Factors affecting the expansion success of bird populations in human-transformed environmentsthe case of the Marsh Harrier Circus aeruginosus in the Ebro Valley

Abstract

Understanding the factors that limit the current distribution of species and populations is crucial not only for ecological and theoretical research but also for predicting the impact of global change on biodiversity and its consequences for effective management and conservation. Using as a study model an expanding population of Marsh Harrier Circus aeruginosus in Spain, this review shows that environmental factors, sociability, dispersal constraints and density-dependent mechanisms may all play an important role in the distribution of common native species. Marsh Harrier breeding numbers fell greatly between 1960 and 1980 mainly due to organochlorine pesticides, the drainage of wetlands and direct persecution. Its recent population recovery may be attributable in part to less use of organochlorine pesticides and less direct persecution. However, this species may also have benefitted from the increase in number of artificial structures such as irrigation ponds and reservoirs in areas of intensive agriculture, which are used as breeding sites and their surrounding croplands as hunting areas. At a local scale, individual variation in density dependence in productivity and settlement patterns may have favoured the dispersal of some individuals to new empty habitat patches, while other birds have aggregated around traditional breeding areas. However, on a large scale (i.e. peninsular Spain), the breeding population of Marsh Harriers seems also to be spatially constricted by factors other than environmental variables. This indicates that potential unoccupied habitat for Marsh Harriers still exists and further expansion in the Iberian Peninsula could occur (at least at the spatial resolution of available studies that do not, however, take into account small-scale processes such as food abundances or pesticides that may restrict the species’ distribution). The potential consequences of this expansion for other species should be taken into account in order to ensure the conservation of overall biodiversity in a world of change.