Factors que afecten l’èxit d’expansió de poblacions d’aus en medis transformats per l’homel’arpella "Circus aeruginosus" a la Vall de l’Ebre Factors affecting expansion success of bird populations in human-transformed environments: the marsh harrier Circus aeruginosus in the Ebro Valley

Resumen

Change in species geographic ranges is a natural phenomenon, but the rate and magnitude of such change has significantly increased due to anthropogenic causes. The rate at which a population spreads/contracts through space is a function of the rates of population growth and dispersal, coupled with density-dependence. Human-induced environmental changes can lead to variation in density-independent components of local birth or death rates through deterministic process (e.g. impoverishment/increase of habitat and food resources), thus pushing populations toward positive/negative growth. Moreover, spatial range constraints, such as dispersal, and density-dependence may also limit population growth and range expansion, beyond environmental characteristics. The main goal of this thesis is to explore the role played by ecological factors such as habitat and food availability, spatial range constraints and density-dependence in the expansion of a species through human-transformed environments. For this purpose, I use as a study model an expanding population of the marsh harrier living in an agricultural landscape of the Ebro Valley (northeastern peninsular Spain). Results obtained in this thesis suggest that environmental factors, sociability, dispersal constraints and density-dependent mechanisms may play an important role in the distribution of a species. Marsh harrier breeding numbers had been greatly reduced in previous decades (1960-1980) mainly due to organochlorine pesticides, drainage of wetlands and direct persecution. Thus, at least part of their increase in recent decades may represent population recovery following reductions in organochlorine use and direct persecution. However, the species may also have benefited from the spread of human-made structures such as artificial ponds and reservoirs related to agricultural intensification, for breeding and from their surrounding herbaceous crops for hunting. At a local scale, individual variation in density-dependence for productivity and settlement patterns may have favoured dispersal of some individuals to new empty habitat patches, while others aggregated around traditional areas. However, at a large scale (i.e., across peninsular Spain), the breeding population of marsh harriers seems to be spatially constricted beyond environmental variables. This result indicates that potential habitat for marsh harriers still exists for further expansions in the Iberian Peninsula (at least at the spatial resolution of our analyses, which does not allow us to consider smaller scale processes that may restrict the species distribution such as local food abundances or pesticides). Potential consequences of this expansion for other species should be considered in order to properly conserve biodiversity in a world of change.