American Society of Naturalists

David H. Klinges and Brett R. Scheffers (Jan 2021)

Revisiting Janzen 1967’s classic: we show that 🌳, ❄️, microhabitats drive 🌡️ overlap on 29 🏔️s more than latitude

Read the Article (Just Accepted)

Mountain passes are higher for those in the tropics…and in forests, and underground

Since the publication of Dan Janzen’s seminal 1967 paper, “Why mountain passes are higher in the tropics”, ecologists have pondered the role that mountains play in creating barriers to dispersal. Janzen suggested that tropical organisms, which oftentimes experience similar temperatures year-round, may have a tougher time crossing a cold mountain peak than temperate organisms, which are tolerant of a range of climates due to their cool winters and warm summers. This effectively makes tropical mountains thermally “taller” than temperate ones of similar height. Yet despite many works exploring the downstream implications of these findings, few have explored other possible drivers of mountain “thermal height” besides changes in latitude. For examples, forests shade out sunlight in the heat of the day and trap warm air at night, thereby offering more thermally stable conditions—might forested mountains be therefore taller than non-forested ones?

Here, researchers at the University of Florida expand upon Janzen’s hypothesis using in-situ measurements globally, and for the first time account for many other geographic variables besides latitude that may influence how much temperature changes across mountains. Their findings suggest that mountains may be thermally “taller” for organisms that live within soils relative to those that live above the ground, as well as for forest-dwellers relative to species in open-air systems. Mountains situated at different latitudes, on the other hand, did not differ substantially in “thermal height”.

Although this work does not discount Janzen’s idea that tropical mountains may act as more difficult barriers to dispersal than temperate mountains, it suggests that more emphasis should be placed on local considerations for determining dispersal. To accompany the classic gradients of latitude and altitude, the authors propose that “vertitude”—for lack of a better word to describe the change in climate from below ground, to the surface, to above any vegetation canopy—ought to be a worthy consideration for biogeography as well.

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Abstract

An extension of the climate variability hypothesis is that relatively stable climate, such as that of the tropics, induces distinct thermal bands across elevation that render dispersal over tropical mountains difficult compared to temperate mountains. Yet, ecosystems are not thermally static in space-time, especially at small scales, which might render some mountains greater thermal isolators than others. Here, we provide an extensive investigation of temperature drivers from fine to coarse scales, and demonstrate that the degree of similarity in temperatures at high and low elevations on mountains is driven by more than just absolute mountain height and latitude. We compiled a database of 29 mountains spanning 6 continents to characterize “thermal overlap” by vertically stratified microhabitats, biomes, and owing to seasonal changes in foliage, demonstrating via mixed-effects modeling that micro- and mesogeography more strongly influence thermal overlap than macrogeography. Impressively, an increase of one meter of vertical microhabitat height generates an increase in overlap equivalent to a 5.26° change in latitude. In addition, forested mountains have reduced thermal overlap – 149% lower – relative to non-forested mountains. We provide evidence in support of a climate hypothesis that emphasizes microgeography as a determinant of dispersal, demographics, and behavior, thereby refining classical theory of macroclimate variability as a prominent driver of biogeography.