Data from: Using beta diversity to inform agricultural policies and conservation actions on Mediterranean farmland

1. Spatial variation in species composition (β-diversity) is an important component of farmland biodiversity, which together with local richness (α-diversity) drives the number of species in a region (γ-diversity). However, β-diversity is seldom used to inform conservation, due to limited understanding of its responses to agricultural management, and lack of clear links between β-diversity changes and conservation outcomes.
2. We explored the value of β-diversity to guide conservation on farmland, by quantifying the contribution of bird α- and β-diversity to γ-diversity variation in low- and high-intensity Mediterranean farmland, before (1995-1997) and after (2010-2012) the Common Agricultural Policy reform of 2003. We further related β-diversity to landscape heterogeneity, and assessed the conservation significance of β-diversity changes.
3. In 1995-1997, bird diversity was highest in low-intensity farmland, where it further increased in 2010-2012 due to a strong positive contribution of α-diversity to γ-diversity. In high-intensity farmland, diversity converged over time to much the same values of low-intensity farmland, with strong positive contributions of both α- and β-diversity. These patterns were largely consistent for total, farmland and species of European conservation concern assemblages, and less so for steppe birds.
4. Beta diversity increased with landscape heterogeneity, particularly related to spatial gradients from agricultural to natural habitats in low-intensity farmland, and from annual to permanent crops (olive groves) in high-intensity farmland. The first gradient was associated with the replacement of steppe birds of high conservation concern by more generalist species, while the second was associated with the replacement between species with lower or higher affinity for woodland/shrubland habitats.
5. Synthesis and applications. In low-intensity farmland, β-diversity was largely stable over time, reflecting a positive conservation outcome related to persistence of landscape heterogeneity patterns required by endangered steppe bird species. In contrast, β-diversity in high-intensity farmland was favoured by increases in landscape heterogeneity driven by olive grove expansion, contributing to enhancement of total bird diversity. Overall, our results stress the value of β-diversity to understand impacts of agricultural policies and conservation actions, but also highlight the need of evaluating β-diversity changes against specific conservation goals.

1. 物种组成的空间变异即β多样性（β-diversity）是农田生物多样性的重要组成部分，其与局域物种丰富度（即α多样性（α-diversity））共同决定区域物种总量（即γ多样性（γ-diversity））。然而，由于学界对β多样性响应农业管理模式的认知不足，且β多样性变化与保护成效之间缺乏明确关联，β多样性极少被用于指导保护实践。
2. 本研究以2003年共同农业政策（Common Agricultural Policy）改革前后（1995-1997年与2010-2012年）的地中海低强度与高强度农田为研究对象，通过量化鸟类α、β多样性对γ多样性变异的贡献，探究β多样性在农田保护中的应用价值。此外，本研究还将β多样性与景观异质性（landscape heterogeneity）相关联，并评估了β多样性变化的保护意义。
3. 1995-1997年，低强度农田的鸟类多样性最高，且在2010-2012年进一步提升，这源于α多样性对γ多样性的显著正向贡献。高强度农田的鸟类多样性随时间推移逐渐收敛至与低强度农田相近的水平，其提升同时得益于α与β多样性的显著正向贡献。上述模式在总体鸟类群落、农田鸟类群落以及欧洲具有保护关注价值的物种群落中基本一致，但在草原鸟类（steppe birds）群落中表现较弱。
4. β多样性随景观异质性升高而增加，这种关联在低强度农田中主要与农业生境向自然生境的空间梯度相关，在高强度农田中则主要与一年生作物向多年生作物（橄榄园）的转变相关。第一种空间梯度对应着高保护价值的草原鸟类被更多广食性物种所替代，而第二种梯度则对应着对林地/灌丛生境亲和力高低不同的物种之间的替代。
5. 总结与应用启示。在低强度农田中，β多样性随时间基本保持稳定，这反映了濒危草原鸟类所依赖的景观异质性格局得以维持的正向保护成效。与之相反，高强度农田中的β多样性因橄榄园扩张带来的景观异质性提升而得到改善，进而推动了整体鸟类多样性的提升。总体而言，本研究结果强调了β多样性在解析农业政策与保护措施影响方面的价值，但同时也指出需结合具体保护目标评估β多样性的变化。