Ecological niche conservatism shapes the distributions of lichens: geographical segregation does not reflect ecological differentiation

We studied the ecological requirements of two closely related species of lichens, Solenopsora cesatii and S. candicans, which grow predominantly on basic rocks in natural habitats. We determined the ecological niches they occupy at the centre of their distribution (Mediterranean Basin) where they occur sympatrically and in areas with a continental climate (Western Carpathians and Pannonia) at the limits of their ranges, where they are geographically segregated, in order to assess the level of differentiation across their distributions. Tests of niche equivalency and similarity revealed that the species colonize similar habitats both in the centre and the margin of their ranges, which indicate niche conservatism across their distribution. Geographical segregation between populations of S. cesatii and S. candicans at the range margin does not reflect niche differentiation, but a lower availability of suitable habitats. For the Western Carpathians, we developed predictive habitat suitability maps using a Climate Space Model based on presence-absence data, which indicated the potential distribution of suitable sites. The reconstruction of habitat suitability under past climatic scenarios indicates presence of suitable sites in Last Inter-Glacial and Mid-Holocene and absence in Last Glacial Maximum. Predictions for the years 2050 and 2070 show horizontal habitat tracking and far fewer suitable sites. This case study is an example of how the conditions at the limits of their range can contribute to the finetuning of the ecological requirements of species.