How does an ecological gradient, like the slope of a mountain, affect the evolution of organisms that interact with it? Does it promote ecological speciation? And does it act the same way in different organisms? These questions are largely still unanswered. Today, my colleagues and I have published a study on the amphibians and reptiles found on a mountain in northern Madagascar, that goes some way towards answering these questions. The study is published Open Access in the journal Ecology & Evolution.
The mountain is Montagne d’Ambre, an isolated volcanic massif in the north of Madagascar that was last active around 10 million years ago. Today, it is cloaked in forest, from dry deciduous forest at the foot, to montane humid forest at the peak. From November 2017 to January 2018, I spent three months on this mountain with a team of Malagasy students and colleagues, as well as my partner, Ella. Our goal: to collect data on the herpetofaunal community composition across the mountain, and population-level sampling of a number of target species, in order to understand speciation and diversity on the mountain.
Our camp on Montagne d’Ambre in the first days of January 2018. Note: there were no rivers running through camp before we set it up.
This was the toughest fieldwork I have ever carried out. Mid-way through the trip I was able to publish a short blog post here. At the time, all was well. Just over two weeks later we evacuated the forest; as Cyclone Ava hit the area, rivers rose sharply, and the high winds were bringing massive trees down, endangering the team.
Regardless of the somewhat harrowing end to the trip, the data from this fieldwork yielded fantastic insights. In the new paper, we report 34 frog species, and 48 reptiles. Among these were several new species, including the frog Stumpffia bishopi and the gecko Lygodactylus tantsaha, and range extensions for others, such as the large white-lipped bright-eyed frog, Boophis albilabris, and the extremely secretive snake Micropisthodon ochraceus.
We recorded the slug-eating snake Micropisthodon ochraceus from Montagne d’Ambre for the first time. This species is extremely secretive, but seems to be one of the most widespread snakes on the island!
We show that the elevational ecotone on Montagne d’Ambre is reflected in both community composition—around 800–1000 m above sea level there is a significant turnover in species composition—and gene flow within species. In two chameleons and two frogs, we find evidence for parallel but uneven levels of genetic differences associated with the ecotone.
Taking this all together, it is clear that the ecotone here is significant enough to be driving divergence associated with ecological adaptation to higher and lower zones. And although we haven’t yet identified any species that actually have undergone ecological speciation across this gradient, we see divergence to different extents in a selection of species that suggests that the ecotone is having impacts on each of them, and we may be looking at species at various stages of the the process of speciation. And that’s an exciting prospect, and makes this mountain a excellent setting for speciation research.
Full citation:
Scherz, M.D., Schmidt, R., Brown, J.L., Glos, J., Lattenkamp, E.Z., Rakotomalala, Z., Rakotoarison, A., Rakotonindrina, R.T., Randriamalala, O., Raselimanana, A.P., Rasolonjatovo, S.M., Ratsoavina, F.M., Razafindraibe, J.H., Glaw, F. & Vences, M. (2023) Repeated divergence of amphibians and reptiles across an elevational gradient in northern Madagascar. Ecology and Evolution, 13(3):e9914. DOI: 10.1002/ece3.9914