Synergic effects of vegetation structure and food supply underlie higher abundance of a farmland specialist bird in organic than in conventional arable fields

Abstract

Organic farming is considered beneficial for farmland biodiversity. However, it is often unclear what specific factors are underlying the biodiversity benefits of this farming practice. The positive effects may be driven by factors acting at the field level represented by, for instance, the absence of pesticides, or by the factors acting at the level of entire farms represented by a higher diversity of crops and intercrops types, and higher share of semi-natural landscape elements. Distinguishing between these two groups of factors is difficult because they are confounded in heterogeneous landscapes consisting of small arable fields. However, their separation is crucial to understand the origins of organic farming effectiveness. The landscape in those Central and Eastern European countries that have undergone collectivization, such as Slovakia, Czechia or Lithuania, consists of large fields cultivated under either conventional or organic regime. These circumstances are suitable to assess the sole influence of in-field factors providing potential biodiversity benefits of organic farming. For this purpose, we selected a study area in the lowlands of the south-western Slovakia, where conventional and organic fields were situated in close proximity. Such a spatial arrangement was suitable to examine the possible influence of farming practice (i.e. organic vs. conventional) itself, minimising the influence of landscape elements. We focused on a single cereal crop, winter wheat, cultivated on both organic and conventional fields. Here, we counted the breeding abundance of the Eurasian Skylark (Alauda arvensis), a farmland specialist bird closely associated with arable fields providing both breeding and feeding habitat for this species, which may be considered as a farmland biodiversity indicator. Skylark counts were higher in organic than in conventional wheat fields throughout the season. This difference was statistically related to a higher abundance of invertebrates in samples collected in organic fields than in conventional fields. However, Skylark counts were also related to sparser crop cover in organic fields, especially at the end of the season. Sparser crop cover may improve nesting conditions and facilitate foraging. Our results indicate that organic farming may have a positive impact even at the field-level. The benefits for the Skylark, and possibly for the other farmland organisms, can be provided by the joint effects of the absence of pesticides and a suitable vegetation structure. Organic farming can thus be a suitable agri-environmental measure within the Common Agricultural Policy even in intensively used landscapes dominated by large fields.

Keywords

• Skylark
• Organic farming
• Vegetation structure
• Invertebrates
• Food supply