Montane regions are broadly considered hotspots of biological diversity. However, not all mountains are equally diverse: those in the tropics stand out due to an exceptional concentration of small-ranged species and rapid species turnover with elevation. High diversity in tropical mountains has frequently been linked to marked thermal stratification, i.e. limited overlap in temperature across elevations. This pattern is expected to select for narrow thermal tolerances, hence reducing dispersal of tropical organisms along elevational gradients and into different temperature regimes relative to their temperate-zone counterparts, where greater seasonality should result in wider thermal tolerances and increased dispersal. The evolutionary consequences of climatic gradients are conditional on their long-term stability. The climatic context and narrow niches of species in tropical mountains are linked to patterns of diversity because they may allow (1) greater species packing along elevational gradients and (2) increased opportunities for evolutionary diversification via either allopatric speciation across mountain passes or valleys, or parapatric speciation along mountain slopes. Additionally, the interaction of narrow elevational ranges of tropical species with spatial variation in climate and the shape and orientation of some mountains (e.g. the Andes) result in linear geographic distributions which are prone to isolation-by-distance and fragmentation, likely drivers of speciation. Although there is evidence that patterns of richness and speciation mechanisms are related to climatic variation along elevational gradients at regional and continental scales, comparative analyses linking local to global patterns and processes are lacking. Our study will use global datasets on attributes describing mountains, geographic distributions of bird species, spatial and temporal variation in climate, and diversification rates to examine patterns in montane avian richness and connect them to evolutionary processes such as speciation and extinction. We will follow a hierarchical approach in which we will describe patterns of overlap in temperature and other climatic variables along local elevational transects across mountain regions across the world, and relate patterns of climatic overlap across elevations with transect-specific features such as slope and orientation, and mountain-level characteristics including size, shape and latitude. Finally, we will link features of mountains with processes influencing avian diversity by asking whether patterns in climatic stability and climatic overlap across elevation correlate with the linearity of geographic ranges of montane birds, turnover in species composition with elevation, and speciation rates of birds using mountain regions as replicates. We anticipate our hierarchical analyses going from local (mountain-slope) to global scales will shed new light on the enduring question of why there are so many species in tropical mountains.
- Assign mountains, generate transects
- Ignacio? - I will do this soon
- Extract variables
- WorldClim—Juan?
- Slope—Ethan?
- Build models / run analyses
- Ethan?
- Drafting
- Daniel?