Supply and demand

摘要

There are currently about 10,000 named bird species on the planet, but only half as many mammals. What dictates the number of species in any given group? In this issue, Price et al (page 222) marry the ecologies of bird species living in close proximity in northern India with their ages to argue that it is not the rate at which new species form but the rate at which new niches are created that limits how many species actually persist. By building a new evolutionary tree linking all 358 songbird species inhabiting a Himalayan slope, the authors find that it has taken an average of 7 million years for related species to end up together on the landscape.