Nitrogen is an essential nutrient for all plants. But, like other carnivorous species, the roundleaf sundew plant (Drosera rotundifolia) has evolved to live in nitrogen-poor environments by supplementing its diet with insects.
Throughout the developed world, industrial activities have caused an increase in nitrogen pollution, so that more of the element is seeping into soils via rainfall.
In Sweden, where the experiments took place, the southern and central parts of the country are more polluted than the north, which has less industry.
Accordingly, the team found that roundleaf sundew plants in southern Swedish bogs are taking up more nitrogen via their roots than those in northern and central bogs, said study leader Jonathan Millett, a plant ecologist at Loughborough University in the U.K.
"They're more full-up," Millett said. "If you've got enough food in the fridge, you don't go to the shops to buy some more."
Braving the Bogs
For the work, the team collected tissue samples from the roundleaf sundew and its insect prey—mostly small flies called midges—from three Swedish bogs.
The scientists also sampled sphagnum moss, to serve as a control plant that receives all of its nitrogen via its roots and none from insects.
Bogs are "not the most pleasant environments to work in," Millett added. "It's impossible to stay dry—no matter how hard you try, you end up with wellies full of water."
Once their fieldwork was done, the team ground up the samples and analyzed the kind of nitrogen contained in each.
The nitrogen atoms drawn in by plants are slightly lighter than the ones in bugs, because biological processes tend to favor the heavier version of the element.
The team could therefore look for chemical tracers to pinpoint whether most of the nitrogen in a sample came from insects or from the soil.
From the data, the team figured out that sundew plants in polluted areas were getting more nitrogen from soil, implying that the predatory plants are laying off bugs.
It's "what people would predict, but no one has measured it before," said Millett, whose study appears in the July issue of the journal New Phytologist.
Nitrogen's "Large and Real" Impacts
Overall, nitrogen pollution has become a global problem that has "large and real" impacts on ecosystems, Millett noted.
At particular risk are carnivorous plants, which have evolved to live only in low-nitrogen environments.
Although it might seem like a boon to have lots more nitrogen handy, the plants' predatory trappings—such as specially shaped, sticky leaves—suck up a lot of energy. This makes the plants weaker and less able to compete with hardier, faster growing plants that will take advantage of higher nitrogen levels.
For instance, Millett suspects that "heather and grasses will start to 'take over' and shade the carnivorous plants, which don't grow too well in more shaded environments.
"The carnivorous plants do tend to do better on an individual basis when there is more nitrogen, but this ... isn't enough to keep up with these more competitive plants."
Aaron Ellison, a Harvard University ecologist, said in an email that the new, "carefully done field study" fits with "other studies of this phenomenon done with other species of carnivorous plants."
Because the roundleaf sundew is so widespread, it's not in danger of going extinct, study leader Millett added. But rarer carnivorous plant species that are hanging on in small populations could be in trouble.
"I would be surprised if nitrogen [pollution] doesn't have an impact on carnivorous plants" as a group, he said.