Efeitos do alagamento sazonal em assembleias de Formigas (Hymenoptera: Formicidae) nas florestas de Terra-firme, Igapó e Várzea na Amazônia Central

Abstract

The Amazon is the largest tropical forest on the planet, covering over 750,000 km2 and comprising approximately 82% of upland forest and 17% of periodically flooded forests, such as igapó and várzea forests. The igapó forest is seasonally flooded by blackwater, which is highly acidic, nutrient-poor, and rich in humic acids. The várzea forest is seasonally flooded by whitewater, derived from rivers originating in the Andes mountain range, and carries a large amount of sediment. The seasonal flooding phenomenon can be considered a natural and cyclical disturbance for many organisms living in flooded forests, such as ants. Flooding leads to a decrease in available habitat, increasing the density of individuals and competition for resources. We aimed to investigate the effect of periodic flooding on the taxonomic and functional structure of terrestrial and arboreal ants, using terra firme forests as controls. Collections of ground-dwelling and arboreal ants were made in 3 transects of 2 km with 10 sampling points in each forest type: upland (non-floodable), várzea, and igapó (floodable), in the region of Lake Uauaçú, near the confluence of the Solimões and Purús rivers, in central Amazoni, Brazil. In the ground stratum, we used 6 soil pitfall traps (3 without bait and 3 with faeces as bait) during the dry season. In the arboreal stratum, we used an entomological umbrella (50 understory trees per transect) during both the dry and flood seasons. We recorded 6,903 ants divided into 7 subfamilies, 35 genera, and 129 species/morphospecies. The number of ground-dwelling ant species sampled in the upland forest was higher compared to várzea and igapó in the ground stratum. In the arboreal stratum, the number of ant species was higher during the flood period compared to the dry season, and the várzea forest showed greater ant richness. The composition of ground-dwelling ants differed between the upland forest and várzea/igapó in both the ground and arboreal strata during the dry and flood periods. Through the classification of functional groups, we found a significant increase in the arboreal predator functional group in the arboreal stratum during the flood season compared to the dry season. Ants that forage and nest in the ground stratum are adversely affected during the flooding period, while generalist omnivorous ants and arboreal omnivorous ants appear to benefit during the flood season. Given that igapó and mainly varzea are among the most threatened ecosystems in tropical forests, understanding the relationship between ant diversity and the natural flood pulse has important implications for conservation. Moreover, as ants are frequently recognized as a keystone group, human modification of forest structure may lead to changes in ant-assemblage diversity that could affect many other elements in the ecosystem.