Stress in wild Greater Rhea populations Rhea americana: effects of agricultural activities on seasonal excreted glucocorticoid metabolite levels

In recent years, wild populations of Greater Rhea (Rhea americana) have declined drastically, due mainly to the conversion of grassland into cropland as a result of intensive, specialized agricultural practices. In this study we test the potential stressful effects of agricultural activities on this ratite by assessing their adrenocortical response. Specifically, we compared fecal glucocorticoid metabolite (FGM) levels of rheas living in two areas under very different land use: grasslands mainly used for cattle grazing and agro-ecosystems intensively used for crop production. Groups of rheas were observed during 45–50 min from a distance of approximately 1 km to avoid any potential disturbance. Therefore, no matching of each fecal sample with a particular sex or individual within the group was possible. Radioimmunoassay of fecal samples from 269 individuals indicated no significant differences in mean concentrations of FGM from the two habitats sampled. In the agro-ecosystem we found no overall effect of agricultural practices on the birds' FGM levels. However, during the dry season FGM concentrations were significantly higher, which may represent a stress response to the low availability of food in the post-harvesting season. In contrast, no increase in FGM levels was registered during the dry season in the grassland, where food was available throughout the year. In this environment the highest increases in FGM levels coincided with the reproductive period, likely due to the frequent agonist encounters between males at this time of the year. Our findings, therefore, suggest that the agricultural practices have to be viewed as chronic environmental stressors for Greater Rhea populations living under such conditions. The present results support earlier research showing detrimental impacts of agricultural activities on this species, which inhabits the most productive regions of South America.