Nitrogen pollution from agriculture and fossil fuels has been seriously damaging grasslands in the UK.
A new European study has claimed that this problem is not only confined to the UK, but across Europe.
It has, therefore, said that current policies to protect ecosystems need a re-think.
David Gowing, one of Stevens' PhD supervisors at The Open University in the UK, said Stevens studied acid grasslands - upland pastures with relatively infertile soils, and she found that in places where more nitrogen is deposited, there are fewer plant species.
According to Stevens, the gradient was so pronounced that one species has been lost for each additional 2.5 kg of nitrogen per hectare deposited every year.
Nitrogen from man-made sources, like intensive farming and cars, causes significant air pollution in the UK, and some is deposited from the air on to the land.
Deposition is highest in densely populated areas, and in Britain ranges from about 5 to 35 kg of nitrogen per hectare per year.
The current approach to protecting wildlife from nitrogen pollution is to calculate critical load values for different ecosystems - how much nitrogen a system can accumulate every year before damage occurs.
Infertile habitats, like heathlands and bogs, are the most vulnerable.
But Stevens' research showed that species are being lost even where deposition is 'beneath' the critical load for grasslands.
"The species aren't going extinct, but if this is happening everywhere, we are moving towards much more species-poor grasslands, and we have no idea what the knock-on effects of that will be," Stevens said.
Last year, Stevens, David Gowing, Nancy Dise and Owen Mountford, and a team of experts from Germany, the Netherlands and France, embarked on a Europe wide project, part of the European Science Foundation (ESF) EuroDIVERSITY Programme.
The project's aim is to see if the effects are the same on a wider range of grasslands, across the entire Atlantic side of Europe.
Seventy new grasslands in at least nine European countries have been added to the picture. So far, the first year's field results seem to adhere to the pattern, showing that species loss is directly related to long-term deposition of nitrogen.
The team has started experiments to see if they can establish how extra nitrogen has these effects. They hope to predict what will happen in the future.
This project is called Biodiversity of European grasslands - the impact of atmospheric nitrogen deposition (BEGIN).
It involves scientists from the Open University, UK; the University of Bordeaux, France; Utrecht University, The Netherlands; the University of Bremen, Germany; Manchester Metropolitan University, UK and The Norwegian Institute for Nature Research, Norway.
The work was presented at the first EuroDIVERSITY conference, held in Paris from October 3 to 5.