Population variability of sparrows in space and time

The abundance of a species through time and across space, and the variability of that abundance, determines the species' persistence within its geographic range. We investigated the relationship between abundance and variability of nine species of grassland sparrows to uncover their population dynamics across their ranges. Sparrow populations consist of centrally located sites of high abundance with relatively low variability surrounded by sites of low abundance with relatively high variability. These sites are distributed across space such that variability decreases and abundance increases with increasing distance from the edge of a species' geographic range. Population numbers as a whole become increasingly variable over time, but only a portion of the total population heavily influences this increase in variability. For all but one species variability accrued in the areas of highest abundance. Thus, for these sparrows, the edge of the range is sparsely populated and variable, but not increasingly so. We suggest that the spatial and temporal behavior of grassland sparrow populations exhibit a ''source and sink'' dynamic where the core of the range 'feeds' the less productive peripheral areas. Conservation of species that exhibit this dynamic would depend on preservation and management of highly productive core areas.