Carnivorous plants change their diet: traps as toilet bowls

In tropical mountains, the number of insects declines with increasing altitude. This intensifies in high altitudes competition between plant species that specialize in catching insects as an important source of nutrients. How creatively some of these plant species have reacted to this situation is shown by an international research team with Prof. Dr. Gerhard Gebauer from the University of Bayreuth in the “Annals of Botany”: In mountain regions on Borneo, some species of the pitcher plant Nepenthes have changed their diet: With their traps, which originally served to capture insects, they catch the excrement of mammals and are thus even better supplied with nutrients than before.

Analyses in the Laboratory of Isotope Biogeochemistry at the University of Bayreuth have made it possible to discover this successful strategy of adaptation to an intensified competitive situation. It was known from previous studies that plants that feed on either preyed insects or animal excrement contain significantly higher levels of the nitrogen isotope ¹⁵N compared to “vegetarian” plants. However, it was unclear which of the two feeding strategies is more beneficial. Bayreuth biologist and isotope researcher Prof. Dr. Gerhard Gebauer and his master’s student Miriam Wickmann therefore analyzed the nitrogen in pitcher plant species that originated from high altitudes in the Malaysian part of the island of Borneo. In these regions, nitrogen gain from insect trapping or animal excrement is an important competitive advantage, as the soils are extremely poor in nitrogen. The result of the analyses: with one exception, all the species studied contained more ¹⁵N in their tissues than the “vegetarian” plant species living in the vicinity. In the tissues of pitcher plants that had switched their diet to animal excrement, the ¹⁵N content was even more than twice as high as in those pitcher plants that stuck to catching insects.

“A high level of the nitrogen isotope ¹⁵N in plant tissue is a clear indicator of an improved supply of nitrogen and other important nutrients. Our studies therefore clearly show that the switch to feces as a new food source was worthwhile. To switch their diet, pitcher plants simply had to convert their traps: They used to attract and trap insects with colors and droughts, but now their sugar-secreting nectaries invite small mammals native to Borneo to deposit their feces inside. Traps have become toilet bowls. This change in function is a surprising example of how plants are able to creatively adapt their diet. Such developments should be studied in more detail in the future. The knowledge gained will help to better protect plants under changed climatic and ecological living conditions,” says Gebauer. Not least, pitcher plants will also benefit from such protective measures: 40 percent of their species are currently classified as critically endangered, endangered or threatened.

International research cooperation:
The study resulted from close collaboration between the Laboratory of Isotope Biogeochemistry at the University of Bayreuth and research partners in Australia, Malaysia and the United States.