Efeito do pastejo, estrutura da vegetação e impactos antrópicos sobre a ocorrência de aves ameaçadas de extinção em brejos e campos úmidos no Pampa brasileiro

Abstract

Many species of grassland birds show preferences, or even dependence, in relation to the structure of the habitat, especially in relation to vegetation. Human activities interact with environmental factors, modifying the habitat structure and, influencing the composition of the bird community. In the Brazilian Pampa, the main anthropogenic activity on the remaining native grasslands is extensive livestock farming, which shapes the vegetation structure, mainly through animal load and management strategies employed. Most of the endangered grassland bird species in this region are sensitive to vegetation structure and inhabit tall grasslands, marshes and wet grasslands. However, these environments have been modified, in addition to livestock farming, by growing crops, such as soybeans and eucalyptus. In this context I made 2 studies: 1) comparing the birdlife of native grasslands under moderate and low grazing intensity and grazing exclusion, with focus on threatened species; and 2) evaluating the effects of vegetation structure, physical dimensions and anthropogenic impacts on the occurrence of nine endangered species, inhabiting marshes and humid grasslands in the central portion of the Brazilian Pampas. The bird richness, varying from 50 to 54 species, was practically the same under different grazing intensities. However, composition and abundances of the bird community were influenced by different grazing intensities. Tall grass species, such as Cistothorus platensis and Emberizoides herbicola were favored by low intensity and grazing exclusion, while broad-habited and short grass species, such as Anthus hellmayri and Vanellus chilensis, were favored by moderate intensity. Low grazing intensity apparently did not affect the occurrence of threatened species dependent on tall grasslands. Despite the importance of excluding grazing for threatened species, the encroachment of woody vegetation under this condition is modifying the environment and species associated to woody vegetation and forest edges, such as Cyanoloxia glaucocaerulea and Campstostoma obsoletum, were recorded. In marshes and humid grasslands, evaluated deeply in the second study, endangered species were favored by greater habitat availability and the occurrence, mainly, of these two types environments and vegetation the associated. Capuchino seedeaters (Sporophila sp.) and flycatchers (such as Culicivora caudacuta), for example, were favored by tall grasses, while Xanthopsar flavus and Limnoctites rectirostris, among others, were favored by gravatas. Practically all species apparently are positively related to a moderate shrub cover. On the other hand, the increase in woody vegetation and the occurrence of anthropogenic impacts, mainly the construction of dams and drainages over the habitat, have negatively impacted on them. These results reinforce the importance of preserved humid grasslands, marshes and native grasslands in general, for the occurrence of endangered grassland birds. The adoption of management strategies that maintain the vegetation structure suitable for the occurrence of these species is essential for maintaining their populations in the region, as well as the respect for legal reserves, permanent preservation areas and the implementation of conservation units.