Canopy physiognomy governs the distribution of peninsular Indian flying lizards in regions of climatic suitability.
Above: Silhouette of an Indian Flying Lizard in its arboreal habitat.
I have always been intrigued by organismal distributions. Why do certain species occur in certain regions? Why do they stop, sometimes abruptly, at certain latitudes where there aren’t any physical barriers to dispersal? So, when I decided to work on peninsular Indian Flying dragons (Draco dussumieri) for my PhD, it was quite natural to ask why Dracos do not cross the Goa gap (15.8° N) in the Western Ghats mountains of peninsular India.
The Western Ghats is a great laboratory to study various aspects of organismal biology. Life here comprises of organisms with remarkably distinct evolutionary histories that have colonized these mountains at different periods in geological time. The distribution of a species in the Western Ghats is largely a factor of when its ancestor first colonized these mountains and its ecological ability to disperse and adapt to a constantly changing environment.
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Chaitanya, R, & Meiri, S. (2021). Can’t see the wood for the trees? Canopy physiognomy influences the distribution of peninsular Indian flying lizards. Journal of Biogeography, 49, 1– 13. https://doi.org/10.1111/jbi.14298
This mountain range is characterized by three prominent biogeographic breaks, the Shencottah pass, the Palghat gap and the Goa gap. While the former two are physical barriers – deep valleys that cut across the mountains horizontally, the Goa gap is not a physical barrier but an invisible boundary that certain organisms have failed to span. Naturally, biogeographers have long been flummoxed by the absence of certain organisms north of this ‘hypothetical’ barrier. What exactly is going on north of the Goa gap that has prevented otherwise vagile organisms such as Flying dragons from crossing over?
Studies in the past have suggested that the Goa gap may be a boundary that demarcates two different climatic regimes in the Western Ghats, the northern region being warmer and drier. While this is true, certain organisms that are dependent on high rainfall such as the Roux’s lizard (Monilesaurus rouxii) have successfully crossed the gap and colonized regions north of it, but not certain others like Draco that could easily persist in dry forests. So, why have the wet-adapted, arboreal Roux’s lizards been able to colonize the drier regions north of the Goa gap when the more climatically pliant Dracos have not?
To solve this riddle, Shai and I compared the ecological niches of the Roux’s lizard and Draco to try and decode the environmental variables that govern their distributions. We expected a complex concoction of reasons for Draco absence, but to our surprise we received a very simple answer: tree heights! Draco could not traverse the Goa gap because trees north of this barrier were not tall enough to support them. Simple statistical analyses of canopy physiognomy in regions of Draco presence (south of the gap) versus absence (north of the gap) revealed great disparity in canopy height and coverage. Further, niche models built based only on climatic variables revealed vast expanses of suitable habitats for Draco north of the Goa gap. This indicated that in regions of climatic suitability, the height of canopies influences the presence of these exclusively arboreal lizards.
During fieldwork, my colleagues and I have noticed that Draco occupy the upper reaches of trees closer to the canopy and only descend about midway through the tree trunk during courtship or to forage for ants. The females come down to the ground only once, when they have to lay eggs. The males never do! Roux’s lizards on the other hand, are often found mid-trunk, often even lower near the base of the tree. They are regularly seen sleeping on low shrubs too. So, despite requiring more rainfall for their persistence, Roux’s lizards have been able to disperse across the Goa gap chiefly because they are not dependent on tall canopies.
Our study belabours the point that in the absence of topographic barriers to dispersal, the environmental regime an organism occupies governs its spatial boundaries. It proposes a new, canopy physiognomy based biogeographic hypothesis for the Western Ghats region that can be tested against other model organisms.
Curiously, King Cobras, Slender Lorises and Hump nosed pit-vipers reach the Goa gap from the south but do not pass over. So, what stops these animals from spanning the Goa gap?
The Peninsular Indian Flying lizard, Draco dussumieri, takes flight. Location: Agumbe, India.
Photographed by Vinod Venugopal.
R. Chaitanya; School of Zoology, Tel Aviv University, Tel Aviv, Israel