Solène Croci

Does Urbanization Filter Birds on the Basis of Their Biological Traits

Abstract Urbanization leads to the biotic homogenization of global avifauna. We hypothesized that urbanization acts as a filter on species traits and, therefore, that urban passerines share biological traits explaining their capacity to tolerate urban constraints. We investigated 18 biological traits of passerines related to their general biology, distribution, breeding, and morphometry. In a regional analysis conducted on passerine data from one Swiss and 11 French cities (regional analysis), we identified urban adapters (tolerant species) and urban avoiders (intolerant species), and compared their traits. In a local analysis conducted on passerine data of 13 woodlands located along a short rural-urban gradient, we identified groups of species associated with particular vegetation structures within or particular landscape structures around woodlands. We associated each of these species groups with a tolerance level to urbanization and compared their traits. Regional analysis revealed that urban adapters prefer forest environments, are sedentary, omnivorous, widely distributed, high-nesters with large wingspans. Urban avoiders seem to allocate more energy to reproduction than do urban adapters, to the detriment of adaptation to new environments such as urban areas. Local analysis did not reveal any link between traits and species tolerance levels. At large spatial scales, urbanization seems therefore to act as a filter on species traits. However, the urban constraints that filter species at such large scales do not seem to be the same ones that determine species distribution at local scales. Analyses of traits are powerful tools to understanding regional community composition between urban and rural areas.

Geographical range as predictor of spatial expansion of invading birds

Many studies have been carried out on predictive traits, such as geographical range, but most of them were related to introduced species and considered the invasion as a whole. The contrasting results previously obtained suggest that studies should take into account the dynamics of the invasion process (immigration, establishment, spread). We hypothesise that the geographical range reflects the species tolerance to new environments, and is related especially to the establishment and spread of the invasion process. First, data on spontaneous invasive birds in France since 1950 were collected to determine the relation between geographical range and spontaneous invasions of French biogeographical areas. Second, the urban context, assumed to be free of immigration, was used to focus on this relation during the establishment and spread of species. For all species recorded, we determined the geographical range from an atlas, measured as the distribution range area RA (occupied area in km2) and the latitudinal range LR (km from north to south). Our results on spontaneous invaders show that the geographical range, especially RA, could help to predict the extent of an invasion once it has started. In the urban context, RA and LR discriminated urban colonists from urban avoiders, which supported the link between the geographical range and establishment/spread success. The geographical range participates, with other traits, in defining an ‘ideal invader’. We suggest that the dynamics of the invasion process i.e., considering each step of invasion rather than an entity, should be an important conceptual tool for future predictive studies.

How useful are urban island ecosystems for defining invader patterns

Biological invasions are pervasive, alter ecosystem sustainability, and can reduce native biological diversity (Vitousek et al. 1996). Ecologists are scrambling to predict where and when new invaders may strike (Lodge 1993; Fagan et al . 2002). Urbanization has created a number of new ecological niches which, after remaining empty for some time, are increasingly being colonized or invaded by vertebrates (Erz 1966). One of the major disputes in evolutionary biology concerns the rate at which species evolve to occupy new niches (Diamond 1986). How quickly are animals adapting to these new habitats and what makes them successful colonists?