Isotopic Niches of Tropical Birds Reduced by Centenary Anthropogenic Impacts

Land use changes in tropical forests intensified during the 20th century, mainly caused by deforestation for agricultural uses, which had an overwhelming influence on bird communities. However, there is a knowledge gap on how these historical anthropogenic changes have impacted habitat use and diet of tropical birds. On this regard, isotopic analysis can be useful since δ13C and δ15N values provides information of habitat and food resource use, and also isotopic niche of species. Here, we aimed to evaluate whether changes in landscape composition caused by centenary anthropogenic impacts have influenced the isotopic signatures and niche widths of birds in a Neotropical biodiversity hotspot, the Atlantic Forest, in southeastern Brazil. Our results showed that all bird guilds (frugivore, granivore, insectivore, nectarivore, omnivore) presented a large reduction (28-70%) in niche width from the twentieth century until contemporary. These reductions are likely associated with historical impacts (e.g. fragmentation and habitat loss), which are responsible for a decrease in the availability of food resources. There was also a reduction in the mean values of δ15N over the years in all guilds, which might be related to the expansion of agricultural areas and the increment in use of synthetic nitrogen fertilizers. Albeit some of the analyzed species are considered diet and habitat generalists, all of them showed to have been strongly influenced by anthropogenic actions throughout the years. Since these birds perform ecological functions in fragmented areas, we suggest an increase in conservation actions that ensure they do not vanish in the near future.