A mensurative experiment to study effects of landscape heterogeneity on biodiversity

Spatial heterogeneity of landscapes is a major factor for sustaining and restoring biodiversity, sometimes more important than the type of farming system. However, landscape heterogeneity is often evaluated by the proportion of habitats without accounting specifically for their spatial arrangement. It is crucial to separate the effect of habitat amount from the effect of spatial organisation to determine if management efforts should focus on increasing the area of habitats or on the design of particular habitat spatial pattern. Semi-natural habitats were shown to play an essential role to maintain biodiversity in agricultural landscapes. By providing a wide range of habitat types, agricultural lands can also enhance biodiversity and the related ecosystem services. The diversity of cropping systems results in a heterogeneous mosaic of vegetation covers which might influence species that use crops during their life cycle, or disperse in the crop mosaic. The heterogeneity of crop mosaic has been poorly studied, to our knowledge. Thus, the role of the spatial arrangement of cropping systems for biodiversity remains unknown. A mensurative experiment was designed in 2011 to analyse and separate the respective effect of the amount and spatial organisation of semi-natural and cultivated habitats on plant and carabid beetle diversity. Spatial analyses of land covers were performed to select twenty 1km² landscapes distributed along a double gradient of habitat amount and length of edges between semi-natural habitats and crops (spatial organisation) in Brittany, France. To evaluate gamma and beta diversity, carabid and plants surveys were performed using a stratified sampling design. In each landscape, two replicates of five habitat types (woodlot, hedgerow, permanent and temporary grassland, wheat crop) were sampled using pitfall traps for carabid beetles and squared plots for plants. The percentage cover and spatial organisation of the different crop types were estimated to integrate the heterogeneity of the crop mosaic in the analyses. We hypothesize that gamma diversity of carabid beetles and plants rise along both axes of the gradient. The amount of semi-natural habitats, as well as the length of edges (that promotes exchanges between habitats), are expected to enhance carabid and plant gamma diversity. The beta diversity, which reflects differences between communities, is expected to increase with semi-natural habitat cover (appearance of less common species). However, interfaces are expected to reduce those differences and so, the beta diversity. The responses are likely to vary according to functional groups, in particular, “crop specialist” carabids and arable weeds