Are mountain passes higher in the tropics? janzen's hypothesis revisited

Abstract

Synopsis In 1967 Daniel Janzen published an influential paper titled "Why Mountain Passes Are Higher in the Tropics." Janzen derived a simple climatic-physiological model predicting that tropical mountain passes would be more effective barriers to organismal dispersal than would temperate-zone passes of equivalent altitude. This prediction derived from a recognition that the annual variation in ambient temperature at any site is relatively low in the tropics. Such low variation within sites not only reduces the seasonal overlap in thermal regimes between low- and high-altitude sites, but should also select for organisms with narrow physiological tolerances to temperature. As a result, Janzen predicted that tropical lowland organisms are more likely to encounter a mountain pass as a physiological barrier to dispersal (hence "higher"), which should in turn favor smaller distributions and an increase in species turnover along altitudinal gradients. This synthetic hypothesis has long been at the center of discussions of latitudinal patterns of physiological adaptation and of species diversity. Here we review some of the key assumptions and predictions of Janzen's hypothesis. We find general support for many assumptions and predictions, but call attention to several issues that somewhat ameliorate the generality of Janzen's classic hypothesis.

Plasticity in thermal tolerance has limited potential to buffer ectotherms from global warming

“…Our broad-scale analysis of thermal tolerance plasticity across ectotherms has many implications for our understanding of the interactions between plasticity and warming. First, it has been suggested that tropical taxa may be more vulnerable to warming than temperate taxa, in part, because tropical taxa are expected to have lower plasticity in thermal physiology [12,14,15]. However, we find little support for decreased plasticity in upper thermal tolerances in taxa from low latitude/ low seasonality habitats.…”

Effect of warming rate on the critical thermal maxima of crabs, shrimp and fish

“…Species that evolved in non-seasonal environments, like the tropics, are less likely to have broad thermal intervals and to acclimate to different temperatures. Thus, tropical species may be more vulnerable to alterations in temperature because their thermal limits may be closer to their optimal temperature (Stillman, 2003;Ghalambor et al, 2006;Deutsch et al, 2008). For this theory to be tested, the thermal limits of a much wider range of animals need to be estimated experimentally.…”

Cryptic species diversity reveals biogeographic support for the ‘mountain passes are higher in the tropics’ hypothesis

“…The CVH thus predicts selection for broad thermal tolerances of temperate species and narrower thermal tolerances of tropical species. Narrow thermal tolerance will in turn select against dispersal into inhospitable climates, resulting in smaller geographical ranges [13,14,19,20].…”

“…The MPHT hypothesis proposes that mountains are more effective physiological barriers for tropical than temperate species, because along elevational transects, the annual thermal regimes of sites in the tropics have less overlap than those in the temperate zone [19]. Consequently, the broad thermal tolerances of temperate-zone species should allow them to disperse more broadly across elevational gradients, whereas narrow thermal tolerances of tropical species should restrict their distributions to relatively narrow elevational bands [19,20].…”

“…By extension, the MPHT hypothesis provides a mechanistic explanation for latitudinal differences in species richness [20]. The MPHT predicts that across tropical elevational gradients, populations with narrow thermal tolerances and limited dispersal ability will have increased isolation, genetic divergence and speciation, ultimately leading to higher species richness at low latitudes.…”