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We investigated whether the degree of exchange with other populations affects the occurrence of density-dependent regulation. We contrasted data from an Icelandic and a Finnish population of breeding wigeons (Anas penelope), the former population being more closed than the later. We looked for density dependence in time-series data and investigated whether breeding success is density dependent and plays a role in population dynamics and regulation. Time-series analysis did not reveal density-dependent regulation in either population. Nor did we find evidence of density-dependent breeding success in either population. However, population growth rate appeared to be strongly dependent on the breeding success in the previous year in the closed population but not in the open population. Our findings underline how important it is to link time-series analysis to the study of potential stabilizing mechanisms in order to understand population dynamics and regulation. We also suggest that it may be a difficult task to achieve sustainability in waterfowl harvesting, the theoretical basis of which is density-dependent population regulation.