Cliffs as priority ecosystems

In December 2022, the 15th Conference of Parties of the UN Convention on Biological Diversity (COP15) was held to discuss halting global biodiversity loss by 2030. To prevent governments from failing again, as they did with the previous global biodiversity target for 2010 (Butchart et al., 2010), the new pact sets more specific conservation goals. For instance, nations are expected to prioritize the protection of fragile ecosystems with high ecological integrity and high biodiversity (UN CBD, 2022). One ecosystem that particularly fulfils these criteria is cliffs. Cliffs, all over the world, harbor a large diversity of endemic plants, endangered species, and rock-dwelling birds and bats (Larson et al., 2000). Specifically, cliffs hold 35–66% of the endemic plant taxa of most countries (Ellenberg, 1988; Larson et al., 2000; Wardle, 1991) and provide vital climatic and ecological refuges for rare species and millennial-old trees, as recently discovered with the oldest living tree in the European Union (Sangüesa-Barreda et al., 2022). Despite cliffs have been among the least human-disturbed ecosystems, many of them are now under pressure from the huge increase in popularity of climbing (deCastro-Arrazola et al., 2021; Kelly & Larson, 1997; March-Salas et al., 2018, 2023). Climbing has turned from a niche sport to a worldwide sensation. In the United States, the number of climbers has increased from <500,000 in the late 1990s to over 10 million. Worldwide there are an estimated 50 million climbers (AAC, 2019; deCastro-Arrazola et al., 2021), and their recreational activity threatens many endemic and rare plants, disturbs rock-dwelling animals, and increasingly affects the unique biodiversity of cliffs (Covy et al., 2019; deCastro-Arrazola et al., 2021; March-Salas et al., 2023). This increase in climbing could also severely affect the ecological integrity of many cliffs and reduce the genetic diversity of cliff species through habitat degradation (Gallagher et al., 2023; Vogler & Reisch, 2011). The loss of genetic diversity would likely jeopardize the potential of cliff species to adapt to climatic changes, which may further increase their extinction risk (Exposito-Alonso et al., 2022), especially considering the limited dispersal abilities of many cliff plants (Larson et al., 2000). Unfortunately, cliffs and their biota are relatively unstudied and have been largely overlooked in global agendas for biodiversity conservation. The European Union's Habitat Directive 92/43/EEC includes calcareous and siliceous rocky slopes and sea cliffs among its priority areas for biological conservation (European Commission, 1992), but the legal framework is restricted to European Union member states, and its implementation has been limited. Meanwhile, in most countries, continuous monitoring of the effects of climbing on cliff biodiversity is lacking, which denotes deficient regulatory control for this ecosystem. To mitigate future losses of species and genetic diversity, more data on cliff biodiversity and more research linking climbing pressure and cliff ecosystem functioning are urgently needed. Gathering information on biodiversity is relevant for already established climbing areas, but it is especially important for undisturbed cliffs likely to be climbed in the future. Uncrewed aerial vehicles could be advantageous for extensive data acquisition and to support the conservation of inaccessible and endangered plant species (La Vigne et al., 2022). However, the use of this technology should be optimized to efficiently record high-resolution images of large cliff areas. Improved image recognition algorithms and machine learning models trained on large data sets may aid information extraction (e.g., species identification, population size, phenology) from such images. The increasing establishment of new climbing areas highlights the need for initial impact assessments. This should include descriptions of the biodiversity on the rock face and on the access way to the cliff and an estimation of the carrying capacity of the system. Site-specific, evidence-based regulations for cliff management are also needed. To attain this, we encourage setting limits of acceptable change (LAC) and specific management guidelines (McCoy et al., 1995; Stankey et al., 1985). To prevent a regulatory mess among countries, regulations and LAC should be globally approved by national governments and a biodiversity action agenda (Chan et al., 2023). Long-term strategies for protection of cliff biodiversity require management plans for rewilding, restoring affected populations (Perino et al., 2019), and ex situ conservation of species at risk (e.g., La Vigne et al., 2022). The successful management of cliffs will also require engaging stakeholders in collective action, which will heighten awareness, ensure long-term environmental and societal benefits (Chan et al., 2023), and bridge the current gap between climbers and land managers. To achieve all this, it is crucial that conservation agendas and the Post-2020 Global Biodiversity Framework include cliffs as priority ecosystem, which would contribute to stemming the loss of biodiversity by 2030.