An alternative hypothesis for the origin of Amazonian bird diversity

Aim To determine the origin of the high diversity of birds and other animals and plants in Amazonia. Previous hypotheses are: palaeogeography hypothesis, river hypothesis, river-refuge hypothesis, refuge hypothesis, disturbance-vicariance hypothesis, gradient hypothesis, pest-pressure hypothesis, intermediate disturbance hypothesis, riverine disturbance hypothesis, models of fine-scale habitat heterogeneity, lake hypothesis, and museum hypothesis. Methods At present there is agreement between areas of high species diversity and sites located over 100 m. As these sites would have been islands during a sea-level rise of this value, it was important to determine the probable distribution of these islands in Amazonia during a marine transgression. For that purpose, I traced the 100 m contour line from topographic maps. Results Two broad marine transgressions would have been produced from the Atlantic Ocean via the Amazon and Orinoco rivers. Two very large islands would have been formed to the north of the Amazon River, and other islands and archipelagos would have been formed along the coastal lowlands of Guiana, and at the periphery of the Amazon basin. The area located between the Solimoýes and Negro rivers and in the lower Branco would have been completely covered by the sea. Main conclusions A substantial part of the high diversity of forest and nonforest birds in lowland Amazonia can be hypothetysed to have originated during sea-level rises of about 100 m in the Quaternary and late Tertiary. These transgressions would have fragmented the Amazonian lowland into a large number of true islands and archipelagos, thus favouring active allopatric speciation. Values appreciably higher than 100 m above the present sea-level during previous periods of the Tertiary would have produced segregation of the biota earlier than the Tertiary–Quaternary boundary. Sea-level rises and vegetational changes (by aridity or cooling) would thus have driven the speciation pump, and local disturbances and other processes, would maintain the diversity.