Testing biotic indicator taxa: the sensitivity of insectivorous mammals and their prey to the intensification of lowland agriculture.
Changes to agricultural policy aim to extensify agriculture and increase biodiversity. However, it is not known how sensitive many taxa are to intensification. Sensitive taxa could be used as biotic indicators, to assess changes over time and the effectiveness of policy changes. We sampled shrews, bats and their prey (beetles, flies and moths) on matched pairs of sites and assessed the response in their abundance to aspects of intensification: increased agrochemical inputs (using abundance on matched organic and conventional cereal fields as a proxy), the switch from hay to silage (using abundance on matched hay and silage fields) and boundary loss (using abundance in the field and near the boundary as a proxy, in cereal and grass separately). We quantified the abundance-derived sensitivity of the taxa in order to assess their use as biotic indicators. There was substantial variation in the sensitivity of taxa to the three aspects of intensification. Most estimates (51%) of sensitivity to boundary loss were significant, but only 8% for increased agrochemical inputs and 16% for the switch from hay to silage. Sensitivity to one aspect of intensification was not significantly related to sensitivity to another. Insectivorous mammals were relatively insensitive to increased agrochemical inputs and the switch from hay to silage, but strongly sensitive to boundary loss. Taxa with significant sensitivity to increased agrochemical inputs included some Carabidae and Diptera. We found fewer significant differences in abundance between organic and conventional farms than previous workers, probably because we controlled for boundary characteristics, suggesting that the quality of field boundaries is important in influencing biodiversity differences between organic and conventional farms. The switch from hay to silage had a positive effect on some Coleoptera and Diptera but a substantial negative effect on Hepialidae (Lepidoptera). Synthesis and applications. The recorded sensitivity of taxa to changes in agricultural practices is highly variable. Therefore, the selection of biotic indicator taxa of agricultural intensification is not straightforward. If surveys of biotic indicator taxa are used to assess the effectiveness of changes in agricultural practice, empirical evidence should be used to select suitable taxa.