Bird-habitat relationship in semi-arid natural grasslands and exotic pastures in the west pampas of Argentina

In the semi-arid grasslands of the west pampas, in Argentina, extended natural grasslands still persist only with cattle grazing. However, in the last years there has been an important increase in the cultivation of African pasture species. We evaluated the incidence of the replacement of natural grasslands by exotic pastures on bird diversity in spring summer and in winter. In five different grassland habitats (two native and three sown pastures), we sampled bird populations using the strip transect method and vegetation variables simultaneously at the same sites. We used multiple regressions to examine the relative importance of habitat variables on richness, abundance and presence of bird species. The replacement of native grasslands (Sorgastral) by sown pastures results in habitat modifications such as changes in green vegetation, percentage of bare ground and distance to trees. When native grasslands are moderately grazed (mixed grassland) plant species richness increases notably relative to other native and exotic pastures. Some vegetation variables were correlated with bird species richness or with some bird populations. However, the habitat variable that best described bird species richness and bird abundance was plant species richness, which varied both with the grazing history of the native grassland and with the type of pasture used as replacement. Consequently, grassland replacement by sown pastures in the west pampas results in changes in bird alpha diversity; decreasing diversity with respect to mixed grasslands, but favoring it in relation to the climax grassland (Sorgastral). The composition of grassland bird communities in natural grasslands would be little affected by exotic pastures replacement. However, since some vegetation variables best represented in some habitats had particular effects on the abundance and presence of specific grassland birds, managers and policy makers should take into account the complexity of the processes associated with changes in land use of the west pampas. This would not only decrease the probability of negative effects on the total bird diversity but also would decrease the risk of local extinction of declining species. (c) 2005 Elsevier Ltd. All rights reserved.