Identifying the evolutionary and geographical underpinnings of global avian mountain biodiversity
Mountain systems are often much more species-rich than lowlands with similar area, particularly in the tropics where they have been categorized as “hotspots” of biodiversity. Explaining why highlands harbor the highest bird richness has received considerable attention, however, no general consensus has emerged. Given their consistently high species richness, tropical mountain systems should stand out in the key processes that ultimately account for spatial variation in species richness: migration and diversification rates. In turn, variation between these arises from several specific, non-exclusive processes that require consideration of both geographic and evolutionary history. Here, we integrate a time-calibrated phylogeny of all extant bird species with their respective geographical and elevational distributions and analyze largely biologically-independent mountain systems as replicates. We use a random points sampling approach according to the scale for which our data is accurate. We extracted historical (diversification rates), geographic (area) and ecological factors (e.g., Temperature, NPP) for each sampling point as well as mountain level variables such as age, heterogeneity, and area. We used generalized linear models that accounted for spatial pseudoreplication under a Bayesian framework for each mountain system. Finally, we designed a hierarchical model to identify the relative influence of historical and ecological factors in explaining standing global avian mountain richness.
Species richness for each of the 47 delimited mountain systems mostly followed a hump-shaped relationship with elevation, with the peak at lower elevations. Estimates of historical drivers of mountain biodiversity varied strongly between each of the 47 mountain systems. For instance, in the South American Andes, diversification rates increased with elevation, but decreased with species richness –suggesting that either diversification happened recently with insufficient time to accumulate species or that migration have removed the effect of diversification. Conversely, the Himalayas diversification rates are coupled with species richness, and decreases with elevation. However, the hierarchical model indicates that diversification rate is strongly and positively associated with elevational species richness. Intriguingly, the results also suggest that mountain area and age do not explain avian biodiversity. Collectively, our preliminary results suggest that idiosyncratic historical factors underpin differences between mountain ranges in bird richness while ecological factors shared between mountain ranges impose some constrains on standing diversity. This study is a significant step towards understanding the mechanisms that have shaped extant mountain biodiversity in a unifying framework.