A review of the distribution and ecology of the elusive Brown Hairstreak butterfly Thecla betulae (Lepidoptera, Lycaenidae) in the Iberian Peninsula

The Brown Hairstreak (Thecla betulae L.) is one of the least observed butterflies of the Palaearctic region, even though its distribution spans from Portugal in the west, to Russia and Korea in the far east. Adults are arboreal and seldom descend to ground level. As a result, this species is mostly monitored via the detection of eggs on the food plant during wintertime. In the Iberian Peninsula, this species was largely unknown until very recently, but a recent burst of regional studies in Spain has begun bridging this gap. However, their focused nature and a still incomplete knowledge on T. betulae in Portugal promoted the need for an integrative study at the Iberian scale. Here, we carried out a full literature review on the distribution, ecology and behaviour of T. betulae in Portugal and Spain. Complemented with field work in Portugal, we revealed an almost continuous distribution in the northern third of Iberia, whilst populations further south are mostly mountain-bound. In order to help with future discovery of new populations, we built a species-distribution model relating its occurrence with bioclimatic variables. This model accurately explains the current known occupation of the territory and highlights other areas where the species may potentially be found. Finally, we found evidence of a broadening of the species’ niche through the local use of an hitherto unknown food plant. This study sets a new knowledge baseline for future works and conservation of T. betulae through southern Europe.