The diversity of plant and animal life in the Amazon basin is well understood: Less known, however, is that the Upper Amazon, a region comprising an arc running along the western border of the river basin, overlapping the lower slopes of the Andes and including the Tambopata National Reserve, is the most biodiverse region of the Amazon and, scientists believe, the Earth. This belt, usually about 30 miles wide, runs from Colombia to Bolivia and rises as high as 5,000ft.
The reasons for the Upper Amazon's stunning diversity are not entirely understood. Scientists believe that one factor is the altitude range as lowland tropical rainforest gradually transforms into premontane cloudforest as the vegetation follows the rising Andean foothills, creating a series of climatic niches favouring increased speciation. Another suggestion is the Pleistocene Refugia theory, although this has fallen out of scholarly favour in recent years. According to this theory, the rainforest retreated during climatic variations over geological time to small areas known as refugia, including the lower reaches of Andean valleys. These refugia essentially acted as biodiversity repositories from which some species spread out again as climatic conditions encouraging rainforest expansion returned.