Abundance and spatial distribution of Greater Rhea Rhea americana in two sites on the pampas grasslands with different land use

Abstract

In Argentina the original landscape structure of the pampas grasslands has changed rapidly due to intensified and specialized agricultural practices. The conversion of grasslands into croplands has been suggested as a threat to Greater Rhea Rhea americana populations. The main goals of this study were to estimate the abundance of Greater Rheas and to analyse the species' spatial distribution at two sites of the pampas region with different land use: agro-ecosystem and grassland. The former was mainly devoted to crop production whereas the latter was mostly used for livestock grazing. Data were collected through 14 aerial surveys from 1998 to 2001, and in 2004. Each survey consisted of six strip-transects per site, spaced at regular intervals. The total area surveyed represented 4% of each study site (113 km2 in the grassland and 95 km2 in the agro-ecosystem). Greater Rhea density (±SE) was significantly higher in the grassland (from 0.22 ±0.11 to 0.86 ± 0.24 ind. km-2) than in the agro-ecosystem (from 0.05 ± 0.05 to 0.12 ± 0.08 ind. km-2). Greater Rheas occupied 51% of the study area in the grassland but less than 5% in the agro-ecosystem. They showed a wide and uniform spatial distribution in the former area, whereas in the latter individuals occurred in small and isolated clusters. These results raise concern about the future of the species because, over recent years, land has been increasingly used for crop production. Hence, if the current rate of change in land use continues, the conservation status of this species will be seriously affected. © BirdLife International 2008.