Abstract
Nearly all remnants of temperate grasslands in southeastern South America are used for livestock ranching and are subject to habitat degradation resulting from this activity. Exploring how habitat features affect the composition of grassland avifaunal communities is a first step to understand how current cattle-ranching management practices impact avian diversity. We used canonical ordination to test for relationships between five habitat variables and the composition of the bird community in coastal grasslands in southern Brazil. We sampled pastures with different heights, from overgrazed short-grass to tall herbaceous vegetation. We recorded 1,535 individuals and 27 species of birds. The first ordination axis indicated a strong contribution of mean vegetation height on the composition of the bird community, whereas the second axis revealed the influence of herbaceous vegetation patchiness and woody vegetation cover. Three groups of species were revealed by the ordination: one more diffuse associated with intermediate and tall herbaceous vegetation, another with short grass, and a third with vegetation patchiness and woody vegetation. Species restricted to tall herbaceous vegetation are negatively impacted from habitat degradation resulting from overgrazing and trampling by livestock, and mowing and burning of tall plants. Occurrence of these species in our study area is related with the presence of swales immediately behind the dune system and where remnants of tall vegetation persist. Birds of pastures with ample cover of short herbaceous plants, including one globally threatened species and six other restricted to short-grass habitat, apparently benefit from local livestock management practices. Woody vegetation possibly functions as a keystone structure, enabling the occurrence in grasslands of avian species that rely on shrubby habitat. Although livestock ranching promotes the diversity of habitats by creating distinct patches of vegetation height in grasslands, current management practices directed to the maintenance of short grass pastures may eliminate an entire subset of species, including regionally threatened taxa, and reduce avian diversity. The maintenance of large patches of tall herbaceous plants is needed to ensure the survival of species reliant on this type of grassland structure in our study area.
Highlights
Afforestation and urbanization are the main causes of habitat loss, whereas overgrazing and trampling by livestock and inadequate burning regimes are the main drivers of habitat degradation (Henwood, 2010; Azpiroz et al, 2012)
Less is known about patterns of habitat use by birds in the Campos unit of northeastern Argentina, Uruguay, and southern Brazil, where the largest grassland remnants are located (Azpiroz et al, 2012)
Some ranchers occasionally set fire to these patches to open up the vegetation, but burning is limited due to restrictions imposed by the Brazilian environmental law and overall dampness of the terrain
Summary
In southeastern South America, where temperate grasslands once covered 762,000 km in southern Paraguay and Brazil, Uruguay, and eastern Argentina, the majority of studies addressing this issue have been conducted in the Argentine Pampas unit (Azpiroz et al, 2012) These studies have shown that variations in grazing and burning intensity affect the height and density of the herbaceous vegetation, determining the occurrence of bird communities associated with tall-grass, short-grass and a mosaic of tall and short vegetation, as well as a series of generalist species capable of using a broader spectrum of vegetation types (Comparatore et al, 1996; Isacch & Martínez, 2001, 2003; Isacch & Cardoni, 2011; Isacch et al, 2005). In southern Brazil, Gressler (2008) showed that species richness was higher in the largest grassland remnant, which harbored threatened and data deficient grassland taxa
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