Air pollution makes carnivorous plants lose appetite

The study was carried out in three Swedish bogs where the amount of nitrogenous precipitation is clearly increasing from the northernmost bog to the southernmost one. The round-leaf sundew, Drosera rotundifolia, is a common plant in northern European bogs. The bog environment has a very poor supply of nutrients like nitrogen. The round-leaf sundew has coped with this by using its colourful and sticky leaves to attract and break down insects that get stuck on the leaves. This adaptation makes it one of the few herbaceous plants that can thrive in bogs surrounded by bog moss.

But the amount of nitrogen supplied via precipitation has increased as a result of human activities, from industries and traffic, for example, which leads to disturbances in the unique ecosystem that a bog represents. The study now being published shows that the sundew is taking up more and more of its nitrogen via its roots as nitrogenous precipitation increases. Capturing flies is simply no longer so important.

-    The plant grows more lushly, with larger leaves, but it no longer has the same need to capture insects for nutrition, says Håkan Rydin, professor of plant ecology at Uppsala University, who, together with his colleague Professor Brita Svensson, has collaborated with British associates.

The researchers gathered sundew plants, insects, and mosses growing on the same sites. They examined the content of various nitrogen isotopes, that is, forms of nitrogen of differing atomic weight. Nitrogen of biological origin – from flies, for instance – has a different mixture of isotopes from nitrogen that comes from rain. They were then able to calculate the portion of nitrogen that the plant took up via its roots in relation to the nitrogen coming in from capturing insects. The results show that the sundew plants that grew in the area with the least air pollution took in 57 per cent of their nitrogen from insects, whereas the corresponding figure for more polluted environments was as low as 22 per cent.

How is it possible, then, for the sundew to alter its diet in this way? The scientists believe it may have to do with the colour and stickiness of the leaves: previous studies have shown that when the amount of nitrogen increases, the leaves become less sticky and also more greenish than red – the red colour is believed to attract insects. The researchers are now going on this summer to increase the number of bogs in their study and also to measure leaf stickiness and colour temperatures.

How important is capturing insects for the plant’s survival in its environment? The capacity to capture insects is a clear competitive advantage in environments that are poor in nutrients, but this occurs at the expense of less efficient photosynthesis, among other things. In other words, other plants may take over. The most serious threat is that nitrogenous precipitation favours plants that grow larger, such as sedge, which means that sundew risks being overshadowed and thus dying out.

The study was funded by the UK National Environmental Research Centre, among others.