Modelling the effects of persecution on the population dynamics of golden eagles in Scotland

Abstract

We used observations of the age structure and breeding productivity of the Scottish population of golden eagles Aquila chrysaetos together with the classic theory of population dynamics to derive current `unmanipulated' estimates of life history parameters. We then used regional differences in age structure associated with differences in persecution intensity to derive estimates of prospective `persecution-free' life history parameters. The different parameter combinations were entered into a population model to simulate their effects on the number of occupied territories over time. Most simulations suggested that with unmanipulated demographic parameters the population should decline. The disparity between these predictions and the observed apparent stability in occupied territories was ascribed to the buffering effect of a lowering in age of breeding in areas where persecution is most intense and that more favourable parameter estimates within the estimated limits may be more realistic. The results indicated, nevertheless, that currently the population is vulnerable to decline as also suggested by the apparent lack of adults to occupy vacant territories. In the absence of the estimated 3–5% annual adult mortality through persecution, modelling suggested the population would increase. Removing estimated effects of persecution on reproductive rate and preadult survival were on their own insufficient to reverse the declines predicted from unmanipulated parameters, although the effect of persecution on preadult survival may be more severe than we estimated. In the absence of persecution we conclude that the population could expand to fill currently vacant but apparently suitable habitat and have a more secure long-term status.