A long-term ecological study has revealed that using high levels of common fertilisers on grassland significantly reduces pollinator numbers and drastically decreases flower abundance.

Conducted by the University of Sussex and Rothamsted Research, the study found that increasing nitrogen, potassium, and phosphorus application on agricultural grassland led to a fivefold reduction in flowers and halved the number of pollinating insects.

Bees were the most affected, with plots free of chemical fertilisers supporting over nine times more bees than those receiving the highest fertiliser levels.

The lead researcher, Sussex University’s Dr Nicholas Balfour, said: “As you increase fertilisers, pollinator numbers decrease – that’s the direct link that​ to our knowledge has never been shown before.

“It’s having a drastic effect on flowers and insects. The knock-on effect goes right up the food chain,” he said.

The study, published in npj Biodiversity, attributes this decline to fertilisers promoting the growth of fast-growing grasses, which outcompete other grasses and flowers. Since pollinators often rely on specific plant species, a reduction in floral diversity directly impacts their populations.

The research was conducted at Rothamsted in Hertfordshire on Park Grass, a grassland strip studied since 1856.

On average, UK grasslands receive around 100kg of fertiliser per hectare. The highest experimental application was 144kg per hectare, which resulted in pollinator declines of 50% or more. Even at the national average fertiliser level, pollinator numbers were 42% lower, and flower abundance was reduced fivefold compared to unfertilised land.

The results were most pronounced on plots treated with nitrogen, the most widely used type of fertiliser. However, plots treated with fertilisers that excluded nitrogen retained relatively higher numbers of pollinators and flowers.

Given that nearly all UK grasslands are fertilised to some extent, with only 1%-2% classified as species-rich habitats, the findings highlight the broader issue of biodiversity loss.

Since the 1930s, the UK has lost 97% of its wildflower meadows, and pollinator populations have declined across the country.

In addition to tracking pollinators—including bees, hoverflies, butterflies, wasps, and flies—researchers also measured grassland productivity by weighing the hay yield at the season’s end.

Over the past decade the demand for agricultural fertiliser has increased.

Prof Francis Ratnieks, an entomologist at Sussex University, said: “I visited Park Grass many years ago and realised the unique opportunity it provided to study the effect of fertilising grasslands on wildflowers and bees.

“Considering the current focus on fertiliser use, and the substantial declines in pollinator numbers over recent years, this study could not have come at a better time, as we seek to understand how landowners can best help bees and other pollinators through open grassland areas,” he said.

The study underscores the challenge faced by farmers: improving biodiversity requires reducing soil fertility, which in turn lowers agricultural yields.

“Our most important and challenging finding is the existence of a trade-off between flower and pollinator diversity and grassland yield,” researchers state in the study. They highlight the need for financial incentives in the UK and EU to support biodiversity-friendly farming practices.

Balfour said: “While reduced yields aren’t typically thought of as a good thing, reducing grassland production intensity has the potential to realise many of the benefits of multifunctional landscape.” These include benefiting pollinators, improved soil health, better air quality and more resilience to extreme weather events.

Dr Philip Donkersley, a senior researcher in ecology and evolution at Lancaster University, who was not involved in the study, said: “What is interesting and novel here is the timeframes. Normally, our studies on this last four to five years. This is approaching 150 years of applying chemical and organic fertilisers, and therefore much more reflective of what has been happening on British farms.”