The carnivorous common sundew (Drosera rotundifolia) traps insects by attracting them with its bright red color, getting them stuck to the glistening drops of mucilage on its leaves, and then curling around them (see it in action here and magnified here) and dissolving them with enzymes. Because it lives in habitats that are poor in nutrients, it has to eat bugs to supplement its diet. But Dr. Jonathan Millett of Loughborough University in Leicestershire, UK, has determined that pollution has caused the sundew to lose its appetite for meat. "If you've got enough food in the fridge, you don't go to the shops to buy some more," the plant ecologist analogizes. He studied the sundews growing in the bogs of Sweden and has found that, due to the burning of fossil fuels, the rain that falls now contains enough nitrogen to sustain the plants without catching flies and midges. Millett and coauthors have published their study in New Phytologist, showing that plants in lightly-polluted areas got 57% of their nitrogen from their insect prey, while those in more heavily-polluted areas got only 22% from insects. How do they shift their diet? When they absorb the nitrogen through their roots, they make their leaves less sticky, trapping fewer prey, and they tone down their bright color, so fewer bugs are drawn to them. How did the scientists identify the sources of the nitrogen in the plants' diet? They did an isotopic analysis of the sundews, the insects, and the moss growing in the area, which allowed them to work out the proportions, since nitrogen of biological origin (bugs) has a different atomic weight than nitrogen deposited in the rain. Although the ability to make this change in their diet allows the sundews to take advantage of the artificial rain of fertilizer that has disturbed their specialized ecosystem, it does not spare them possible extinction. Carnivorous plants have to spend lots of energy on their specialized equipment, so even if they are using the adaptations less, "...they still have to bear the residual costs of being carnivorous, and other plants without these will be better able to survive. So it’s quite likely we’ll see less abundance and perhaps local extinctions from carnivorous species. The individual plants get bigger and fitter, but the species as a whole is less well adapted to high-nitrogen environments and will lose out over time,” explains Millett. Because sundew is so widespread it is unlikely to be killed off by nitrogen pollution, but the same cannot be said of other carnivorous plants existing in smaller populations.
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