Effect of Different Control Operations

Killing removes a percentage of possums from the population. Possum numbers are reduced immediately. The survivors will have an increased reproductive and survival rate immediately after the numbers are reduced because of a reduction in density-dependent regulating factors. To calculate the theoretical rate at which a population will build up it is necessary only to impose the basic rate of increase curve on the remnant population. Following a 70% kill, a population with a maximum rate of increase of 1.4 will take a minimum of 10 years to return to 90% of its former level (Fig. 2). The increase in the first year after killing 70% of the population will be only 25% (or a rate of increase of 1.25). The maximum rate of increase of 1.40 will not be reached because population numbers have not been reduced to minimal. If the kill is 90%, the period of recovery is extended to 14 years. If the maximum rate of increase is only 1.2, the recovery time is much longer.

Inhibition of reproduction (or sterilisation) of both sexes leaves animals alive to compete for mates, food, nesting sites, and other resources, and therefore does not result in increased reproductive and survival rates of non-sterilised animals. If the sterilised animals are fully competitive and equally distributed they are theoretically capable of suppressing reproduction in non-sterilised members of the population to a degree equal to the percentage of the population sterilised. This will still apply even though possums may be polygamous, provided the same proportion of dominant individuals is sterilised as for the population as a whole.

Temporary sterilisation (i.e. sterilisation effective for only one breeding season) of 70% of the population reduces numbers only by suppression of reproduction for one breeding season. It will take a population with a maximum rate of increase of 1.4 only 3 years to reach 90% of its former level (Fig. 2).

Permanent sterilisation of 70% of the population in a single operation will suppress reproduction in the first breeding season as for temporary sterilisation, but will continue to suppress reproduction in subsequent breeding seasons, page 229

Fig. 2. Theoretical response of possum populations to 70 percent control for one year.

Fig. 3. Theoretical response of possum populations to 70 percent control for three successive years.

page 230 although its effect will reduce as the number of sterilised possums declines (through natural mortality). It will take a minimum of 23 years for the population to reach 90% of its former level (Fig. 2).

The degree of sterilisation required to exterminate a population using expression (3) is 97% for a population with a maximum rate of increase of 1.4. This may be achieved by sterilisation of 70% of the population for three successive years (Fig. 3); i.e. 70% in the first year, 70% of the remainder in the second year, and 70% of the remainder in the third year. In practice the time to reach 90% of the population's former level as in Fig. 2, and to reach extinction as in Fig. 3, should both be shorter than shown. This is because the term for survival of sterile males and females does not take account of the increasing age of the sterilised animals. The term needs to be age specific.