Influence of herbaceous elements on butterfly diversity in hedgerow agricultural landscapes

Landscape factors influencing butterfly diversity in five agricultural landscapes with hedgerows were investigated in western France. Two main questions were addressed: (i) how important are linear (hedge bank, road and lane verge) versus non-linear (grassland) herbaceous elements for butterfly diversity? (ii) Do number and spatial arrangement of these elements influence butterfly diversity? The contribution of each type of herbaceous elements to butterfly diversity was investigated by analyzing diversity at different levels: element, element type (road verge, hedge bank, grassland) and landscape. The spatial arrangement of herbaceous elements was measured by two indices: (i) the mean distance between each grassland and its nearest neighbor (ii) the grain size of the landscape (mean field size). Linear herbaceous elements provided higher butterfly diversity than grasslands. Moreover, butterfly diversity was positively correlated to the surface area in linear herbaceous elements. As they are the remaining low input and lightly managed herbaceous elements in current agricultural landscapes, they contribute greatly to the maintenance of butterfly diversity. Grain size of the landscape was not correlated to butterfly diversity, however, mean distance between nearest grasslands was strongly positively correlated to butterfly diversity. These results show that species movement in the five landscapes was neither prevented by the field size of the studied landscapes nor by the distance between grasslands. The advantages of the landscape heterogeneity measures used in this study and of the method of diversity partitioning, for biodiversity studies in agricultural landscapes, are discussed. (C) 2002 Elsevier Science B.V. All rights reserved.