Invasive risk assessment and expansion of the realized niche of the Oriental Garden Lizard Calotes versicolor species complex (Daudin, 1802)

Correlative species distribution modelling (SDM) can be a useful tool to quantify a species’ realized niche and to predict its potential distribution for non-native ranges. The agamid lizard Calotes versicolor s.l. belongs to the most widely distributed reptile taxa worldwide. In the past, C. versicolor s.l. has been introduced to several countries, including regions in the Oriental, the Neotropical and the Afrotropical realms, where strong negative impact on the local fauna is assumed. Due to the complicated taxonomy and the existence of several cryptic species, which are covered by this taxon, we used C. versicolor sensu lato and its four subtaxa (C. versicolor sensu stricto, C. irawadi, C. vultuosus, C. farooqi) as target species to (1) compute correlative SDMs for C. versicolor s.l. and its subtaxa and project them across the globe to highlight climatically suitable areas of risk for future invasion and (2) based on the ecological niche concept, we investigate if the species complex expanded its realized climatic niche during the invasion process. We use two different SDM approaches, namely n-dimensional hypervolumes and Maxent. N-dimensional hypervolumes are a non-hierarchically ranked approach, which is a useful tool to investigate the expansion in the realized niche, while Maxent, a hierarchically ranked model, is used to focus on potentially suitable areas
for future invasion. We calculated two final models for C. versicolor s.l., one based on records from the native range and one based on records from the native and invaded range, as well as one model for each subtaxon. Our results show a geographic expansion into novel climatic conditions as well as an expansion in the realized niche. Our results reveal that C. versicolor s.l. is currently inhabiting 13% of its potential range but could find suitable climatic conditions on a global surface area between 14,025,100 km2 and 53,142,600 km2. Our predictions reveal large areas of highly suitable climatic conditions for the Oriental, Australian, Afrotropical and Neotropical realms, whereas only small regions of the Palearctic and Nearctic realms provide moderately suitable conditions. Further, some localities, especially those with a high amount of human traffic like ports or airports, might act as multiplicators and might therefore be a stepping stone into further areas.