This experiment assessed the effects of manipulating conventional management practices (cutting, inorganic fertilizer application and aftermath grazing) on the butterfly and bumblebee communities in intensively managed grasslands. The study is novel in that it investigated multiple management techniques in combination, looked at the responses of more than one pollinator group, and in parallel, assessed the floral resources driving community structure with the whole experiment comprising a high level of spatial replication carried out over 4 years.
performance of grass and sown treatments
Overall, the relative performance of the grass-based and sown treatments varied between the pollinator groups studied. Bumblebees were most abundant and diverse in the sown treatments and were virtually absent from the grass plots. Adult butterflies were also most common and speciose in the sown plots; however, the most extensively managed grass treatments were nearly as good at supporting butterflies. In contrast, the sown plots were poor sites for butterfly larvae, probably reflecting the lack of suitable host plants. Treatment × year interactions were found for both the bumblebees and the butterflies; however, the interpretation of these interactions is likely to be subject to several competing explanations that we could not discriminate between in this study. A potentially major factor driving the variability in treatment quality between years was the relatively small size of the field margin plots used in this study (individual plots being 10 × 50 m). At a landscape scale, the contribution that such plots can make to support bumblebee colonies or to provide food plants for butterflies may be limited, as population changes for individual species observed in this study were unlikely to be driven by resources provided by the experimental treatments (e.g. pollen and nectar within flowers). Instead, factors which could not be controlled by the experimental design are likely to have been driving population change of an individual species, e.g. population cycles, loss or change in management of habitats outside of the study plots or responses to weather conditions. As population sizes of individual species change between years, potentially in an asynchronous way, their contribution to total abundance, species richness or diversity within each treatment would also change. Such differences may be driving the high levels of between-year variability in the ability of the nine treatments to support butterfly and bumblebee populations. However, the identification of consistent patterns across years, particularly the superiority of the sown treatments 8 and 9, does provide a basis for recommending these treatments as specific management recommendations, independently of inter-year fluctuations in their value.
Total bumblebee abundance varied markedly across years, with 2004 being a particularly good year for English bumblebees (as the warm dry summer of 2003 may have allowed greater nest provisioning) and 2005 being very poor (as the wet summer of 2004 may have suppressed nest provisioning). High levels of inter-annual fluctuations are characteristic of bee communities (Williams, Minckley & Silveira 2001) and reflect the effects of climate, forage and nesting resource variability. Even with the inter-annual variation, the value of the sown treatments was not fully realized until the second year following establishment, which can be seen for T8, and both sowings of T9. The introduction of sown species enhanced the availability of bumblebee floral resources (especially Trifolium spp., Lotus corniculatus and Vicia sativa) as has been observed in other studies (e.g. Holzschuh et al. 2007), and cultivation (i.e. ploughing, preparation of a seed bed and drilling) also encouraged the establishment of other ‘magnet’ species such as Cirsium, Ranunculus and Taraxacum spp. found in the seed bank (Carvell 2002). The abundance and species richness of bumblebees in T8, however, returned to low levels, similar to the grass plots, in years 3 and 4 post-sowing. Treatment 9 plots were not intended for use after 2 years due to risk of pernicious weed establishment after the biennial kale had set seed, leaving bare patches. The treatments based on the existing sward (T2 and T5–7) were expected to show some succession with the cessation of fertilizer inputs, at least in 2003 and 2004. After 2004, little further successional change was expected, and hence, it is more representative of biodiversity benefits from establishing such treatments over the duration of an Environmental Stewardship agreement. For T1–3 and T4, all of which continued to receive fertilizer, there was little successional change in the plant community; thus, all years can be assessed as representative.
Floral and nectar resource diversity are known drivers of bee community structure (Potts et al. 2003). Floral abundance, species richness and the availability of nectar (and pollen) flowers, especially in the form of legumes, were all drivers of bumblebee community composition (Table 2). Overall attempts to increase the floral diversity of the managed grass plots over a 4-year period to provide bumblebee resources had little effect, and this result is reflected in the consistently low number of nectar-providing species found in all grass plots.
The extensively managed grass-based plots (T6 and T7) and the sown treatments had the highest butterfly biodiversity. These plots contained many butterfly nectar plants such as Cirsium, Ranunculus, Senecio, Veronica and Vicia spp. (Supporting Information, Appendix S5b) and provided good shelter (sward height > 20 cm compared to other grass plots T1–T5, < 15 cm; Supporting Information, Appendix S5b). Abundance of butterflies and butterfly flowers was highly correlated, as were butterfly species richness and flower species richness (Table 3); this is consistent with other studies (Krauss, Steffan-Dewenter & Tscharntke 2003; Pywell et al. 2004), suggesting that both aspects of plant communities could be manipulated together to benefit butterflies. The provision of nectar resources from non-legume forbs were important drivers, as reported for arable farm studies (e.g. Pywell et al. 2004).
Butterfly abundance and species richness increased with time and this suggests that the value of the grass-based treatments also increased through time. Although there was high inter-annual variation for all butterfly parameters, sown T8 performed well relative to grass treatments in the establishment year, and even better in the second year. In year 1, the plots were mainly barley with some Vicia sativa, and in year 2, there was an increase in grasses, Lotus, Medicago Lupulina and Cirsium species. It is only after the second year that the treatment was cut which resulted in the loss of floral resources. The utility of the sown treatment for butterflies may only be short-term, as the plot composition changes with time and floral resources are lost as plots become dominated by grasses and/or are cut. Treatment 9 was more variable in its value for butterflies compared to treatment 8 and the unpredictability may reflect factors such as poor establishment and/or persistence of some sown species between different years.
In contrast to adult butterflies, the sown plots performed poorly for butterfly larvae in all years owing to a general absence of commonly utilized host-plants (e.g. Agrostis, Lolium and Holcus spp., Supporting Information, Appendix S5c). By contrast, the more extensive grass-based plots had a higher abundance of host-plants, thereby providing more oviposition opportunities for adult butterflies.
Although bumblebees and adult butterflies responded to our management treatments, this may not necessarily relate to population growth but could reflect attraction of pollinators to new resources. However, the measures of butterfly larvae distributions do relate more directly to reproduction.
policy relevance and recommendations
Pollinator biodiversity in improved grasslands can be effectively enhanced by introducing novel flower-rich habitats; however, the benefits are likely to be relatively short-term as sown mixtures become increasingly dominated by pernicious weeds. Longer-term enhancement can be achieved through re-sowing of margins, but this has implications in terms of time and resources needed by farmers. Re-sowing may also be necessary to control pernicious weed problems (e.g. Cirsium arvense), which may present a disincentive to adoption and would therefore require suitable farmer incentives, such as agri-environmental scheme payments or similar schemes. While the sowing of crop mixtures can provide forage and structural resources for pollinators, if harvested it would not be of sufficient agronomic value to feed to livestock.
An alternative option, which favours butterflies but not bumblebees, is to reduce management intensity on areas of existing grassland. Stopping fertilization together with the implementation of a single cut and/or low-intensity grazing to produce a more heterogeneous sward structure has the potential to produce good quality habitats for some butterflies (e.g. Erhardt 1985). This approach represents a straightforward option for farmers to implement relative to the sown treatments. Management regimes that reduce disturbance to swards during the growing season will deliver significant benefits to other grassland invertebrates (Woodcock et al. 2007). In practical terms, the management of T1–5 can be readily incorporated into conventional silage making systems, as the timing of cutting coincides with conventional cutting times.
What emerges is that there is no single solution to enhancing biodiversity in grasslands across a wide range of taxa as their habitat requirements vary markedly. There are always likely to be trade-offs between different species groups depending on how their life-history traits relate to the modified habitats (Kruess & Tscharntke 2002). In this case, extensive grass-based treatments support butterflies but not bumblebees. In addition, the dense swards of the extensive plots provide bird food, in the form of beetles (Woodcock et al. 2007), but remain relatively inaccessible for birds (Cole et al. 2007). Even within a taxon, there are contrasting habitat requirements with some butterflies requiring dense swards for shelter (e.g. Aphantopus hyperantus and Pyronia tithonus) and others requiring more open habitats (e.g. Polymmatus icarus and Lasiommata megera) (Fry & Lonsdale 1991).
The dual approaches of the Entry Level Stewardship (ELS) and Higher Level Stewardship (HLS) in the UK Environmental Stewardship (Defra 2005) are potential tools which could adopt both modifications to grassland management: sowing and extensification. ELS aims to encourage a large number of farmers across a wide area of farmland to deliver simple yet effective environmental management which have wide biodiversity benefits. The extensification of some existing improved grasslands, by applying the management methods tested in treatments 6 and 7 in this experiment, could have major implications for the conservation of widespread and common butterfly species, although the benefits for threatened species requiring elaborate protection measures, and for bumblebees, would be questionable. HLS aims to deliver significant environmental benefits in high-priority situations and areas, and requires more complex management (than ELS) where land managers need advice and support. The introduction of sown margins has been successful in supporting greater numbers and diversity of bumblebees in arable systems (e.g. Carvell et al. 2004), but has yet to be fully tested within improved grassland systems. The adoption of sown margins in the HLS, could have impacts on both butterflies and bumblebees, but the benefits are likely to be localized as HLS is highly geographically targeted. One advantage of careful placement of flower-rich margins in grasslands is that existing habitat patches could be linked together, enabling pollinators to forage and disperse along linear features.
Without widespread intervention in the management of improved grasslands, many pollinator species are likely to continue to decline (Tscharntke et al. 2005). Agri-environment schemes are compulsory in EU Member States, and represent a potentially powerful tool to halt and reverse the loss of pollinators in agro-ecosystems. While the effectiveness of some scheme options has been questioned (Kleijn et al. 2006), the ability of schemes as a whole to support farmland wildlife is generally accepted (Potts et al. 2006). Given the large proportion of agricultural land in Europe used intensively for grazing and silage, there are potentially wide-scale biodiversity benefits if pollinators can be introduced back into these systems. Pollinators not only have a high intrinsic value in themselves, but deliver essential ecosystem services; butterfly larvae also represent a key food group for many farmland birds (Wilson et al. 1999), and therefore, they would have additional benefits in supporting food chains.