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.