Africa (Bloomer and Bester 1992), and Lord Howe Island (Hutton 1998). Foxes have
been removed from 39 islands of Alaska (Ebbert and Byrd 2002). Pigs were successfully
removed on Lord Howe as well as Santa Catalina Island, California, and some of the
Mariana Islands. Goats have also been removed from Mauritius and some Pacific islands.
However, despite their large size, both pigs and goats are difficult to eradicate
because they live in difficult terrain; intensive efforts to eradicate goats on Lord Howe
left a core of six living on precipitous cliffs (Parkes et al. 2002). In general, animals
that are hunted will change their behavior, becoming more shy and using refuge habi-
tats so that a disproportionate effort is required to kill the last remaining animals
(Choquenot et al. 1999; Forsyth et al. 2003): it took 1000 hunter-days to kill the last
four goats on Raoul Island, north of New Zealand (Parkes 1984). Other species are
difficult to remove because of their particular habitat uses and adaptations. Thus, the
Indian musk shrew (Suncus murinus) is an insectivore expanding rapidly across Asia,
Africa, and many islands, where it competes with endemic skinks and geckos.
Difficulties in removing this species include their ability to withstand anticoagulant
poisons and the need to use live bait for traps (Varnham et al. 2002). Brown tree
snakes (Boiga irregularis) have been particularly difficult to remove on Guam, as were
wolf snakes (Lycodon aulicus) on Mauritius, because of their ability to hide in small
holes (Rodda et al. 2002).
Rats and mice have been removed from several small islands. In general, small species
(rodents) with high rates of increase were removed successfully from islands of less
than 1000 ha, an exception being Campbell Island, south of New Zealand, which is
11,000 ha. Larger species can be removed from larger areas.Biological control
Biological control, so effective against insects, has a poor record against pest wildlife.
One of the few successes is the use of Myxomaagainst rabbits. It holds the density
of rabbits in Australia to about 20% of their uncontrolled density despite a decline
in virulence of the virus and of susceptibility of the rabbit, both a product of mas-
sive natural selection.
The chances of finding a biological agent to control vertebrates are always low,
largely because the pathogen must be highly host specific and highly contagious.Population control by manipulating fecundity has several advantages over simply killing
animals, but it also has problems of its own. It was first suggested as a control method
by E.F. Knipling in 1938 (Marsh 1988) but was not applied for another 20 years. Its
first use was against the screwworm fly (Cochliomyia hominivorax), a serious pest of
livestock in the southeast of the USA. Subsequently it has been used against a num-
ber of insect pests in various parts of the world.
The use of contraceptive techniques for population control has been reviewed by
Marsh (1988) with respect to rodents and lagomorphs, by Turner and Kirkpatrick
(1991) with respect to horses, and by Bomford (1990) for vertebrates in general. Bomford
showed that although contraception has often been advocated as a useful control method
against vertebrates, and tried from time to time, there is no clear and well-documented
example of unqualified success. “Many tests of fertility control have not been robust
enough to allow clear conclusions. Experiments have often failed to include treat-
ment replicates, or have relied on small samples. These results cannot be analyzed
statistically to estimate the probability of a treatment effect” (Bomford 1990).360 Chapter 20
20.6.2Control by
manipulating fertility