Spatial and temporal variation in demography of a house sparrow metapopulation

Abstract

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 (λ) for three transitions between years, using a projection matrix model. 2. The mean growth rate of the metapopulation was positive (λ = 1·05). 3. Large variation was found in space and time in the asymptotic growth rate. Even though the standard deviations of the estimates were large, λ was significantly greater than 1 on one island in 2 out of 3 years. In contrast, λ significantly less than 1 occurred on an island on five occasions. 4. Decomposition of the magnitude of the spatial and temporal components of λ showed that one year (1995) had a particularly negative impact on λ. In contrast, one island had a great positive effect. 5. These differences in λ were most closely related to variation in the juvenile survival rate and in the fecundity rate, which, in turn, was closely related to variation both in space and time in the proportion of the eggs that produced a fledgling. 6. As expected from the large spatio‐temporal variation in λ, the sensitivities and the elasticities of λ to variation in the different elements in the projection‐matrix also differed both in space and time. When λ was small, it was most influenced by variation in adult survival rate. In contrast, for large λ, it was most sensitive to variation in the recruitment 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 metapopulation, and that the ratio of juveniles to adults can be used to characterize populations as sources or sinks. 8. These results demonstrate large variation in space and time in the demography of a small passerine metapopulation. These differences result from a combination of regional effects on the whole metapopulation, and factors that occur at one particular island in a single year.