Overwinter egg survival drives dynamics of alpine Parnassius smintheus butterflies

We use population monitoring (mark-recapture) and experimental manipulation of winter conditions (snow), over 6 years, to determine the role of over-winter egg survival on the dynamics of alpine Parnassius smintheus butterflies in the Rocky Mountains of Alberta, Canada. Annual population change of butterfly abundance between consecutive summers was highly correlated with the survival of over-wintering eggs during the intervening winters. Experimental removal of snow cover resulted in very low, or no, survival of eggs in each year. Augmentation of snow amount by the use of snow fences increased egg survival. These patterns support long-term (26-year) population monitoring of P. smintheus, where the interaction between winter temperatures and snow cover are the best predictors of population change. Results also suggest that loss of snow cover, due to climate change, may pose a challenge to these alpine butterfly populations.