Stefano Mammola: Revisiting Janzen’s hypothesis using cave-dwelling spiders

In this insight, Dr. Stefano Mammola discusses his paper “Extending Janzen’s hypothesis to temperate regions: a test using subterranean ecosystems” as well his experience as a subterranean ecologist.

What’s your paper about?

This paper is about testing the underlying assumption of Janzen’s hypothesis in caves. Published in 1967 under the evocative title “Why Mountain Passes are Higher in the Tropics”, Janzen’s hypothesis is an important conceptual framework that mechanistically links climate, local adaptation and dispersal propensity, to explain how climatic variability shapes biodiversity patterns. Concerns over the fate of biodiversity in a changing climate have revived research into Janzen’s thoughts, as exemplified by a recent comprehensive test of this hypothesis in tropical versus temperate areas. Building upon this ground, 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 cave-dwelling spiders of the genus Troglohyphantes, thermal tolerance decreases with increasing level of subterranean specialization. We then linked thermal specialization to dispersal propensity, proving that specialized spiders with narrower thermal tolerance have smaller elevational ranges across which they encounter a similarly shorter temperature excursion. Lastly, consistent with our previous predictions of habitat change estimated for future climate change scenarios, we showed that such ecological specialization greatly limits the spider dispersal, posing serious hindrance to their survival in a climate change perspective.

What is the background behind your paper?

A cave-dwelling spider of the genus Troglohyphantes. Photo courtesy of Francesco Tomasinelli (
A cave-dwelling spider of the genus Troglohyphantes. Photo courtesy of Francesco Tomasinelli (

We’ve been studying alpine cave spiders for many years now. In particular, our model organisms are spiders of the genus Troglohyphantes, a group whichunderwent a remarkable radiation in subterranean habitats. The genus comprises more than 130 species with different degrees of subterranean adaptations distributed across the main European mountain ranges. In a recent work, we used species distribution models (SDM) to explore the potential future distribution of some of these spiders in a climate change perspective. We predicted a substantial decline in their future distribution, a result that raised concerns for their long-term survival. Yet, this result was exclusively based on correlative evidences; thus, we set up thermal tolerance tests for corroborating model projections. By combining physiological data on thermal tolerance, distribution data, morphological measures related to subterranean adaptation, as well as temperature data acquired in the natural habitats of these spiders, we were ultimately able to test the Janzen’s hypothesis in caves and build a conceptual framework for anticipating the differential responses of subterranean species in a changing climate.

Who should read your paper?

Anyone, of course: celebrities, key authorities, the average man in the street… perhaps even scientists. In all seriousness, we hope that this article will break the barrier of subterranean biology and may be applicable to scientists interested in understanding what is happening to our planet and to the global biodiversity.  Despite using cave spiders as models, our results can be read from different perspectives and the general conclusions may apply to different organisms, especially if considering the tight connection between species and climate. Most of us like caves for their mystery, their beauty, for the large bat colonies they harbour, and for the remains of Neanderthals. That’s where the general understanding usually ends, but we would like to push the reader beyond these ideas: subterranean ecosystems are much more than that. It’s recognized that due to their environmental constancy and simplified communities, caves are spectacular natural laboratories in which to explore a wide variety of biological questions. This research is nothing but a little example of how we can use cave organisms to test general eco-evolutionary hypotheses.

What is the best thing about being an ecologist?

The first thing that comes to my mind is fieldwork and the possibility of spending a lot of time outdoors looking at fascinating species and ecosystems. But ultimately, I think it’s much more. Ecological systems are utterly complex: thousands of species, millions of interactions, chaotic fluctuation in the abiotic conditions; all these elements mixed in the same cauldron with a perfect equilibrium. Although we’ll never really understand this beautiful mess, even getting a little insight into it can be really rewarding.

What’s your current position?

Stefano Mammola doing fieldwork in a cave. Photo by courtesy of Francesco Tomasinelli (
Stefano Mammola doing fieldwork in a cave. Photo by courtesy of Francesco Tomasinelli (

I’m in a bizarre position at the moment: a postdoc bridging the University of Torino in Italy, within dr. Marco Isaia’ research team, and the Finnish Museum of Natural History of Helsinki, within the LIBRe research group lead by Dr. Pedro Cardoso. My scholarship covers the expenses for the period abroad, which is designed to improve my CV, with the condition that I’ll apply for some European projects at the end of my stay. It’s a very nice opportunity, and I’m really enjoying my time here in Finland – such a great country, although with not many caves!

What are you currently working on?

I primarily study subterranean ecology. Currently, my research mainly focuses on the effects of climate change in subterranean ecosystems. This is an understudied topic, offering a fertile ground for novel discoveries. I’m also interested in spider ecology and taxonomy, conservation biology, and more.

What article are you most proud of?

I’m very fond of “Record breaking achievements by spiders and the scientists who study them”, published in PeerJ. Certainly not the most serious research in history, but I really enjoyed working on it.

What do you do in your spare time?

I love climbing, alpinism, speleology, and outdoor activities in general. At the moment, perhaps my greatest passion is chess. Recently, I’ve been charmed by this relatively new variant called “4 player chess”. This highly complex game features four players fighting on an enlarged chess board, either playing as teams of two versus two or all against each other. A friend of mine defined the act of playing this game a “drunken art”. Somehow, I see in it several parallels with the (drunken art of doing) ecology.

You can read Stefano’s paper here and the free plain language summary here.

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