Multiannual Fluctuations in Willow Ptarmigan Lagopus l. lagopus-does the Genetic Variation of Nesting Females Enhance the Effect of Predation?

Willow ptarmigan Lagopus l. lagopus is a popular game bird which fluctuates in abundance. The causes of these fluctuations, however, remain controversial, but several studies have emphasised the effect of predation. Predation not only reduces the number of breeding birds, but does it in such a way that genetic variation among chicks at hatching becomes reduced, causing reduced viability and increased mortality among chicks. We present an extended predation hypothesis in which the multiannually fluctuating population dynamics of the willow ptarmigan are better explained by a model including both predation and genetic variation of territorial nesting females than by predation alone. A simple model including the heterozygosity of nesting territorial females and the percentage of females suffering egg predation explained 95% of the observed fluctuations in chick production on an inland study area during five years, whereas predation alone only explained 72%. The data may suggest a non-additive relationship between predation and genetic variation of nesting females which enhance the effect of predation. Observed and calculated chick production per two adults deviated on average by only 0.38 chicks. In another inland population, showing multiannual fluctuations for almost 20 years, observed and calculated chick production deviated on average by 0.58 chicks, and the model explained 61% of observed fluctuations in chick production, whereas predation alone only explained 28%. In an island population, however, the full model explained 45% of observed fluctuations in chick production. This was about the same as predation alone (44%). It is discussed whether the better fit of the full model than the model including predation alone between observed and calculated chick production obtained in the two inland populations in contrast to the island population, may be caused by the different predator communities.