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The Common hamster in Western Europe is a typical inhabitant of cereal fields and was considered a pest species,less than 50 years ago.However,the species is currently highly threatened and in risk of local (and sometimes nationwide) extinction in Europe.Thus,in Western Europe,Common hamsters became an iconic species for nature conservation in farmland areas,but populations continue to decline despite conservation efforts.In order to examine why it is so difficult to conserve this former pest species,deterministic and stochastic population models were constructed in order to better understand the population ecology of the species.Data from a 10 year study of the Common hamster in the Netherlands and from a number of other published studies in (mainly Western) Europe were used to parameterize first a deterministic model simulating the dynamics of the hamster population in time steps of one day.Variation in litter size and the irregular occurrence of "optimal hamster years" were subsequently added to the deterministic model to construct a stochastic population model.This model was then used to simulate six scenarios representing different timing of agricultural practices,more specifically the timing of the cereal harvest (ranging from early harvest on the 8th of June to no harvest at all) .Our results indicate that under current common agricultural management of cereal fields in Europe,the likelihood that females produce enough offspring for a sustainable population is almost negligible.We conclude that conservation projects should focus on delaying the harvest date until at least the end of August.