Environmental Indicators of Biological Urbanization

Abstract

The objectives of this chapter on biological urbanization (the biological process resulting in exploitation by living beings of urban habitats and the successful establishment of populations of such organisms in urban habitats from ancestral populations living in natural or rural man-made habitats) were to (1) identify environmental indicators of urbanization; (2) rank these indicators in terms of indicator ability; and (3) test for spatial and temporal consistency in indicator ability. Four different measures of urbanization have been proposed: (1) year when a rural population became urbanized, as judged by direct observations of the first reproducing individuals in the urban habitat; (2) the difference in population density between urban and rural habitats with higher density in urban habitats reflecting more ancient and hence more advanced urbanization; (3) at least one urban population having higher population density in urban than in nearby rural populations of the same species; and (4) records of reproduction in city centers. The three largest mean effect sizes (Pearson’s product-moment correlation coefficient) reflecting likelihood of urbanization of birds were 0.47 for predation-related factors, 0.46 for geographic range and population density, and 0.29 for body size. The year of urbanization for different populations of the same bird species had a repeatability of 0.44 among sites. The difference in population density between nearby urban and rural habitats had a repeatability of 0.26 among the same sites. Selection for adaptation to the novel urban environment included (1) a reduction in dispersal propensity and hence a reduction in morphology associated with efficient flight or movement; (2) changes in vocalizations of birds and other organisms to cope with altered urban noise; (3) changes in songs and positions in the vegetation used for song related to changes in a predator community between rural and urban habitats; (4) changes in flight distance due to change in predator community; and (5) a change in flight behavior in response to the proximity of humans toward perpendicular flights away from the walking direction of humans to avoid human disturbance.