Predation on artificial bird nests along an urban gradient: predatory riskor relaxation in urban environments?

Urbanization - the anthropogenic conversion of natural ecosystems into huma n-dominated ecosystems - has occurred on global scales. The human-dominated landscape presents particular challenges to researchers because the effect s of urbanization on ecological processes are not well understood. We inves tigated the influence of urbanization on predation by conducting an artific ial nest experiment along an urban gradient of six sites ranging from natur al to urbanized ecosystems. Previous hypotheses suggest that predation pres sures in urban environments will either 1) increase because of the high abu ndance of exotic species which act as predators or 2) decrease due to the l ack of natural predators. To determine relative predation pressures among s ites along the urban gradient, we monitored the fates of 16 artificial avia n nests at each of the six sites for a total of 96 nests in each year (1996 . 1997). We analyzed the dependency of nest fate (depredated or undisturbed ) on intensity of urbanization (sites along the urban gradient), nest heigh t (ground, above-ground), and year using loglinear models. The frequency of nest predation was strongly dependent on site along the urban gradient, in dicating that urbanization intensity was an important determinant of nest f ate. Predation pressure exhibited an overall decline from natural to urban sites in both years, suggesting that urban environments have low predation pressures relative to natural areas. The predatory relaxation in urban envi ronments may partially explain the greater abundance of some species in urb an environments, particularly urban exploiters such as european starlings S turnis vulgaris, house sparrows Passer dol,domesticus, and rock doves Colum ba livia.