Changing patterns in plant and animal communities along elevational gradients have interested biogeographers and macroecologists since Humboldt’s pioneering studies in the Andes 200 years ago. It was long assumed that species richness would decline monotonically with elevation, reflecting decreases in temperature and primary productivity. However, species richness has actually been shown to peak at intermediate elevations in 70% of past investigations. Only 20% of prior studies have shown the predicted monotonic richness declines with increasing elevation or otherwise divergent patterns.
One of the problems is that the analysis of tropical species-rich assemblages are often confounded by an insufficient amount of properly identified species. No one is to blame for this. We simply don't have the necessary number of experienced taxonomists and especially for arthropod groups we are facing a rather large amount of cryptic species. I might start sounding like a broken record but I maintain my statement that DNA Barcoding could be the tool needed to overcome these limitations, and I have a new study to back this up.
Colleagues have conducted a study with the largest number of species from one order (Lepidoptera) from a rather small geographic region. They sampled and analysed over 14000 geometrid moths which turned out to represent more than 1850 species. The area was a forested elevational gradient from 1020 – 3021 m in the southern Ecuadorian Andes. The authors state that all sampling sites were actually undersampled which is inevitable in such species-rich areas. Consequently they did some extrapolation and concluded that the total number of geometrid species is likely closer to 2350. The study was comparative as the area was sampled twice. The first time only morphology was used to identify species and the second time DNA Barcoding was included. The result - species richness at sites rose by 32–43%, and the beta diversity component increased by 43–51%.
To put these numbers in context - this small area of the Ecuadorian Andes is home to more than twice as many Geometrids moths as in all of Europe (<1000 species) or even Borneo (~1100 species). In addition, the study focused on the presumably most diverse intermediate elevation range. There will be more (maybe not as many) species awaiting discovery below and above the studied range.