Figure 3.

Increasingly selective harvest leads to a decreased critical rate of environmental change. The y-axis shows the proportion of simulations in which the population became extinct before 600 time steps had been completed, and the x-axis gives the amount by which the environment changes every time step. Harvest is random for a degree of selectivity of zero and is very selective when the degree of selectivity is 4 (see electronic supplementary material, figure S1). Because of the stochasticity in the model there is not a single critical rate for each set of parameter values, but this plot shows that a much lower rate of environmental change causes extinction when selectivity is high. All data compiled from 80 model runs at each combination of parameter values for 600 time steps, with the harvest rate set to 0.2, a base fecundity of 3 and a carrying capacity of 1000. The dashed line shows the proportion of simulations becoming extinct when the harvest rate is set to zero—as can also be seen in figure 2, random harvest of males only (the situation when selectivity = 0) appears to protect the population to some degree against environmental change, probably because of the ‘Hydra effect’ recently described by Osmond et al. [34].