Stefano Mammola, Elena Piano, Florian Malard, Philippe Vernon, Marco Isaia
In order to predict the consequences of climate change for current biomes, scientists seek to investigate possible factors that could drive the distribution of species across space and time. A seminal contribution in this realm is the ‘Mountain Passes Hypothesis’, proposed by Dan Janzen in 1967 to explain why biodiversity is generally higher in tropical mountains. Janzen’s hypothesis, published under the evocative title “Why Mountain Passes are Higher in the Tropics”, provides a mechanistic framework to link climate seasonality to the thermal specialization and dispersal capacity of species. While the Mountain Passes Hypothesis was specifically developed to explain why biodiversity is generally greater in tropical versus temperate mountains, this framework could be theoretically transferred to a larger variety of habitats displaying reduced climatic seasonality, for example soils and deep sea waters. Yet the ecological complexity of these ecosystems, as well as the challenge posed by the scale at which this hypothesis could be tested, have largely prevented ecologists from comprehensively testing the underlying assumptions of Janzen’s hypothesis in non-tropical habitats.
In this work, we focused our attention on cave ecosystems, unique natural laboratories with largely constant temperature. By means of an integrative approach, we applied and validated Janzen’s predictions by demonstrating that in obligate cave-dwelling spiders, thermal tolerance—as derived from laboratory tests—decreases with increasing level of subterranean specialization. Then, we linked thermal specialization to dispersal propensity, by proving that specialized spiders with narrower thermal tolerance have smaller elevational ranges across which they encounter a similarly shorter temperature excursion. Finally, consistent with our previous predictions of habitat change estimated for future global warming scenarios, we showed that such ecological specialization greatly limits spider dispersal, posing a serious hindrance to their survival in response to climate change. Our contribution provides the first extension of Janzen’s hypothesis to cave ecosystems, but it could be potentially generalised to the larger variety of habitats displaying a similarly reduced thermal seasonality. More importantly, our work develops a conceptual approach to understanding the effects of past and future climate changes on biodiversity patterns.