Scale dependence of biotic homogenization by urbanization: a comparison of urban bird communities between two continents

Recent studies showed contrasting results about the homogenizing force of urbanization on bird community composition at large and regional scales. We studied whether urbanization promotes the homogenization of wintering bird communities and if this is scale-dependent. We used qualitative and quantitative similarity indices in comparing communities. Processes governing bird community dissimilarity were examined with the partitioning of Sörensen index in species turnover and nestedness. We made bird surveys in town centres and suburban habitats of three cities located in the Pampean region of Argentina and in the boreal region of Finland using a single-visit study plot method. Species richness did not differ amongst the town centres between the continents, but it was higher in the suburban areas of Argentina than in Finland. At the continental scale, we found a higher similarity amongst the town centres than amongst the suburban areas; whereas at the regional scale similarity between town centres was comparable to similarity between suburban areas. The use of a quantitative index produced a higher similarity between town centre communities of both countries than when using a qualitative index. Dissimilarity between habitats in Argentina was related to nestedness, and to species turnover in Finland. Our results indicate that it is necessary to consider the spatial scale and include abundance data when analysing the homogenization of bird communities promoted by urbanization. Moreover, processes of community dissimilarity between urban habitats may differ with latitude.