Thermal adaptation of amphibians in tropical mountains. Consequences of global warming

Résumé

Temperature is likely to be one of the most important abiotic factors given how it affects the physiology of the whole organism and as a consequence, it has an essential role in ecology and evolution. However, how the geographical (and temporal) variation of temperature is related to physiology still raises many questions. Several macrophysiological hypotheses have been proposed to explain the variation patterns of thermal physiological parameters across ecological gradients. Among them, Janzen’s (1967) 'seasonality’ hypothesis is probably one of the most relevant as it has awakened a great interest in other areas besides physiology itself, such as biogeography, ecology, evolution and conservation biology. Janzen proposed that tropical species are specialists to thermally stable environments and therefore would be more limited to dispersing altitudinally (up or down mountain) than temperate species because of evolved physiological barriers. These biologically based dispersal constraints may act as a selective mechanism promoting isolation in the populations and thus fuelling speciation rates in tropical mountain ranges, considered the most diverse hotspots in the world. The present thesis explores the evolution in the thermal sensitivity of amphibians across a broad tropical elevational range (4200 meters) in the tropical Ecuadorian Andes and propose the likely environmental causes (altitude and microenvironment) driving the extraordinary diversity in physiological parameters across the gradient. This information also provides essential insight for predicting which species or populations are most vulnerable to global warming. Through estimates of thermal sensitivity in larval and adult amphibians we show, through comparative methods, how thermal sensitivity and tolerance limits diverge along the elevational gradient. We demonstrate that environmental variation at the individual scale is important when testing some of the main macrophysiological hypotheses being better predictors of thermal physiological diversity in amphibians. We demonstrate that amphibians’ thermal physiology is strongly influenced by their thermal environment but also some of its variation may be limited by inherent constraints. A further main finding is the empirical demonstration that physiological barriers in tropical mountains are ‘higher’ upwards than downwards, refining Janzen´s paradigm that altitude functions as a barrier through cold tolerance whereas warm evolution occurs contrarily only transversally (horizontally) through habitat selection. Finally, this thesis suggests that lowland tropical amphibians are more vulnerable to an increase of temperature than their upland counterparts, because they are currently experiencing environmental temperatures close to their physiological optima and heat tolerance. Yet, the use of microclimatic information predicts how, in some cases, highland species may be also vulnerable to suffer heat stress and will therefore need to search for thermal shelters to avoid extreme heat events.