Ononis tridentata L. subsp. crassifolia (Leguminosae) is a narrow endemic plant restricted to gypsum outcrops in south-east Spain. Its habitat and populations are currently threatened by anthropogenic disturbance. Because of the paucity of information concerning its distribution, abundance and response to disturbance, we assessed its status and evaluated the impacts of quarrying, ploughing, grazing and afforestation. Distribution and population size were estimated by field surveys, censuses and mapping. We measured cover, plant volume, fruit and seed production, seed predation and seedling recruitment to asses any effects of disturbance. The species’ area of occupancy is 1.6 km2 and its extent of occurrence 337 km2, in 29 habitat patches and 16 populations between 705 and 1,125 m altitude, and its population size was estimated to be 531,605. Quarrying, ploughing, overgrazing and afforestation negatively affected the species, in this order. We recommend this subspecies be categorized as Vulnerable on the IUCN Red List. A species recovery plan is required, and the ecological restoration of altered areas would mitigate negative effects on the species and improve the overall conservation of gypsum habitats.
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