Vanishing butterflies linked to nitrogen surplus

More than half of butterfly species in Switzerland are potentially threatened by excess nitrogen from vehicle emissions, industry, and agriculture.

Researchers at the University of Basel have discovered a link between the unintentional fertilization of soils through nitrogen emissions and low diversity of butterflies. 

Professor Valentin Amrhein, who led the team, says that the impact of nitrogen enrichment on butterflies has been underestimated. When it comes to butterfly diversity, the experts believe that nitrogen appears to play a similarly extensive role as climate change. 

Previous studies have shown that excessive nitrogen levels lead to denser vegetation with fewer plant species. 

“We wanted to find out whether a nitrogen surplus also indirectly affects the diversity of butterflies via this change in vegetation,” explained study lead author Dr. Tobias Roth.

The researchers investigated the diversity of plants and butterflies at nearly 400 sites throughout Switzerland by analyzing data from Biodiversity Monitoring Switzerland. The results clearly showed that plots with more nitrogen had lower plant and butterfly diversity. 

“As caterpillars, some butterfly species need certain plant species as food, or are dependent on a certain microclimate,” said Dr. Roth. 

With excessive fertilization, warm and dry places become cooler and damper as they are shaded by denser plant growth. The nitrogen surplus limits the prevalence of a large number of butterfly species in Switzerland that prefer open and dry sites, particularly those that are endangered. 

“Nitrogen from the air is likely to be an important factor in the reason why these species are endangered,” noted Dr. Roth.

While the researchers do not see a simple approach for improving the situation, there may be some potential in technical advances.

“In the past, slurry was sprayed on farmland, for example, and some of this was transferred to other areas of land by the wind,” said Dr. Roth. Today, he says, drag hoses are used increasingly to apply the slurry directly to the soil, which reduces nitrogen input in areas where it is not intended. 

According to the researchers, there is ultimately no way around environmentally friendly consumer behavior when it comes to reducing unwanted nitrogen input. This could be achieved, for example, through the reduction of vehicle emissions and livestock farming. 

The study is published in the journal Conservation Biology.

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By Chrissy Sexton, Earth.com Staff Writer