Figure 4.

Increasing the minimum age for harvest reduces extinction probability. In this model, males and females become sexually mature at age 2 and harvest is restricted to these mature individuals. If the minimum age for harvest is increased (as in the centre and right hand columns), the effect of selective harvest on extinction risk seems to be largely nullified. This is because the ‘high-quality’ males that would otherwise be removed from the population have an opportunity to breed and pass on their genes. Management of trophy hunts whereby older males only are targeted is already recommended for lion populations, albeit to reduce the effects of infanticide and lack of paternal care associated with removal of breeding age males [35], and similar schemes should perhaps be considered for other hunted animals. Age-based management requires either an easy way of telling an animal's age or close management of a population whereby individuals are followed through time. Neither of these are likely to be possible for many harvested populations, however: these include many mammal populations which are not intensely managed as well as, for example, insect populations where males with large secondary sexual traits are the focus of collection. For these populations it is difficult to recommend a simple management intervention that will avoid the effects detailed here, but close monitoring and reactive management is likely to help and to give warning of declining numbers and potential problems. All extinction probabilities calculated from 80 replicate runs of the simulation, with base fecundity = 3, the rate of environmental change set to 0.005 per time step, strong sexual selection (strength of sexual selection = 5) and strong condition dependence (α = 4).