Influência da carga animal na comunidade de aves campestres no sistema ecológico Campo de Solos Rasos, sudeste da América do Sul

Abstract

The southeastern Grasslands of South America presents a rich diversity of birds and include an important area covering Brazil, Uruguay, Argentina and Paraguay. The pressure caused by overgrazing is considered one of the main causes of threat to native grasslands. Studies that quantify these pressures are necessary for supporting management proposals that ensure the maintenance of the local biota.We aimed to verify how cattle stocking management (low, intermediate and high stocking rates) would influence upon environmental variables and also on the bird community in the grasslands of the ecological system Shallow Soil Grassland (SSG). We conducted our study insouthwestern Rio Grande do Sul, Brazil and in the departments of Rivera, Taquarembó and Salto, Uruguay, in grasslands of the ecological system of SSG. We surveyed birds during the Austral spring-summer (2014-15 and 2015-16). We used point counts to identify birds and a standardized method to sample environmental variables (percentage of vegetation cover, water, dung, and exposed soil; height of vegetation and lateral obstruction of vegetation) in 320 sample units disposed in three different cattle stocking managements: low, intermediate and high. We present our results in two chapters, the first as a manuscript to be subbmited to a research journal and the second as a booklet to spread the obtained results to the largest number of people directly involved. For the environmental variables, we observed that grasslands with high and intermediate cattle stocking presented greater amount of dung when compared to low cattle stocking. Moreover, the average height of vegetation was significantly lower in fields with high cattle stocking. The species richness and abundance of birds, in the sample units, showed differences between the management types (P< 0.001, for both), however, birds’ community did not present clear associations between species and the diferent cattle management types. Vegetation height and dung cover were the environment variables with the greatest influence in the SSG´s grasslandsbird community. The Eared Dove (Zenaida auriculata) was the species with highest relation with dung and the Sedge Wren (Cistothorus platensis) showed the greatest relation to vegetation height. We recorded five species classified in any of global or regional threat categories. Two species were observed in all three management: the Greater Rhea (Rhea americana, almost threatened globally) and the Burrowing Owl (Athene cunicularia, almost threatened in Uruguay).Only two species in the management with low and intermediate cattle stockingthe Sedge Wren (Cistothorus platensis, vulnerable in Uruguay) and the Ochre-breasted Pipit (Anthus nattereri, globally vulnerable; vulnerable in Brazil and Uruguay). And one species exclusively to the intermediate management type: the Pampas Meadwlark (Sturnella defilippii, globally vulnerable; extinct in Brazil and endangered in Uruguay). We conclude that in the SSG grasslands, the extensive livestock management is a necessary ally for the maintenance of the threatened bird species thatmostly depend on a taller vegetation structure, present only in grasslands with low or intermediate cattle stocking.