Large-scale synchrony in the dynamics of capercaillie, black grouse and hazel grouse populations in Finland

We analysed 1964-1983 data on population numbers of capercaillie, black grouse and hazel grouse in 11 provinces in Finland. The three species are known to display cyclic dynamics with a periodicity of approximately six yr. Synchrony in species-specific population fluctuations was scored by calculating cross correlations with zero lag among all the provinces. Provincial data were also used to assess the relation between climatic factors and synchrony in the population fluctuations. To enable the statistical analysis, the number of the 12 climate variables was reduced to three non-correlating principal components. A bootstrap method was used both for parameter estimation and for assessing statistical significance level when comparing the tetraonid synchrony with the climate data. For all three species we found large-scale synchrony in population fluctuations. The level of synchrony decreased, however, with geographical distance among the provinces. When a partial correlation analysis is done correcting for the weather derived principal components, correlations with synchrony and distance remained with two of the three principal components. When corrected for distance, there is correlation between synchrony and climate for capercaillie and black grouse. We conclude that spatial phenomena, such as climatic homogeneity within closely located areas and dispersal of individuals are responsible for maintaining large-scale synchrony.