Effect of livestock grazing on terrestrial ant diversity in subtropical habitats of Argentina, with a special interest in the fire ant Solenopsis invicta

L. CALCATERRA, S. CABRERA, F. CUEZZO, I. PÉREZ-JIMÉNEZ, J. BRIANO

Ouro Preto, Minas Gerais, Brasil

Simposio; XIX Simpósio de Mirmecologia e I Simpósio Franco-Brasileiro de Mirmecología; 2009

The maintenance of species diversity in modified and natural habitats is a central focus of conservation biology. Habitat degradation and invasive species are the two greatest threats to global biodiversity. In this regard, the Iberá Nature Reserve is a highly diverse macro-ecosystem in northeastern Argentina that protects one of the largest freshwater wetlands in South America. However, key invertebrate groups such as the ants remain almost unknown. As one of the most extensive forms of land use, grazing is a principal threat to biological diversity. Livestock grazing is one of the major disturbances to this macro-ecosystem; however its effect on ant biodiversity is unknown. The objective of this work was to study the effect of grazing on the structure and composition of terrestrial ants in two subtropical habitats (savanna and grassland) with a focus on the response of the red fire ant Solenopsis invicta to grazing. Unbaited pitfall traps were used to capture worker ants in 25 grazed and 20 non-grazed sites located in both habitats. Fifty-six ant species were collected from the four studied situations; however, 90% of the 1,766 workers captured belonged to only 10 species. The most represented genera were Pheidole with 15 species (26.8%) and Solenopsis with seven (12.5%). The savanna was twice as rich in species as the grassland; however, grazing did not affect species richness. Generalized Myrmicinae dominated grazed and ungrazed habitats. Only savanna possessed two associated ant species according to the analysis of indicator species. Ant species in grazed sites were less equitably distributed; consequently, they were dominated by a lower number of species with higher populations. Mean biomass was two-fold higher in the grassland grazed sites than in strict conservation sites. Solenopsis invicta was the most common and widely distributed ant, being captured in 61% of the samples. It was also numerically dominant representing the 23.7% of the captured individual, but Camponotus punctulatus punctulatus showed the highest biomass with 27.4 mg (27% of the total). The occurrence and abundance of the invasive fire ant S. invicta were favored by grazing only in grassland situations. Although the grazing intensity was relatively low (especially in the grassland), the presence of cattle seems to have also influenced the occurrence of wild mammals. Overall, results indicate that extensive grazing in the Iberá Nature Reserve has a low effect on overall structure and composition of the ant assemblages in subtropical habitats, but a high impact on specific functional groups or taxa promoting occurrences and substitutions of some groups and more abundant populations.