How important is meat to a curious carnivorous plant?

Drosophyllum lusitanicum, also known as the Portuguese sundew or dewy pine, is unusual even for carnivorous plants in that it lives in dry environments. Typically, carnivorous plants live in nutrient-poor wetlands, so does it really gain much from carnivory?

Normally if you want to find a carnivorous plant in the wild, the best places to head are where it’s sunny and wet. In water-logged and nutrient-poor soils you can find carnivorous plants like sundews or pitcher plants, supplementing their diet with nutrients from their insect prey. But Drosophyllum lusitanicum, the dewy pine or Portuguese sundew is different. It lives in dry, fire-prone, habitats. It’s also a bit of a taxonomic oddity. It’s definitely not a pine, nor is it a sundew. It’s from a different family to sundews, the Drosophyllaceae. Though D. lusitanicum stands out as an oddity, it’s not a complete freak.

Drosophyllum lusitanicum, the Portuguese Sundew
Drosophyllum lusitanicum Photo: Fernando Ojeda

“While it is unique in many ways, Drosophyllum also uses the same trapping mechanism (adhesive trap) as a wide range of other carnivorous plants (e.g. Drosera, Byblis, Roridula, Pinguicula, Triphyophyllum and Philcoxia also use adhesive traps),” said Laura Skates, who worked on the paper studying the plant “An ecological perspective on ‘plant carnivory beyond bogs’: nutritional benefits of prey capture for the Mediterranean carnivorous plant Drosophyllum lusitanicum“. 

“These genera all mostly evolved independently (with the exception being the adhesive traps and stalked glands in Drosophyllum and Triphyophyllum [Dioncophyllaceae], which are sister families. Stalked glands in these two carnivorous plants are actually very similar in structure, being highly complex and the only ones irrigated by both xylem and phloem vessels. Though there are important differences between these two genera: Triphyophyllum is carnivorous only at initial stages, producing few linear, Drosophyllum-like leaves in a rosette of otherwise lanceolate, non-carnivorous leaves. Then, it grows into a liana which does not produce carnivorous leaves. By studying Drosophyllum and comparing it to these other groups of plants with adhesive traps, we can get a better understanding of how and why these carnivorous plants evolved.”

Skates and colleagues decided to examine the nitrogen in the Drosophyllum leaves to see how much of the nitrogen came from its prey. Nitrogen, Skates said, is important. “There are three reasons why we focus on nitrogen: 1) it is an essential element for all living organisms, 2) it is usually at very low concentrations in the soil where carnivorous plants grow, and 3) it naturally occurs as two stable isotopes (14N and 15N).”

The difference between 14N and 15N is only its weight; 14N has seven protons and seven neutrons. The seven protons make the element nitrogen, as opposed to carbon or oxygen. The seven neutrons make the nucleus stable, but some isotopes nitrogen has an extra neutron. This heavier form of nitrogen is still stable, but the extra neutron can be a useful tag. “In a typical food chain, ‘normal’ plants will have the lowest 15N:14N ratio and the proportion of 15N will increase as you move up the food chain from plants to herbivores to omnivores to carnivores. This is because 15N is heavier and more easily retained in the body, while 14N is lighter and more easily moves through metabolic processes to eventually be excreted,” said Skates.

“In the case of carnivorous plants, we expect that non-carnivorous plants will have the lowest 15N:14N ratio and insects will have the highest 15N:14N ratio. As carnivorous plants can get nitrogen both from the soil and from insects, we expect that carnivorous plants will have a 15N:14N ratio somewhere in between non-carnivorous plants and insect prey, depending on how much they rely on prey to get their nutrients.”

“To measure the amount of 14N and 15N in the plant leaves and the insects, we crush each sample into a dry powder, place the powder into a small tin capsule, and then run it through an EA-IRMS (Elemental Analyser coupled to an Isotope Ratio Mass Spectrometer).”

The results showed that the D. lusitanicum plants were indeed getting nitrogen from their insect prey, but that proportion varied from one place to another. At Puerto de Gáliz, the insects contributed only 36% of the nitrogen in the plant. At Montera del Torero it was 75%, with Sierra Carbonera in between the two. “It was fascinating to see the differences between the three sites – not only in the insect prey but also in the soil properties. This goes to show that there is no ‘one-size-fits-all’ when it comes to the nutritional benefits carnivorous plants can gain from prey – there are lots of factors at play, including prey availability and soil chemistry,” said Skates.

It wasn’t just nitrogen that the plant was gaining from its prey. The authors found that carbon was another input to the plant. This result was not a surprise, Skates said. “Some other studies have shown that carnivorous plants can gain some carbon from their prey, so we did expect that Drosophyllum might take up some carbon from prey alongside the nitrogen uptake. We would suspect that the carnivorous plant is taking up lots of other nutrients at the same time (but with nitrogen, we can at least measure this uptake using stable isotope techniques).”

Drosophyllum is part of a forthcoming PhD thesis by Skates where she will also examine some other plants. “I am currently writing up my PhD research on Byblis species from Western Australia! It is fascinating to compare different carnivorous plants which all have a very similar trap. The adhesive-trapping carnivorous plants provide some exciting opportunities to compare carnivory, both ecologically and evolutionarily. I would love to spend more time researching Drosophyllum in the future if an opportunity arises, especially to look further at its similarities and differences with other sticky-leaved carnivorous plants.” 

The biggest problems for the research were logistics and finances, which usually come with international research, Skates said. “The project was an international collaboration, involving multiple researchers from Australia, Spain and Germany. The plant, soil and insect samples were collected at three sites in Spain by Dr Paniw and Prof Ojeda, with necessary permits from the Andalusian, regional government (Consejería de Medio Ambiente, Junta de Andalucía). This permission was very important to note because Drosophyllum is a red-listed species. The samples were then sent to the BayCEER Laboratory of Isotope Biogeochemistry, at the University of Bayreuth, Germany. I travelled from Australia to Germany to learn about stable isotope techniques from Prof Gebauer, and to prepare the samples for stable isotope analysis.”

“All of this wouldn’t have been possible without financial and institutional support – I would like to thank the International Carnivorous Plant Society for providing funding for the analyses of the plant, insect and soil samples, and the Australian Flora Foundation for supporting my travel to Germany to complete the isotopic analyses. Financial support for fieldwork was provided by project HERRIZA (CGL2015-64007-P, MINECO-FEDER) from the Spanish Ministry of Science, Innovation and Universities.”

The results of the study show how the chemical analysis of a plant can yield results telling us its place in an ecosystem. Skates said that while insects are important in their own right in conservation, the research showed their importance to the Portuguese sundew to survive too. “Our results show that insect prey is a substantial part of the diet of Drosophyllum plants in their natural habitat, so without access to abundant insect prey, the plants may not be able to thrive. For this and other reasons, a whole-ecosystem approach is really important for carnivorous plant conservation.”