Rarity is a fundamental concept in conservation biology, as rare species are usually thought to have a greater likelihood of extinction than their relatives. There is, however, a large variance in the factors leading one species to be considered “rare” (historical, geographical, ecological, biological…), and therefore in the modus operandi of the potential treats associated to them. Low abundance, small range size, and habitat specialization may predispose species to local extinctions, but they may affect species differentially. In complex ecosystems like mountains, with strong altitudinal gradients and contrasted habitats, the strength and frequency of those factors are expected to vary over such heterogeneous landscape. A first step for an efficient conservation policy should therefore consider a survey of different types of rarity at different environments, followed by their characterization in terms like evolutive diversity, chorology, and robustness provided by biological traits.
We used the flora of the central Pyrenees, one of the highest mountain ranges in Southern Europe, to explore what kind of rarity is most common in mountain systems, and how rarity patterns change along the altitudinal gradient. We first scored about 2000 taxa according to chorology, altitudinal range extent, local abundance and distribution, peripheral position, and habitat. Then we evaluated them according to thresholds of “small range”, “low abundance”, and “habitat specialization”, and their combination, in order to generate groups of “form of rarity” (sensu Rabinowitz 1981). Finally, we analyzed species diversity (number of taxa and phylogenetic diversity) in each group along altitude.
Results/Conclusions

Most plants occur at small population sizes. Low local abundance, thus, is the main factor accounting for rareness, followed by small range at the local scale examined. Categories including the “habitat specialization” factor, however, proportionally contain a higher diversity at upper taxonomic levels. As the frequency of the three factors examined increases with altitude (particularly habitat specialization), it is more likely to find rare plants at higher than lower altitudes, which are especially vulnerable to habitat change. About 80% of plants with peripheral populations in the Pyrenees would be included in some category of rareness. The link between marginal position and rareness is especially important among Boreoalpines and the Iberian Mediterranean endemics. The Southern face of the Pyrenees constitutes, therefore, a mix and a barrier for very contrasted biogeographical European elements, and a very suitable place to monitoring future changes of biodiversity, as many rare species should disproportionately respond to climate change.