Estimating the population size of the endemic lichens Anzia centrifuga (Parmeliaceae) and Ramalina species (Ramalinaceae) on Porto Santo (Madeira archipelago)

Due to isolation, islands are known to harbor a high number of endemics. Although most lichen species are widespread, a number of genera are well-known for the large number of endemics. Often, those endemic taxa have a low population size and are vulnerable to ecosystem change. We carried out a survey of all seven endemic lichens of the island of Porto Santo (Madeira, Portugal, 42 km(2)) in order to generate data for a IUCN Red List assessment. Six km 2 of suitable habitat for the species were searched and mapped at 100 m resolution. The main habitat for the species consisted of volcanic outcrops, mainly basalt peaks and lava flows on the slopes. All accessible areas-circa 90% of the peaks and 50% of all outcrops-were surveyed by the authors during one week as a volunteer project. The population size of Anzia centrifuga was estimated to be 50-100 individuals. It occurred only on exposed, stable, vertical, N to W facing rock faces above 240 m, restricting the potential habitat to less than 1.0 km(2). Ramalina nematodes occurred often abundantly on most of the larger exposed ridges. Ramalina confertula and R. portosantana each occur on several rock outcrops in the N part of the island. Ramalina erosa, R. jamesii and R. timdaliana were restricted to an area often less than 1000 m 2 in the surroundings of their type localities. After application of the IUCN criteria, all studied endemics fitted well into category Critically Endangered although no immediate threats seem present.