Close Encounters

Weeds with a bad reputation have value for pollinators and biodiversity

Ecologists have found that weeds are far more valuable in supporting biodiversity than we give them credit for.
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Governments are recognising the importance of supporting pollinators as an aid to agriculture. The UK government recommends planting some species, such as clover, alongside crops to encourage pollinators. But new research shows that some weeds listed as injurious provide better forage for pollinators. This month, Nicholas Balfour and Francis Ratnieks have published their findings in the Journal of Applied Ecology.

The British Government lists five plants as injurious. These are plants that are native to the British Isles but are considered to be a hazard for agriculture. Balfour and Ratnieks compared the biodiversity value of common ragwort, creeping thistle and spear thistle with plants recommended for pollinators, such as red clover and wild marjoram. 

A golden ragwort flower against a green leafy background shining like a nine pointed star.
Common ragwort. Jacobaea vulgaris syn. Senecio jacobaea. Image: Canva.

The ecologists studied six pasture or ex-pasture sites where they watched pollinator visits to determine their preferences. They then combined this with flower-visitor data from the Database of Pollinator Interactions.

In the Database of Pollinator Interactions, four times as many pollinator species and five times more conservation-listed species have been recorded visiting the three insect-pollinated weeds. Of the 387 plant species analysed in the database, in terms of pollinator species recorded, the weeds were ranked 4th (Cirsium arvense), 6th (Jacobaea vulgaris), and 13th (Cirsium vulgare). Similarly, the Database of Insects and their Food Plants showed that twice as many herbivorous insect species are associated with the five weed species.

Dr Nicholas Balfour said in a press release: “There now exists a substantial body of evidence which shows that weeds are a vitally important resource for pollinators. 

“The three insect-pollinated species have open flowers that allow access to a wide variety of pollinator species, and they produce, on average, four times more nectar sugar than the DEFRA recommended plant species.

“Pollinators are crucial to maintaining global biodiversity, ecosystem resilience and agricultural output. However, there are significant concerns about pollinator declines, and the long-term decline of flowers in our landscapes is considered a key factor.”

“We appreciate that agricultural weeds can cause yield losses in arable and pastureland. However, we’ve shown that they can also be of great value to both flower-visiting and herbivorous insects – and shouldn’t be underestimated when it comes to supporting our natural biodiversity.”

Freedom of information requests to public bodies such as councils indicated that Natural England and Highways England spends around £10 million controlling injurious weeds.

The majority of local councils indicated that they actively control ragwort, thus classing it in the same bracket as invasive, non-native species such as Japanese knotweed (Reynoutria japonica), likely due to the Ragwort Control Bill 2003. 

Dr Balfour added: “It is alarming that the many public bodies are using tax-payers’ money and volunteers to actively remove ragwort. This plant was found to support the most conservation-listed insect species in our study.

“The implementation of the Ragwort Control Bill probably deserves greater scrutiny, especially given that the evidence underpinning it is questionable.

“Our results clearly show that weeds have an underappreciated value in supporting our natural biodiversity. Unfortunately, current UK agricultural policy encourages neither landsparing for, nor landsharing with, weeds.”

The authors are now calling for policymakers to look at how existing policies are implemented and reconsider the role of weeds in future agri-environmental policy.

ORIGINAL ARTICLE

Balfour, N.J. and Ratnieks, F.L.W. (2022) “The disproportionate value of ‘weeds’ to pollinators and biodiversity,” The Journal of Applied Ecology. https://doi.org/10.1111/1365-2664.14132

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