Ecological influences on the dynamics of a field vole metapopulation

Although metapopulation theory is widely used in basic and applied ecology, there are still few empirical studies that explore the relationships between dispersal, patch size, and the persistence of natural populations. Here, we reanalyzed data from a six-year study of a spatially structured field vole (Microtus agrestis) metapopulation in the Tvarminne archipelago, Finland. Our goal was to address several issues relevant to metapopulation studies: (1) the relationships between within-subpopulation dynamics, dispersal behavior, habitat quality, and metapopulation dynamics; (2) the generality of one of the most common conclusions of metapopulation theory-that smaller and less frequently inhabited islands are less important for metapopulation dynamics; and (3) the comparison of different methods for understanding and predicting dynamics in "metapopulation-like" systems. Our results suggest that this vole metapopulation is driven by extinctions and colonizations of island subpopulations. However, contrary to expectation, colonizations by voles from tiny, ephemeral skerry subpopulations were about as important for metapopulation persistence as were colonizations from the more persistent subpopulations on large islands. This pattern resulted from less stable vole densities on smaller islands, combined with increased emigration preceding subpopulation extinctions. Either spatially implicit Levins models or incidence function models provided reasonable predictions of the structure and function of this metapopulation, but parameters fitted to incidence functions varied dramatically among years. Our results suggest that models of real metapopulations need not become highly detail oriented or spatially complicated to provide good predictive power. However, applications of metapopulation theory require careful consideration of how underlying ecological and behavioral mechanisms will shape metapopulation dynamics of particular species and situations.