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In intensive agricultural environments, arable field margins are important habitats as reservoirs of various beneficial wild species. Many studies of species diversity in field margins have focused on the local habitat level. However, relationships between the network of the margin habitats and species diversity are less studied. Edge density index of field patches is a class-level landscape metric used as one measure of habitat network. This study focused on edge density index and its impacts on plant species richness and community turnover among the margins in agricultural landscapes. We examined how effectively the index of edge density can be used as a class-level indicator for species diversity in margins. This study introduces a method for indicating plant species diversity of the margin habitats by using an easily measured spatial indicator without comprehensive analysis of land use/land cover. A total of 29 landscape-square pairs, each consisting of two 25ha squares, were sampled across southern Finland. Vascular plant species were sampled in each square. In GIS, field patch edges were divided into two types: those within crop fields, referred to as within-field edges, and those between crop fields and non-crop lands, referred to as non-crop field edges. By comparing the roles of three landscape groups (the two different edge types as well as all edges grouped together) in explaining plant species richness (at alpha and gamma level) and community turnover (at beta level) in the sampled landscape squares, we found that density index of non-crop field edges surrounded by contrast land use is an effective indicator of species diversity at beta and gamma level. This study suggested that field margins surrounded by contrast land use shall be considered as a priority landscape element in agricultural edge network design, whereas simply using the density of all field edges to indicate species diversity without separating the different effects of various edge types shall not be recommended.
