Ecological constraints and distribution of the primitive and enigmatic endemic Mexican butterfly Baronia brevicornis (Lepidoptera: Papilionidae)

Baronia brevicornis Salvin (Lepidoptera: Papilionidae) is one of the most enigmatic butterflies in the world and possibly represents the most ancient lineage among the superfamily Papilionoidea. Its geographic distribution is remote from that of all its potential close relatives and many of its biological and ecological characteristics are unique among the suborder Rhopalocera. One of its particularities is that the occurrence plots of this species seem to be independent, each representing individual populations, despite the fact that the host plant: Acacia cochliacantha Humboldt and Bonpland ex Willdenow (Fabaceae), is one of the most common Mexican Fabaceae species. Our results show that no B. brevicornis populations occur if the host plant does not cover at least two-thirds of the locality. Even in the most favourable zones, the landscape occupancy of the butterfly does not exceed 2.5% of the available habitat even when its host plant covers 50% of the area. The average density of adults was 840 individuals/ha in favourable habitats, frequently on areas of around 3 ha, below of 1400 m. Using the BIOMOD2 package and the largest available set of abiotic conditions for Mexico implemented in the WorldClim database, we propose a revised potential distribution and discuss the results of our model with field occurrence data. Evolutionary and conservation issues are discussed in the light of our results.