Spatial and temporal variation in demography of a house sparrow metapopulation

1, In order to compare spatial and temporal variation in the demography of a metapopulation of house sparrows living on four islands off the coast of Northern Norway, we computed the asymptotic population growth rate (lambda) for three transitions between years, using a projection matrix model. 2, The mean growth rate of the metapopulation was positive (lambda = 1.05). 3. Large variation was found in space and time in the asymptotic growth rat e. Even though the standard deviations of the estimates were large, lambda was significantly greater than 1 on one island in 2 out of 3 years. In cont rast, lambda significantly less than 1 occurred on an island on five occasi ons. 4, Decomposition of the magnitude of the spatial and temporal components of lambda showed that one year (1995) had a particularly negative impact on l ambda. In contrast, one island had a great positive effect. 5, These differences in lambda were most closely related to variation in th e juvenile survival rate and in the fecundity rate, which, in turn? was clo sely related to variation both in space and time in the proportion of the e ggs that produced a fledgling. 6, As expected from the large spatio-temporal variation in lambda, the sens itivities and the elasticities of lambda to variation in the different elem ents in the projection-matrix also differed both in space and time. When la mbda was small, it was most influenced by variation in adult survival rate. In contrast, for large lambda, it was most sensitive to variation in the r ecruitment rate. 7, We suggest that conditions during the breeding season determine whether a population acts as a source or sink in this temperate small passerine met apopulation, and that the ratio of juveniles to adults can be used to chara cterize populations as sources or sinks. 8, These results demonstrate large variation in space and time in the demog raphy of a small passerine metapopulation. These differences result from a combination of regional effects on the whole metapopulation, and factors th at occur at one particular island in a single year.