Bird populations become less diverse at higher mountain elevations - but they diversify faster. This finding is reported in a paper published online this week in Nature, which challenges long-held assumptions that species richness should be highest at mid-elevations, and that high local diversification rates are necessary to support rich species populations.
Mountain ranges play a key role in driving evolution. They have complex terrains and varied, often highly separate, habitats - enabling the genetic isolation of populations that can lead to the development of new species. At the same time, these qualities make mountains notoriously difficult to study.
In their new study, however, Ignacio Quintero and Walter Jetz looked at the distribution, diversity and evolution of 8,470 different bird species (around 85% of all known bird species) on the world’s 46 major mountain ranges. Although many previous studies reported that species diversity peaks at mid-elevations, the authors used a novel sampling approach that compensates for the overrepresentation of wide-ranging species at mid-elevations, finding instead that across all mountain ranges, species richness decreases linearly with increasing elevation. At such higher elevations, environmental conditions become more extreme - with larger daily temperature fluctuations, and higher levels of wind and solar radiation.
Examining evolutionary relationships among different bird species, the authors found that the more sparsely populated assemblages of species at higher elevations were associated with greater rates of species diversification - flying in the face of the idea that less diverse biota are caused by recent lulls in the rate at which new species arise. Instead, it would seem that intense, ongoing processes of species diversification are necessary to support the unique and highly adapted biotas found at higher elevations.
They also find that assemblages of species living on mountains that had undergone high rates of past temperature change were found to have diversified more rapidly, highlighting the importance of climate fluctuations in driving mountain-based evolution.