Are mountain passes higher in the tropics? Revisiting the climate variability hypothesis suggests microgeography more important than latitude

Background/Question/Methods

The climate variability hypothesis posits that relatively stable climate, such as that of the tropics, induces distinct thermal bands across elevation that render dispersal over tropical mountains difficult compared with temperate mountains. Yet small-scale landscape features buffer thermal variance locally, which may play a greater role in driving mountain thermal regimes than latitude. Although the climate variability hypothesis has been studied extensively, few works have employed thermal data that capture microclimate– such as conditions within forest, soils, and topographically rugose regions– instead relying upon weather tower data or satellite estimates. Here we provide an extensive investigation of temperature drivers from fine to coarse scales to revisit classic assumptions concerning species’ distributions and range limits. We compiled an open-access database of empirical temperature data on 29 mountain ranges spanning six continents (cumulative 11,775,331 measurements and 524 sampling years) to characterize thermal overlap (similarity in temperature at high and low elevations) as it varies across vertically stratified microhabitats, biomes and owing to seasonal changes in foliage. We then constructed a series of mixed effects models to compare the roles of macro-, meso-, and microgeography in driving mountain thermal regimes.

Results/Conclusions

We demonstrate that the degree of similarity in temperatures at high and low elevations, and therefore how much a mountain acts as a physiological barrier, is more driven by vegetation cover, snow depth, and height above or belowground than by latitude of the mountain. Impressively, an increase of 1 m of height above ground generates an average increase in thermal overlap equivalent to a 5.3º change in latitude. In addition, forested mountains have reduced thermal overlap–149% lower– relative to non-forested mountains. Thus, rather than focusing on how macroclimate across latitude influences thermal tolerance, biogeographers should more seriously consider the importance of local-scale climate even when posing macroecological questions. We provide evidence in support of a climate hypothesis that emphasizes microgeography as a determinant of dispersal, demographics, and behavior. We use this research as a case study of how integrating some of the fundamental intuitions from meteorology may cause us to reconsider widely-held ecological assumptions.