introduced disease, shooting, and trapping reduced the cats. The petrel breeding suc-
cess increased from 0 –23% (1979 – 84) to 100%; chick mortality decreased from 60%
in 1979 – 84 to 0% in 1990. Comparisons with breeding on Prince Edward Island and
within a cat-free enclosure on Marion Island identified the cats as the cause of the
initial high mortality and the reduction in cat numbers as the reason for the increase
in recruitment (Cooper and Fourie 1991).
The next example deals with non-target species. The insecticide fenitrothion is a
well-known organophosphorus pesticide, but its effects on song birds and other non-
target animals are little known. The Forestry Commission in Scotland wanted to use
fenitrothion to control the pine beauty moth (Panolis flammea), and was required to
undertake environmental assessment of the effects of spraying on non-target species.
For 3 years Spray et al. (1987) monitored the effect on forest birds.
Their design comprised two pairs of plots, each plot measuring about 70 ha. The
elements of each pair were matched by soil type, age of planting, and tree composi-
tion. One element of each pair was sprayed, all plots being monitored before and
after spraying to detect annual variation of the density of breeding birds, short-term
changes in abundance within 5 days of spraying, and breeding performance of the
coal tit (Parus ater). They detected no significant difference in these variables
between the insecticide-treated plots and the experimental control plots.Animal welfare is an important consideration in any control operation. An animal
has the right to be treated in a humane manner whether it is to be protected or con-
trolled. Unfortunately the notion of humane treatment is often the first casualty of
turning a species into a pest. That is particularly noticeable when the species is an
exotic. The wildlife manager’s paramount responsibility in any control operation is
to ethical conduct rather than to operational efficiency.
Control methods can be divided into those aimed at directly increasing mortality,
those aimed at directly reducing fertility, and those that act indirectly to manipulate
mortality, fertility, or both. The success of an operation is not gauged by the reduc-
tion in the density of the target species but by the reduction in the deleterious effects
of the target species. In all cases the prime responsibility of the wildlife manager is
to determine whether the control adequately reduces deleterious effects and whether
its benefit exceeds its cost.Control by increasing mortality may be direct, as in poisoning, trapping, or shoot-
ing, or it may be indirect as in biological control through pathogens.Direct killing
Five simple principles guide the control of a target population living in an environ-
ment that remains reasonably constant from year to year. These are largely independent
of a population’s pattern of growth and they emphasize the conceptual similarities
between control and sustained-yield harvesting.
1 When a constant number of animals is removed from the population each year
the size of the population will be stabilized by the control operations unless the annual
offtake exceeds the population’s maximum sustained yield (MSY).
2 The level at which the population is stabilized by the removal of a constant
number each year is equal to or greater than the density from which the MSY is
harvested.358 Chapter 20
20.6 Methods of control
20.6.1Control by
manipulating
mortality