Diet and trophic niche overlap among a native waterbird and two non-native herbivores in Pampas grasslands

Abstract

The use of areas by native and non-native herbivores in sympatry has frequently led to conflicts between farmers and native animals. Dietary studies integrating methodologies usually provide a better quantified understanding of resource sharing and dietary overlap. This study aims to evaluate diet and niche overlap among three species that occur in sympatry in southern Brazil: a native herbivorous bird in South America, the southern screamer (Chauna torquata), domestic sheep (Ovis aries) and cattle (Bos taurus). Three complementary methodologies were used: microhistology and metabarcoding of faeces and stable isotope analysis (SIA) in faeces and whole blood. Bipartite networks were constructed for the three dietary assessments. No significant isotopic niche overlap was found between the native bird and the domestic herbivores by SIA in blood, while the three methods applied to faecal samples showed limited overlap between the bird and the non-native ungulates. Conversely, SIA in faecal samples revealed similarity in the feeding habits and niches of domestic herbivores. The complementary use of the three different techniques allowed the evaluation of the herbivore diet from different perspectives, according to strengths and limitations of each method: while metabarcoding indicated the use of a larger number of plant groups in different proportions and identified several of the components at the species level, Cyperaceae species were only identified by microhistology. Conversely, metabarcoding performed better than microhistology in the detection of some species of Onagraceae and Poaceae. SIA in blood elucidated different proportions of resources assimilated by herbivores, while faecal SIA revealed plants with similar photosynthetic cycles in the diet of the non-native herbivores. Through bipartite network analysis, modules emerged (e.g. SIA = 0.2, 95% IC = 0.1–0.19), confirming the differential use of some groups of resources. The combination of the three techniques, enhanced by comparative bipartite network analysis, was critical for obtaining a detailed understanding of the vast range of potential plants used and interactions among herbivores. The detected patterns of dietary partitioning also confirm that the southern screamer is not competing with cattle and sheep and can even provide ecosystem services through the biological control of aquatic weeds.