Global evaluation of current and future threats to drylands and their vertebrate biodiversity

Drylands, encompassing nearly half of the Earth's terrestrial surface, are vital ecosystems supporting endemic species and providing essential services to a third of the global human population. Despite their importance, drylands are often overlooked in global conservation frameworks and development priorities. This oversight is concerning, given their increasing vulnerability to human land-use pressures, including extensive agriculture, alternative energy sources, overgrazing, invasive species, and climate change. These pressures are expected to intensify in the coming decades, leading to severe land degradation and biodiversity loss. Estimates suggest that up to 20% of drylands are already degraded or at risk, exhibiting significant increases in bare ground, vegetation loss, and water declines. Furthermore, the introduction and spread of non-native species, often facilitated by human land uses, exacerbate these challenges. Initiatives like afforestation, while well-intentioned, can also inadvertently exacerbate these drivers through habitat and resource alterations. Climate change poses an additional significant threat, with projected increases in extreme heat events and aridity pushing species beyond their physiological tolerance limits. The combined effect of habitat loss, fragmentation, and climate change severely diminishes the adaptive capacity of dryland species. This paper aims to provide a comprehensive assessment of current and future threats to drylands and their vertebrate biodiversity, identifying broad-scale conservation priorities and informing the development of effective land management and conservation strategies.

Numerous large-scale scientific examinations and recommendations have significantly advanced global conservation efforts in the past 20 years. However, much of this focus has concentrated on megadiverse tropics, neglecting other important biomes, including drylands. Previous research highlights the unique biodiversity and ecological importance of drylands, while also emphasizing their underrepresentation in global conservation initiatives. Studies have documented the sensitivity of drylands to human land-use pressures, predicting intensified threats in the coming decades. The literature also acknowledges the significant contribution of land degradation to desertification and biodiversity loss in dryland ecosystems, documenting substantial declines in vegetation cover and water resources. Furthermore, research has demonstrated the vulnerability of dryland species to invasive species and the compounding effects of climate change.

The study utilized data from the UNEP-WCMC to define drylands, categorizing them into hyperarid, arid, semiarid, and dry subhumid subtypes based on an aridity index (precipitation/potential evapotranspiration). Protected area coverage was assessed using the World Database on Protected Areas (WDPA) and OpenStreetMap, considering all IUCN categories and uncategorized areas. The analysis was conducted at two levels: (1) protected areas designated for biodiversity conservation (IUCN categories I-IV) and (2) all protected areas, including those permitting multiple land uses (IUCN categories V-VI and uncategorized). The distribution and threat status of terrestrial vertebrate species (amphibians, birds, mammals, and reptiles) with at least 50% of their global ranges in drylands were evaluated using data from IUCN, BirdLife International, and GARD. Species were categorized based on their range protection (0%, 0-10%, 10-100%) and IUCN threat status. Representation targets for protected area coverage were set (100% for species with ranges <1000 km² and 10-100% for others). Threat types affecting dryland species were assessed using the IUCN threat-classification scheme. Current and future land-use pressures were analyzed using the Land Use Harmonization (LUH2) project, focusing on cropland, rangeland, pasture, and urban land-use states under different shared socioeconomic pathways (SSPs: SSP1-2.6, SSP2-4.5, and SSP5-8.5). The study calculated total land cover area of 25 km² grids and additional fractional land areas under future scenarios. Analyses were performed using ArcGIS and R.

The study revealed significant disparities in protected area coverage between drylands (12%) and non-drylands (21%). Most dryland vertebrate species had less than 10% of their ranges protected within IUCN categories I-IV, indicating inadequate habitat for maintaining viable populations. A substantial proportion of species (30% of amphibians, 7% of birds, 16% of mammals, and 27% of reptiles) had no overlap with protected areas ('gap species'), including many endemic species. Reptiles were identified as the most diverse group with high levels of endemism and low protection. Assessment of species by representation targets revealed widespread inadequate protection, particularly for narrow-ranging species. The analysis of threats identified agriculture as the most significant anthropogenic threat across vertebrate groups, followed by timber harvesting, infrastructure development, invasive species, and climate change. Different threats emerged as more prevalent for different groups (e.g., water management for amphibians, climate change for birds, overexploitation for mammals, and infrastructure development for reptiles). Projected land-use changes under different SSP scenarios indicated that by 2100, most drylands would experience some degree of land conversion due to increases in cropland, pasture, and urban areas, even under optimistic scenarios.

The findings highlight the critical need for enhanced conservation efforts in drylands. The substantial shortfall in protected area coverage, coupled with projected future land-use changes, poses a significant threat to dryland biodiversity. The inadequate protection of endemic and narrow-ranging species is particularly concerning, as these species are highly vulnerable to extinction. The various anthropogenic threats, including agriculture, infrastructure development, and invasive species, interact with climate change to amplify the risks to dryland ecosystems. Even optimistic scenarios projecting decreased agricultural land use do not negate the negative impacts on drylands due to factors like the expansion of alternative energy sources. The significant knowledge gaps regarding threats to many species, especially reptiles, underscore the need for comprehensive assessments to inform effective conservation strategies.

This study demonstrates the critical need for incorporating drylands into mainstream conservation and development priorities. The substantial under-protection of dryland biodiversity, coupled with impending land-use changes, necessitates the development of targeted conservation strategies. This includes expanding protected area networks, implementing stricter protection measures, and recognizing community-based approaches. Future research should focus on refining threat assessments for data-deficient species and investigating the effectiveness of different conservation interventions in diverse dryland contexts. This research provides a crucial baseline for prioritizing conservation action in these often-overlooked ecosystems.

The study’s coarse spatial resolution in land use projections may mask local-scale variations in land-use change impacts on biodiversity. Data availability limitations, including incomplete WDPA data for some regions and knowledge gaps regarding threats to many species, particularly reptiles, may have influenced the results. The analysis did not account for species-specific responses to different land-use types, potentially underestimating or overestimating the impact of certain changes.