Rural–urban gradient and land use in a millenary metropolis: how urbanization affects avian functional groups and the role of old villas in bird assemblage patterning

摘要

Urbanization generally leads to a complex environmental gradient, rangingfrom almost undisturbed natural areas to highly modified urban landscapes.Here we analyse the effects of a rural–urban gradient on breeding birdcommunities and functional species groups in remnant natural andsemi-natural areas of Rome. A total of 69 breeding bird species were foundin the study area. Species richness decreased with increasing urbanizationat two spatial scales: the point count station and the landscape scales.Evenness showed a negative trend from periphery to city centre, whereas fordominant species the opposite was true. Functional species groups respondedto the urbanization gradient with functional group-specific patterns. Thosegroups linked to open habitats (nesting and habitat functional groups)decreased in abundance along the rural–urban gradient, whereas thoseassociated with forests exhibited a mixed trend. Generalist species'occurrence increased with urbanization. As for predators and granivorousspecies, we found a negative relationship with urbanization whereas foromnivorous species the opposite trend was true. The distribution of oldvillas (large-sized remnant green areas) in the inner city areas influencedspecies composition along the studied gradient, usually showing higherspecies richness than surrounding fragments. Agricultural areas hostedricher and better balanced bird assemblages in respect to those found inurban and forested areas. Our findings proved that an urban gradient plays amajor role in structuring bird communities, although the extent anddistribution of land use categories was another factor that influenced avianassemblages. The presence of historical villas also influenced birdassemblages, making it possible to preserve high bird diversity even ininner city-areas.