Predator plants may cut back on flies if they can access key nutrients elsewhere, according to research.
Scientists studying carnivorous sundew plants in Swedish bogs found that nitrogen deposition from rain reduced how many insects the plants trapped.
Pollution from transport and industry causes nitrogen-rich rain, meaning more reaches the ground in some areas.
"If there's plenty of nitrogen available to their roots, they don't eat as much" says Dr Jonathan Millett.
Dr Millett from Loughborough University led the study, which was funded by the National Environment Research council (NERC) and is being published in the New Phytologist journal.
The findings are based on tests on the round-leaved sundew plant, drosera rotundifolia. By measuring the amount of nitrogen of insect origin and comparing it to the amount taken up by the plant's roots, scientists could examine the proportions of each taken by plants in different locations.
They found that plants living in lightly polluted areas got 57% of their nitrogen from their prey. In more heavily polluted areas that figure dropped to between 20% and 30%.
But the species as a whole is not thought to be benefitting from this alternative source of nutrition. In fact, the discovery could prove to be bad news for the sundew in the longer term.
"Basically, it's like adding more fertiliser," said Dr Millett.
"For an individual sundew it looks like its better. They're bigger and they'll probably be fitter and do better, but the problem is that they have to divert resources into being carnivorous."
Carnivorous plants actually benefit from nutrient-poor environments, because they have less competition from other plants.
Their animal-digesting abilities seem to have evolved as a way to survive in these habitats, and the plants need a great deal of energy to "run" the complicated traps they use to capture and digest their prey.
If other plants move in as nitrogen levels in the soil increase, these predatory mechanisms could prove more of a hindrance than a help.
"When there's more nitrogen available... the non-carnivorous plants can 'out-compete' them," says Millett.
"You might get some local extinctions at very high levels of nitrogen deposition."
Sensitive carnivoresThe outlook for the future of carnivorous plants, more generally, seems bleak, although they are not as thoroughly researched as other, easier-to-study plant groups.
"People kind of suspected that carnivorous plants would be susceptible to pollution, because they tend to be quite sensitive to changes in their environment," says Dr David Jennings from the University of Maryland, who completed a study on the conservation threat to carnivorous plants last year.
"They don't really have very well-developed root systems, so any changes there can cause a lot of harm," he says.
Continue reading the main storyThe majority of carnivorous plant species assessed by the International Union for the Conservation of Nature (IUCN) are listed as threatened.
Although this IUCN figure is based on only 17% of carnivorous plant species, it is a concerning sign, says Dr Jennings. He is concerned both by the known threats to these plants and by the gaps in our knowledge.
For example, we know that Venus flytraps are under threat in the US, he says.
"But it's [relatively easy] to keep track of them, or set aside land in national parks where they can be more protected."
But, he says, so little is know about some species, that it is difficult to work out how to protect them.
The researcher says that Nepenthes plants are of particular concern. These are more commonly known as pitcher plants, because of the structure they use to catch prey. They rely on nutrients from insects, which they catch in a large cup full of fluid, from which the animal cannot escape.
Continue reading the main storyGet involved:
Plant-hunts and other conservation efforts take place across the UK and you are invited to take part:
The "rim" of the cup or peristome is often colourful, it is thought, to catch the eye of passing bugs. Its waxy surface makes it difficult for prey to clamber out.
Above this deadly cup looms an innocent-looking lid, which has just been found to play a key part in prey capture.
"[Nepenthes plants tick] the boxes for a lot of those major threats," says Dr Jennings.
"They're desirable for collectors and poachers; they're often among the most highly endemic, and they face other threats from habitat modification and habitat loss."
As we face the threat of losing some of these plants forever, experts say there are still many discoveries to be made.
The latest of these reveals how the lid of the Nepenthes gracilis pitcher plant can act like a catapult - launching prey to their doom.
Dr Ulrike Bauer is a carnivorous plants expert at Cambridge University and lead author of a report on the plants, which is published in the Public Library of Sciences (PLoS) ONE journal.
"It all started with the observation of a beetle seeking shelter under a Nepenthes gracilis lid during a tropical rainstorm," she says.
"Instead of finding a safe - and dry - place to rest, the beetle ended up in the pitcher fluid, captured by the plant."
The scientists found that, in dry conditions, insects can walk on the underside of the leaf. But in the rain, any insects sheltering there will be catapulted into the digestive fluid of the pitcher plant by the movement of the lid.
This capture process is assisted by special wax crystals on the lower lid surface, which provide enough grip for the insects to wander onto the underside of the leaf but not enough for them to hold on when a raindrop strikes the top.
The scientists filmed their experiments on a high speed camera.
How a plant uses rain to catch insects
Dr Bauer is curious about what other surprises these plants may have in store.
"I seriously think what we know today is only the tip of the iceberg" she tells BBC Nature.
And, she adds, pitcher plants make great models for other fields of study.
"There are three families of completely unrelated yet functionally amazingly similar pitcher plants: Nepenthaceae, Sarraceniaceae and Cephalotaceae," she explains.
"They are about as related to each other as we are to flatworms, and yet their traps bear astonishing resemblance; a perfect system to study the drivers of convergent evolution."
But the scientist fears that we may never get the chance to learn everything that we could from these plants.
"Nepenthes are not only threatened by habitat loss, but also by over-collection and climate change" she says.
"Some of the less accessible species with very local distribution might well be gone before science even takes note of their existence, let alone studying their secrets."
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