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

Urbanization generally leads to a complex environmental gradient, ranging from almost undisturbed natural areas to highly modified urban landscapes. Here we analyse the effects of a rural–urban gradient on breeding bird communities and functional species groups in remnant natural and semi-natural areas of Rome. A total of 69 breeding bird species were found in the study area. Species richness decreased with increasing urbanization at two spatial scales: the point count station and the landscape scales. Evenness showed a negative trend from periphery to city centre, whereas for dominant species the opposite was true. Functional species groups responded to the urbanization gradient with functional group-specific patterns. Those groups linked to open habitats (nesting and habitat functional groups) decreased in abundance along the rural–urban gradient, whereas those associated with forests exhibited a mixed trend. Generalist species' occurrence increased with urbanization. As for predators and granivorous species, we found a negative relationship with urbanization whereas for omnivorous species the opposite trend was true. The distribution of old villas (large-sized remnant green areas) in the inner city areas influenced species composition along the studied gradient, usually showing higher species richness than surrounding fragments. Agricultural areas hosted richer and better balanced bird assemblages in respect to those found in urban and forested areas. Our findings proved that an urban gradient plays a major role in structuring bird communities, although the extent and distribution of land use categories was another factor that influenced avian assemblages. The presence of historical villas also influenced bird assemblages, making it possible to preserve high bird diversity even in inner city-areas.