Assessing the value of beetle banks for enhancing farmland biodiversity

Abstract

The perennial herbaceous understorey found in well-maintained hedgerows and grassy field margins provides resources or refuge for a highly diverse fauna. Invertebrates are especially important as they underpin food chains, with many species being valuable polyphagous predators that feed on arable crop pests. However, agricultural intensification has caused many of these non-cropped areas to become degraded or lost, resulting in very high field area to edge ratios. Farmland biodiversity has declined markedly as a consequence. Predator assemblages dependent on margins within their lifecycles are often less abundant and diverse, with slower colonisation of feeding sites. The temperature-buffered conditions found within dense tussocky grasses in margins are particularly crucial for predator overwinter survival. 'Beetle banks' are grass-sown ridges designed to replicate and increase the availability of this kind of habitat, in a simple and inexpensive form. The original design was for single island strips extending across large arable fields, where predators are most impoverished, to effectively shrink the field in terms of spring dispersal into the crop. Early experimental banks still exist, and many more have been sown over the decade since the concept was first put into practice through the recommendation of advisory organisations. However, long-term management strategies for their successful upkeep are not clearly identified. With a known establishment date, beetle banks provide a means of exploring potential successional change or biodiversity development within new habitat. I assessed age-related differences in the botanical composition of beetle banks, considering potential degradation as overwintering sites, and resource provision for other wildlife. A dense structure was retained, despite age, thus banks continue to be functional for predators for over a decade at least. Increasing summer floral diversity with age may also benefit other insects. Weed presence was no worse than conventional margins, and so control should be relatively straightforward. Associated with this work was an evaluation of whether levels of polyphagous invertebrates, previously reported as high in new sites, were sustained in older beetle banks. I discovered little difference in densities per m' between beetle banks and conventional field margins, regardless of age, while boundary-overwintering carabid densities increased though time. Predator diversity was also similar between habitat types through the year. A large-scale trapping experiment was undertaken to evaluate the spatial-temporal patterns of predatory Carabidae in fields adjacent to beetle banks through the crop season. A novel spatial analysis was used to explore the distributions of different species groups. I found evidence to support the description of a 'wave' of boundary-species emerging from refuges and dispersing across the field. In contrast, field-inhabiting species were slow to develop from field centres and may be of less value for pest control. Beetle banks appear as valuable as conventional boundaries for aiding carabid dispersal into crops. Subtle microclimate, prey distribution or edaphic factors probably accounted for the spatially and temporally fluctuating activity-densities of beetles observed through the season. In addition, the abundance of predatory Empididae was similar within a beetle bank and a hedgerow, with a low, homogeneous presence at increasing distances from them, coinciding in time with serious cereal pests. Thus beetle banks may also contribute useful habitat resource for these little studied insects. 1 hypothesised that simple sown grass strips would contain a lower diversity of other invertebrates when compared to older, botanically complex habitats, although this difference might lessen with age. There was evidence to support this view. Beetle banks were found to contain useful, albeit lower, densities of game bird chick-food, when compared with conventional field margins; additionally furnishing nesting cover for adults birds. Severe declines in wild game are attributed to losses of these invertebrates vital for chick survival, as well as inadequate provision of nesting and brood-rearing habitat. Orthopteran species richness was similar between beetle banks and conventional margins, although there were compositional differences in capture; and older banks were increasingly speciose. Grasshoppers favoured mid-field banks, whereas bushcrickets tended to prefer hedgebottoms. Additionally, Lepidopteran species composition was investigated. As expected fewer butterflies were observed in beetle banks than hedge banks on conventionally managed farmland, but the grass swards clearly were of value, with butterfly presence related to floristic species richness and diversity. Better management, such as incorporating conservation headlands alongside beetle banks, and protection &om agronomic activities in the field, may be a means of further enhancing the resources that beetle banks provide for these invertebrates of increasing conservation concern. A supplementary chapter of this thesis describes the findings of a questionnaire survey sent to a cohort of farmers in southern England, to elucidate their current perceptions, opinions and use of the beetle bank design.

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