Is a restricted niche the explanation for species vulnerability? Insights from a large field survey of Astragalus tragacantha L. (Fabaceae)

Understanding the environmental drivers of the ecological niche of rare plant species is crucial for conservation programs. We have examined the ecological niche of Astragalus tragacantha L. This rare plant is the main component of the relict west-Mediterranean phrygana, a plant community impacted by urbanization and landscape degradation which are major threats for biodiversity of Mediterranean littoral. During our field work in Provence (southern France), we studied habitat characteristics, floristic composition and carried out an exhaustive population census. The census revealed less than five thousand individuals, a strong habitat fragmentation and a high ratio of foliar necrosis. Niche analyses were conducted considering either all individuals or separated size stages. Analyses based on all individuals did not support the hypothesis that a too narrow niche by itself explains the vulnerability of A. tragacantha. However, niche modelling revealed differences among size stages supporting a restricted regeneration niche and a wider ecological range in the past. Our comprehensive field survey underlines a worrying situation for A. tragacantha and sets a baseline for conservation.