Felipe Morales-Armijo, Andrea Sobrevilla-Covarrubias, Eduardo Estrada-Castillón, Adrián Escudero, J F Scheepens, Juan Lorite, Martí March-Salas

1. Cliff ecosystems provide refuge to 35-66% of the world’s endemic plants. However, they face growing threats from climbing. Evidence suggests that untouched cliffs harbor approximately twice the plant richness compared to climbed cliffs, with increasing impact as climbing intensity increases. Unfortunately, the origin and extent of the climbing impact has not been assessed so far.
2. We recorded cliff vascular plants and lichens at the protected natural area of El Potrero Chico (Mexico) before and after the establishment of new climbing routes. Subsequently, we re-recorded the routes at various time-points after openings while controlling the number of ascents. Additionally, we examined whether original cliff vegetation abundance influences the extent of climbing impact, and whether the surroundings of the new routes were also affected.
3. We found that the establishment of new climbing routes exerted the strongest negative effects on cliff plants, reducing species richness by 38%, while subsequent climbers’ ascents generated a minimal impact on richness. Worryingly, route opening affected not only species richness in the route itself, but also the surroundings of the climbing routes. Cliff plant abundance decreased by 60.6% within the bolted climbing routes, whereas it rapidly decreased by 42.3% in the surrounding area. However, this impact depended on the original vegetation abundance of the untouched cliffs. Lichen cover showed a gradual decrease, indicating that cliff-dwelling lichens are affected not only by the opening of the route but also by subsequent climbers’ ascents.
4. Synthesis and applications: Given the almost non-existent regulation of outdoor climbing activities in most countries, we urge the implementation of a conservation management protocol that defines clear strategies to regulate climbing activities and preserve pristine cliffs. On untouched cliffs with narrow endemic, rare, or threatened species, we propose banning the establishment of new climbing areas. On cliffs lacking protected or unique species, dynamic management actions should be implemented, setting a maximum number of climbing routes that can be established, and defining Limits of Acceptable Change as climbing intensity increases. The proposed conservation management should help to halt the loss of unique cliff biodiversity and safeguard pristine cliff ecosystems.