Our approach is twofold. First, with 1964-1983 data on red squirrel (Sciurus vulgaris) in 11 provinces in Finland we show that population fluctuations in different parts of the country are largely synchronous. Second, using two differing model types for producing the dynamics of populations we set up to examine the synchronizing effect of an extrinsic disturbing factor, the Moran effect. We show that in a spatially structured population system a randomly occurring stochastic perturbation reducing reproduction success is indeed capable of synchronizing subpopulations. The synchronizing effect is achievable with a wide range of probabilities of occurrence and strengths. However, when the Moran effect occurs in most years its synchronizing power wanes, despite the strength of the effect. Allowing regionality in the variance of the strength of the Moran effect reduces the synchronizing capacity of the perturbation. The Moran effect is also capable of producing a wide range of population dynamics from very simple premises.
