An analysis of butterflies in Switzerland found elevated nitrogen levels were linked with reduced species diversity and fewer individuals, suggesting that excess nitrogen could be an underestimated driver of insect declines.
Human activities such as the burning of fossil fuels and the use of fertilizers pump excess nitrogen into soil, air and water, but little is known about how increased deposition of this element affects insects and other animals. In a study published April 8 in Conservation Biology, researchers examined the effects of nitrogen deposition on butterfly diversity in Switzerland.
Nitrogen is an essential element for all living things, but too much of this good thing boosts the growth of certain plants and changes the structure of plant communities, which can have cascading effects on ecosystems.
"[Nitrogen deposition] is not as well known as climate change, but it's a factor that is really affecting biodiversity at a large scale," said study lead author Tobias Roth, an environmental sciences research associate at the University of Basel and a project leader at Hintermann & Weber AG.
Insects are facing global declines from climate change, habitat loss, pesticide use and light pollution, among other factors. But according to Roth, only a few studies have considered the effects of nitrogen on insects.
As part of the study, Roth and his colleagues conducted a review of previous research that examined environmental factors related the number of butterfly species. Of the 32 studies evaluated, none measured nitrogen deposition.
"We think [nitrogen deposition] is a bit overlooked when people try to explain diversity patterns of butterflies," Roth told The Academic Times.
To investigate how nitrogen affects butterflies, Roth and his colleagues used data from almost 400 sites that are part of a project called Biodiversity Monitoring Switzerland. To arrive at butterfly estimates, surveyors walked along 2.5-kilometer paths and counted the butterflies seen during seven outings throughout summer and fall. For accompanying plant surveys, botanists recorded all plant species growing along these same transects.
The researchers also gathered other information about the sites, such as temperature, climate, elevation and type of habitat, and they took nitrogen deposition data from a previous study that mapped levels of this nutrient across the whole of Switzerland.
The analysis showed that high nitrogen deposition was strongly associated with fewer butterfly species. When the researchers looked at individual butterfly species, nitrogen deposition was associated with fewer individuals, an effect that was strongest for rare and vulnerable butterflies, suggesting they could be most at risk from increased nitrogen.
"It's well known that nitrogen deposition is affecting plants, and it's well known that butterflies depend on plants, so we expected an effect, but the surprising thing was that the effect was so consistent and as strong as temperature or climate," Roth said. "We didn't expect that."
According to the researchers, the findings suggest that reducing plant density and nitrogen levels through animal grazing or mowing could help promote butterfly conservation. The team hypothesizes that elevated nitrogen mainly affects butterflies indirectly through changes in plants.
"When there is more fertilizer, more nitrogen, the vegetation will be denser, and the microclimate will be cooler," Roth explained. Because insects depend on warm temperatures for development, "When vegetation becomes more dense, less butterflies can survive."
"The other thing is that nitrogen is mostly favoring dominant plant species," he added. "A few dominant plant species will increase, but all the other species will decrease. So, generally, species richness of plants is quite strongly decreasing."
Before becoming butterflies, many caterpillars are picky eaters, feeding on only a particular plant species. That means, "The less [plant species] you have, the less butterflies you have," Roth said.
The study, "Negative effects of nitrogen deposition on Swiss butterflies," published April 8 in Conservation Biology, was authored by Tobias Roth, University of Basel and Hintermann & Weber AG; Lukas Kohli, Hintermann & Weber AG; Beat Rihm, Meteotest; Reto Meier, Federal Office for the Environment; and Valentin Amrhein, University of Basel.