Bird diversity in a subtropical South-American City: effects of noise levels, arborisation and human population density

In Brazil, most conservation-related studies on birds focused on the impact of forest fragmentation or the value of refuges (Willis 1979; Aleixo 1999a; Martensen et al. 2008), but studies of birds in areas of high human density are relatively recent (see Franchin 2009 for review). Bird studies of Brazilian or other Latin America cities have concentrated on surveys of urban parks and university campi (Feninger 1983; Matarazzo-Neuberger 1992; Mendonça-Lima and Fontana 2000; Valadão et al. 2006), comparisons between urban and rural areas (Ruszczyk et al. 1987; Reynaud and Thioulouse 2000; Leveau and Leveau 2004; Leveau and Leveau 2005), or surveys of only a small part of a city (Voss 1979a,b; Torga et al. 2007). The focus on species richness has shown that some parts of the cities are very poor bird habitat, and that other parts can sustain quite large numbers of species. However, this generalization could be made about almost any biome, including mega diverse habitats, such as tropical forests and coral reefs. To contribute to the understanding of cities to biodiversity conservation in sub-tropical areas, we evaluated the distribution of bird assemblages throughout the city of Porto Alegre, which is the largest and most urbanized city in Rio Grande do Sul State, Brazil. Because a single local feature, such as a city park, will not be able to sustain viable populations if it is not connected to other suitable areas, we studied the whole city as a mosaic of habitats resulting from constructions of different areas and heights, natural and highly modified parks, street arborisation, cemeteries, gardens etc. Our principal objective was to determine if the composition and diversity of bird assemblage is affected by urban noise, tree density and human-population density within the city.Cities are highly modified environments in which the only areas that resemble natural landscapes are urban parks with low human population density. Attempts are frequently made to maintain high bird diversity in cities for aesthetic or educational reasons. However, it remains unclear whether local site characteristics are important in determining bird assemblage composition or whether simplification of the assemblage is an inevitable consequence of the changes associated with human population density. From May 1998 to December 1999, we undertook bird counts at 521 points in Porto Alegre, Rio Grande do Sul, southern Brazil. Our main goal was to understand the pattern of distribution of the bird species richness and density within the city and determine which variables most affect species assemblages. We recorded 132 species belonging to 43 families that are common in Rio Grande do Sul and obtained quantitative data on 121 species in survey sites. The two most abundant species (House Sparrow, Passer domesticus and Rock dove, Columba livia) were exotics. Analysis based on a reduced subset of 134 points surveyed in spring/early summer suggested that there was a North–south gradient in assemblage structure. Variation in assemblage structure was also affected by the number of trees, urban noise and human population density. However, human population density had a much smaller effect on richness and assemblage structure than variables subject to management, such as tree density and noise levels. These results suggest that complex communities may be maintained in densely populated urban areas of sub-tropical South-America given adequate urban planning