Using long-term data on two forest rodent species [the bank vole Clethrionomys glareolus (Schreber, 1780) and the yellow-necked mouse Apodemus flavicollis (Melchior, 1834)] from the Białowieża Primeval Forest (E Poland), we decompose the annual density-dependent and density-independent structures into their seasonal components. For this purpose we adopt a state-space modelling approach explicitly incorporating sampling stochasticity. As density-independent factors we use the North Atlantic Oscillation (NAO) - a proxy variable for the overall climatic condition - and data on annual seed production. We find a weak effect of the NAO in the annual models for both species as well as during the winter in the seasonal model for C. glareolus. The effect of the NAO disappears, however, when seed-crops are incorporated into the models (for both the annual and the seasonal - suggesting that NAO primarily affects seed production). Seed production enters the models with a positive effect during the winter only, suggesting that the among-year variation in rodent density is primarily accounted for by differences in seed-production, particularly oak seeds. For A. flavicollis, a slightly positive effect of hornbeam also appears in the summer dynamics. The obtained results are discussed on the basis of earlier studies on the same populations, on the same species studied elsewhere as well as on the basis of general ecological insight.
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