Can’t see the wood for the trees? Canopy physiognomy influences the distribution of peninsular Indian Flying lizards

Aim: In the absence of topographic barriers to dispersal, spatial boundaries of species are largely governed by the environmental regimes they occupy. Flying lizards (Agamidae: Draco) from peninsular India have surmounted prominent geographic barriers, but their northern distribution abruptly ends at the ‘Goa gap’, a latitudinal boundary separating the wet and dry regions of the Western Ghats. Given their exclusively arboreal habits, we posit that canopy physiognomy determines Draco distribution in regions of climatic suitability. Location: Peninsular India. Taxa: Draco dussumieri, Monilesaurus rouxii. Methods: We analysed occurrence data collated from multiple sources against a suite of climatic and canopy-specific predictors to model the occurrence of Draco. We used an information theoretic approach to binary logistic regression that incorporates presence/absence data, along with a maximum entropy-based algorithm that models ecological niches in space using presence-only data. Climatic data were obtained from CHELSA and WorldClim. We compared models for Draco dussumieri with the only other largely sympatric, arboreal agamid, Monilesaurus rouxii, which spans the Goa gap. Finally, we statistically tested how regions Draco inhabits differ in canopy physiognomy from the regions in the Western Ghats where it does not occur. Results: Canopy height was the most influential predictor of Draco presence in regions of climatic suitability under both the logistic regression and maximum entropy models. While climatically suitable regions occur north of the Goa gap, these regions harbour significantly lower canopies with lesser coverage. Despite a narrower range of climatic suitability, Monilesaurus has spanned the Goa gap, presumably because it is less dependent on canopy traits. Main conclusions: Lower canopies with lesser coverage act as a biogeographic barrier for the dispersal of Draco into regions of climatic suitability. In addition to being a climatic boundary, the Goa gap serves as a vegetational barrier that demarcates regions with significantly dissimilar canopy physiognomies. Our study highlights the importance of quantifying environmental regimes that organisms occupy to better understand distribution patterns. Our results implicitly predict the effects of deforestation on Draco distribution in the peninsula and therefore, influence its conservation strategy.