Density- and age-dependent reproduction partially compensates culling efforts of invasive non-native American mink

Management strategies of wildlife species must pay due regard to density dependent changes in vital rates. Knowledge of density dependent relationships is sparse for most species but such knowledge ought to inform adaptive management. Using data from a large-scale, 6 years of control effort of the invasive non-native American mink (Neovison vison) in Scotland, we analysed density dependent changes in reproduction as revealed by placental scar counts in culled females. Control strongly reduced mink density but it varied substantially over time and space, reflecting variation in when control was initiated in each river section. We used hurdle statistical models to simultaneously relate the probability of conception, litter size and female age to prevailing mink density in river sections where the female was culled. Both the probability of conceiving a litter (average 0.81) and litter size (average 5.52 pups) increased as the density of females, but not males, declined. In addition, there was a senescent decline in both components of fecundity, which given culling of mink and subsequent reinvasions, resulted in a younger population, adding further to density dependent compensation in fecundity. There was no evidence of depensation, even at the lowest density. The predicted combined impact of changes in density and age structure could lead to an increase in fecundity of up to 2.1 pups per female occupying or reinvading the controlled area. Control strategies must be sufficiently adaptable and robust in order to overcome this compensation and suppress densities of mink and other invasive mammals.