Invertebrates in field margins: taxonomic group diversity and functional group abundance in relation to age
Sown, temporary field margins are a common agri-environment scheme (AES) in the Netherlands. Despite their wide application, though, there has been scarcely any long-term monitoring of the succession of invertebrates. In the field margins of 40 farms, invertebrate diversity and the abundance of three functional groups were assessed in relation to age. The diversity in terms of number of species groups was found to increase with the age of the margins. The abundance of herbivores and detritivores also showed a positive correlation with the age of the margins. However, the abundance of predators decreased with increasing age. Older margins showed a higher total vegetation cover and fewer plant species, also resulting in lower plant species evenness. We suggest several changes to the current AES regulations. For the conservation of invertebrate diversity, longer-lasting field margins are desirable. In addition, old margins are favoured by detritivores, a group that has particular difficulty finding suitable habitats in agricultural landscapes. However, such margins are less favourable from an agricultural perspective, as they appear unsuitable for high abundances of potentially useful predators and the high vegetation cover attracts many potentially harmful herbivores. To circumvent this, the AES might be extended by incorporating hay-making, which would reduce standing biomass and might lead to more predators and fewer herbivores.