Data from: Sensitivity of the farmland bird community to crop diversification in Sweden: does the CAP fit?

Crop diversification has been introduced as an environmental strategy in the ‘Greening’ of the EU Common Agricultural Policy (CAP) for 2015–2020. The primary target of crop diversification is soil and ecosystem resilience, but claims for potential benefits for farmland biodiversity are also common. However, understanding of relationships between the number (compositional heterogeneity) and spatial arrangement (configurational heterogeneity) of crop fields and biodiversity is generally poor, making such claims relatively unfounded. In this study, we monitored crop and farmland bird diversity on 178 farms across Sweden's main agricultural areas. From a pre-implementation assessment, we show that >97% of the assessed farms would not be required to change their management under the CAP crop diversification measure (minimum of three crops for farms with 30+ ha), suggesting that this measure has generated little change on Swedish farms. While accounting for non-crop elements and farming system (conventional or organic), we show that crop structural diversity (i.e. the management and vegetation structure of crops) rather than crop diversity senso lato positively affected richness of non-crop breeding bird species with stronger effects in arable, compared with forest-dominated landscapes. No such effects were observed among field-nesting farmland bird species. Organic farming had little influence on farmland birds with positive effects only in the most arable-dominated landscapes and for field-nesting species only. In forest-dominated landscapes, organic farms even held lower field-nester densities compared with conventional farms, possibly due to the dominance of grasslands on organic farms that in these landscapes support lower densities of field-nesting species compared with cereals. Policy implications. Our study illustrates the importance of a consideration of structural instead of species diversity of crops for biodiversity, in this case farmland birds. We also underline the absence of such a distinction in current EU Common Agricultural Policy Greening, while simultaneously setting levels on crop diversification too low resulting in little to no change in landscape-scale crop diversity on Swedish farmland. We recommend that future efforts to manage farmland biodiversity should include ways of increasing the structural diversity of crops at the scale of farms and landscapes.

作物多样化作为环境策略，已被纳入2015-2020年欧盟共同农业政策（Common Agricultural Policy, CAP）的“绿色化”改革框架。作物多样化的核心目标是提升土壤与生态系统韧性，但关于其对农田生物多样性存在潜在益处的主张也颇为常见。然而，当前学界对农田斑块的数量（组成异质性，compositional heterogeneity）与空间布局（配置异质性，configurational heterogeneity）和生物多样性之间的关联认知普遍不足，使得这类主张缺乏坚实的科学依据。

本研究针对瑞典主要农业区域的178个农场，开展了作物与农田鸟类多样性监测。通过实施前评估，我们发现超过97%的受评估农场无需依据CAP的作物多样化措施调整经营模式（30公顷以上的农场需至少种植3种作物），这表明该措施在瑞典农场中几乎未产生实际性变化。

在控制非作物要素与耕作制度（常规或有机耕作）变量的前提下，我们的分析结果显示：作物结构多样性（即作物的经营管理方式与植被结构）而非广义作物多样性（crop diversity senso lato），对非作物生境繁殖鸟类的物种丰富度存在显著正向影响，且该效应在以耕地为主的景观中强于以森林为主的景观。而针对营巢于农田的鸟类物种，未观察到类似的正向效应。

有机耕作对农田鸟类的整体影响较为有限，仅在以耕地为主的景观中对营巢农田鸟类存在微弱正向作用。在以森林为主的景观中，有机农场的营巢鸟类种群密度甚至低于常规农场，这可能是因为这类景观中的有机农场以草地为主要生境，而相较于谷类作物，草地所能支撑的营巢鸟类种群密度更低。

政策启示
本研究阐明了在农田生物多样性管理中，应优先关注作物结构多样性而非物种多样性的重要性（本研究以农田鸟类作为研究对象）。同时，我们也指出当前欧盟共同农业政策绿色化改革未对上述两类多样性作出区分，且设置的作物多样化门槛过低，导致瑞典农田的景观尺度作物多样性几乎未发生任何变化。我们建议，未来的农田生物多样性管理工作应纳入提升农场与景观尺度作物结构多样性的相关举措。