Avian species richness in tropical South America

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Abstract
The most extensive biomes of tropical South America, in particular the Amazonian Forest, are the richest in total number of bird species and number of endemic species. On the other hand, relative diversity of species and endemic species (number of species per 10,000 km2 of biome) is highest in areally restricted and/or isolated biomes like the Pacific Colombian‐Ecuadorian Forest, the Andean Montane Forest and the Pantepui Montane Forest. Local species richness is highest in Amazonia, especially in upper Amazonia where 400 to 430 forest bird species (over 500 species, if water‐birds and migrants are included) occur at individual localities. Very high species richness characterizes also the avifaunas of the forests which extend, from upper Amazonia, far eastward in the hilly country both to the north and to the south of the central Amazon Valley. By contrast, local species diversity is considerably reduced in central Amazonia. The avifaunas at individual Amazonian localities are five to six times richer in species than north Temperate Zone avifaunas. “Excess” tropical species richness is explained by several factors, viz. tighter species packing, additional tropical guilds and increased size ranges of tropical guilds. Interspecific competition has a prominent role in determining the range limits of numerous parapatric bird species in the lowlands as well as along the slopes of the Andes. The total number of forest bird species at individual sites decreases with elevation in the Andes closely paralleling a similar trend of decreasing forest stature. During Cenozoic time (Tertiary‐Quaternary), tropical and high latitude avifaunas have been affected by similar vicariance processes (which probably caused extensive faunal diversification), viz. paleogeographic changes in the distribution of land and flooded areas, separation by broad rivers, and climatic‐vegetational fluctuations (leading to the isolation of restricted populations in temporal ecological refugia).