have been often unsuccessful due to the near impossibility of finding ancl killing the last 1-2% of the population, especially in rugged terrain.
1976 Bay, E. C., C. O. Berg, H. C. Chapman & E. F. Legner. 1976. Biological control of medical and veterinary pests. In: "Theory and Practice of Biological Control," p. 457-79. C. B. Huffaker (ed.). Academic Press, Inc., New York, London. 788 pp.
Bellows, T. S., Jr. & T. W. Fisher, (eds) 1999. Handbook of Biological Control: Principles and Applications. Academic Press, San Diego, CA. 1046 p.
Hoddle, M. S. 1999. Biological control of vertebrate pests. P. 955-974. In: Bellows, T. S., Jr. & T. W. Fisher, (eds) 1999. Handbook of Biological Control: Principles and Applications. Academic Press, San Diego, CA. 1046 p.
1986 Legner, E. F. 1986. Importation of exotic natural enemies. In: pp. 19-30, "Biological Control of Plant Pests and of Vectors of Human and Animal Diseases." Fortschritte der Zool. Bd. 32: 341 pp.
2000 Legner, E. F. 2000. Biological control of aquatic Diptera. p. 847-870. Contributions to a Manual of Palaearctic Diptera, Vol. 1, Science Herald, Budapest. 978 p.
1974 Legner, E. F., R. D. Sjogren & I. M. Hall. 1974. The biological control of medically important arthropods. Critical Reviews in Environmental Control 4(1): 85-113.
Pimentel, D. 1955b. Biology of the Indian mongoose in Puerto Rico. J. Mammal. 36: 62-8.
Pimentel, D. 1955c. The control of the mongoose in Puerto Rico. Amer. J. Trop. Med. Hyg. 4: 147-51.
Van Driesche, R. G. & T. S. Bellows, Jr. 1996. Biological Control.. Chapman & Hall, NY. 539 p.