In this post Katherine Orford discusses her recent paper ‘Modest enhancements to conventional grassland diversity improve the provision of pollination services

You can also read a blog post from the Associate Editor who handled this paper, Peter Manning here: Pastures new for pollinators?

Grassland diversity

Species-rich grasslands were once widespread across Western Europe. However, post-war agricultural intensification has resulted in wide-scale conversion of these diverse grasslands into intensive and profitable systems for livestock rearing. Modern grassland management typically entails high fertiliser application rates and frequent defoliation by intensive grazing and cutting regimes. Such practices have resulted in significant declines in species diversity with swards often dominated by competitive commercial grasses species including Lolium perenne (perennial rye-grass) and Holcus lanatus (Yorkshire fog). It has been estimated that only 2% of grasslands have escaped agricultural intensification in the UK.

As the human population is forecast to rise from 7 billion today to 9 billion in 2050 and per capita meat consumption predicted to rise from 32 kg to 52 kg per year over the same time frame, there is an argument to maintain the agricultural value of grasslands. In the majority of cases, intensive and profitable grass production is seemingly incompatible with maintaining high levels of biodiversity. However, if we consider that grasslands cover 30-40% of European agricultural areas, just modest enhancement to grassland biodiversity, whilst maintaining agricultural performance, could have extensive benefits. Practical and widely adoptable methods need to be developed for agri-environment schemes (EU and national government funded schemes of the Common Agricultural Policy), which will provide incentives for farmers to enhance their grassland diversity.

There is a requirement for agri-environment schemes to simultaneously deliver multiple benefits to natural communities. There is growing evidence that increases in plant diversity can deliver a range of benefits, such as amount and stability of production, soil quality and diversity of higher trophic levels and ecosystem services (ecosystem processes that benefit society) such as pollination. The cascading impact of increasing conventional grassland plant diversity on pollination services to surrounding habitats is the focus of this study.

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Pollination in action. Image from Flickr by ‘mike193823319483’. Licensed under the Creative Commons.

The study

The bottom-up relationship between grassland plant communities and pollination services was assessed using a field-scale experiment in which conventional grassland seed mixes and sward management were manipulated on grassland plots. This was complemented by surveys on ten conventional, working farms which possessed a natural gradient of plant diversity within south west England. In a further experiment, three types of plants; Fragaria × ananassa (strawberry), Silene dioica (red campion), and Vicia faba (broad bean), were used to test the pollination service provided by the different grasslands. Fruit production and seed set of these plants, which were placed adjacent to the grasslands of the ten farms, was measured. This gave an indication of the effect of the grassland diversity on crop production and wild flower reproduction via spill-over of the pollinator communities to the surrounding habitats.

Results

We found that increasing grassland plant species richness, by the addition of both legumes and forbs, was associated with significant enhancements in the functional diversity of grassland pollinator communities. This gave an indication of the potential of the pollinator community to pollinate a diversity of plants in the landscape. Increased pollinator functional diversity was associated with a more constant pattern of visits to flowers by the pollinators over time and arguably a more stable ecosystem service.

Increased richness of the grasslands was associated with an increase in the pollination of the experimental species strawberry and red campion, but not broad bean. We found that enhanced pollinator functional diversity of the more diverse pastures was a potential mechanism for improved pollination of the strawberry plants.

Investigating individual pollinator interactions with grassland plant species (creating a plant-pollinator visitation network) we found Taraxacum sp. (dandelion) and Cirsium arvense (creeping thistle) to have the highest pollinator visitation frequency and richness, supplying a pollen and nectar source. However, these species have little agricultural benefit; as ever there is a trade-off between environmental and agricultural considerations. Therefore we highlight Cichorium intybus (chicory) as a potential target species for pasture seed mixes as it was well visited by pollinators and also has deworming properties for cattle as well as a deep tap root bringing up nutrients from the soil and supporting the soil structure.

Conclusion

Agricultural systems are engineered by humans; if we truly understand how they are assembled and how they function they could be manipulated to support the biodiversity that contributes to sustainable ecosystem services. The work presented here demonstrates that modest enhancements to grassland sward diversity with legumes and forbs, which are achievable with the expertise and resources available to many grassland farmers, could potentially improve pollination services across the landscape.

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