Monday 16 March 2015

The survival of wild Grassland Birds on Cattle ranches and Soybean plantations in Uruguay and Brazil.

The temperate Grasslands of South America are home to a unique assemblage of Bird species, which are increasingly being affected by habitat modification as wild Grasslands are converted to agricultural usage. As a consequence many South American Grassland Bird species are now at risk, with 50 species considered to be threatened in Brazil and 24 in Uruguay. While conservation in South American tropical forests has become a matter of international interest in recent years, very little attention has been paid to the fate of South American grasslands, with a consequence that they have received relatively little protection; only 2.2% of all protected areas in Brazil cover temperate Grasslands and only 1.7% in Uruguay.

Wild Grasslands are being converted to a variety of usages in South America, including arable monoculture, particularly Soy (Glycine max), but also corn (Zea mays), Oats (Avena sativa), Ryegrass (Lolium multiflorum), Wheat (Triticum vulgare), Barley (Hordeum vulgare), Sunflower (Helianthus annuus) and Rice (Oryza spp.). Many areas have also been converted to Cattle pasture; these areas are considered to be more natural and likely to provide a better environment for Grassland Bird species, but are also seeded with introduced Grass species to improve the pasture, particularly Ryegrass (Lolium multiflorum) and Lovegrass (Eragrostis plana). Significant areas of Grasslands have also been converted to forestry, particularly of Eucalyptus and Pine (Pinus spp.).

Previous studies of these environments have suggested that natural grasslands are better environments for native Birds than Barley or Sunflower fields or Cattle pasture in Uruguay and that Cattle ranches were better habitats for Birds than croplands in Argentina.

In a paper published in The Condor on 14 January 2015, Thaiane Weinert da Silva of the Laboratório de Ornitologia at the Museu de Ciências e Tecnologia and Programa de Pós-Graduação em Zoologia at the Pontifícia Universidade Católica do Rio Grande do Sul, Graziela Dotta of the Laboratório de Ornitologia at the Museu de Ciências e Tecnologia and Programa de Pós-Graduação em Zoologia at the Pontifícia Universidade Católica do Rio Grande do Sul and of the Conservation Science Group at the Department of Zoology at the University of Cambridge, and Carla Suertegaray Fontana, also of the Laboratório de Ornitologia at the Museu de Ciências e Tecnologia and Programa de Pós-Graduação em Zoologia at the Pontifícia Universidade Católica do Rio Grande do Sul describe the result of a study of Bird abundances and species diversity on Cattle ranchlands and Soybean farms in Uruguay and Brazil.

All of the sites chosen were areas with deep soil layers with essentially flat topographies and occasional undulating hills (coxilhas). These sites were chosen for their similarity, in order to exclude variations in Bird populations caused by topography rather than land-use. The Cattle ranchlands were considered to be semi-natural, with a range of native Grasses. The Soybean sites were all former Cattle ranchlands in their second and third years of arable cultivation. At these sites genetically modified Soybeans were grown as a monoculture with treatment with glyphosate (N-(phosphonomethyl)glycine); a systemic herbicide which kills non-immune plants by inhibiting amino acid synthesis, and which is widely criticized by environmentalists for its environmental persistence and ability to enter water systems, particularly since the advent of resistant crops which can encourage heavy use. 

Soybeans were grown in the spring and summer at these sites, with Wheat being grown at the two Uruguayan sites during the autumn and winter, while Ryegrass was grown out of season at the Brazilian sites. None of these sites was ploughed, and the Soybean sites contained variable amounts of uncultivated Grassland deemed unsuitable for Soybean production and standing fallow; this ranged from 8% Grassland at a site in Santana do Livramento county in Rio Grande do Sul, Brazil, to 17% grassland at a site at Vichadero in Uruguay.

Da Silva et al. found 75 species of Birds living at Cattle ranch sites, of which 38 species are considered to be South American Grassland species (the remainder being more generalist species able to survive in a variety of environments). The Soybean sites were found to host 57 species, including 30 South American Grassland species. Where more than 30 individuals were present it was deemed possible to measure the comparative population densities between the two environments; of the 56 species for which this was done 50 species showed no preference, while five, the Rufous Hornero (Furnarius rufus), Firewood-gatherer (Anumbius annumbi), Great Kiskadee (Pitangus sulphuratus), Grassland Yellow-finch (Sicalis luteola) and Rufous-collared Sparrow (Zonotrichia capensis) favoured Cattle ranchland while one species, the Eared Dove (Zenaida auriculata), was present at higher densities in Soybean cultivation sites.

Only five species classified as Threatened or Near Threatened were observed during the study. The Greater Rhea (Rhea americana), Burrowing Owl (Athene cunicularia) and Black-and-white Monjita (Xolmis dominicanus), were observed on both Cattle ranches and Soybean plantations, whereas the Sedge Wren (Cistothorus platensis) and Saffron-cowled Blackbird (Xanthopsar flavus) were seen only on Cattle ranches.

Sedge Wren (Cistothorus platensis). Graziela Dotta in da Silva et al. (2015).

The species composition in the two environments was essentially similar, but the Cattle ranches had a greater number of Bird species, higher population densities of more species and more Threatened species than the Soybean plantations, suggesting that this environment was more favourable. Da Silva et al. note that this was the first such study of Birdlife in Soybean plantations in South America, despite the fact that this is the fastest growing form of cultivation on the continent. They suggest that future studies should seek to incorporate areas of Soybean cultivation which lack natural Grassland areas, in order to determine how Birds fare in these areas.

See also…

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