Contribuição de nutrientes e implicações tróficas da deposição sazonal de fezes da andorinha-azul Progne subis em um megadormitório no Rio Negro, Amazônia Central

Abstract

Some land animals, such as birds and bats, have high concentrations of nutrients in feces, and large fecal loads occur when many of these animals gather at breeding sites or roosts. In these situations, a single species can influence the composition and dynamics of an entire biological community. In the Rio Negro, Central Amazon, an island is used as a 'megaroost' by hundreds of thousands of individuals of the migratory bird Purple martin, Progne subis, during its wintering. The nutrient poverty in the ecosystem of this megadorm makes it a natural laboratory to verify the effects of fecal deposition of the huge seasonal contingent of P. subis on the dynamics of nutrients and trophic relations of aquatic biota associated with the roost. We evaluated the effects of seasonal contribution of allochthonous resources represented by the droppings of martins in the local assemblages of aquatic macroinvertebrates and fish, as well as in the composition of nutrients of the island’s soil. We used allometric relationships to estimate the nutrient supply based on the abundance of individuals who slept on the island, time of stay, fecal biomass and concentration of nutrients in the feces of birds. We sampled aquatic macroinvertebrates and fish and calculated faunistic indices of richness, abundance and biomass and species composition compared to a nearby island with physical characteristics like that of the roost. We also analyzed the stomach contents of fish to verify the possible direct consumption of fecal material. The seasonal fecal intake of martins on the island was estimated at 697 kg/ha of dry feces on average, including 9,8 kg/ha of nitrogen. The chemical and physical characteristics of the roost island soil were marginally different from the control island soil. There was no difference in the abundance of individuals, species richness and composition of aquatic macroinvertebrate communities between the two islands, and there was no relationship between the number of birds that roosted on the islands and the faunistic attributes of these animals. The roost island fish community differed from the control island, only three kilometers away. We observed that there was direct consumption of puma feces by some individuals of fish species that were among the most abundant of the two islands and which forage in different strata of the water column. This indicates that different components of the fish community can benefit from the seasonally available resource on the megaroost and help maintain a high diversity of fish and dietary behaviors there. More studies with a more intensive sampling regime are necessary to further elucidate the relationships of the migratory bird P. subis with the ecosystems used during its migration.