Avian Diversity in Humid Tropical and Subtropical South American Forests, with a Discussion About Their Related Climatic and Geological Underpinnings

Abstract

The tropical and subtropical regions of South America occupy ca. 14,250,000 km2, of which 8,000,000 km2 are covered by humid forests. Among these, Amazonia, the Atlantic Forest, the forests of the Eastern Andean Slopes, and the Pacific Forest stand out. About 1014 species of birds (324 non-passerines and 690 passerines) are endemic to these forests. Their lineages are believed to have been impacted by five major geological and climatic events in the Neogene and the Pleistocene: the uplift of the Andes, the formation of the Isthmus of Panama, glaciations, sea-level rise, and alternating period of differing humidity. In this contribution, inspired by an analysis of bird distribution patterns inferred from superimposed species ranges, I discuss possible processes that may have influenced biological diversification in tropical and subtropical South America. Data demonstrate that the Andean uplift divided the original pan-Amazonia into a trans-Andean region (the Pacific forest) and a cis-Andean region (present-day Amazonia), and, about 10–7 Ma, induced the formation of large Amazonian rivers. There is evidence that Amazonia and other tropical forests remained predominantly forested over time, even during the cooler and drier glacial periods. Marine transgressions during the upper Tertiary and lower Quaternary, and the formation of the Amazonian rivers following the rise of the Andes, likely fragmented the forest and operated as mechanisms to generate new species. Moreover, the closing of the Panama Isthmus and connections between Amazonia and the Atlantic forest, and the southern Andean and the Paranaense/Atlantic forests, led to the migration of taxa from other biogeographic regions—substantially increasing diversity in these biomes. Biogeographical patterns can promote insight and lead to the generation of hypotheses about the processes that have shaped present-day patterns of avian biodiversity in the region, but testing the latter will require multidisciplinary studies that integrate geology, paleoecology, molecular phylogenetics, and phylogeography. Moreover, despite the historical processes responsible for the extraordinary richness of the tropical and subtropical avifauna, available data point out to a serious problem affecting bird populations in humid forests of South America—especially in Amazonia, the lower Eastern Andean Slopes, and the Pacific Forest: a severe reduction in the diversity of birds. This drop in diversity is appreciable not only in disturbed habitats, but also in national parks and reserves, suggesting that climate change may be involved. Amazona festiva, A. dufresniana, Pionites melanocephalus, P. leucogaster, Deroptyus accipitrinus, Pyrrhura lepida, P. perlata, Aratinga weddellii, Orthopsittaca manilatus, Guaruba guarouba Touit stictopterus, Amazona tucumana, Pyrrhura albipectus Touit melanonotus, T. surdus, Leptodon forbesi