Investigating the diet of carnivorous plants

Earlier this year we reported on how it looked like bladderworts were farming their food, as much as hunting it. A paper submitted before those came out adds to the story, by examining the 'trophic level' of bladderwort and butterwort prey.

Recent research into carnivorous plants has shown they can be a lot less carnivorous than we thought. Bladderworts, might even be farming algae so they can have a snack in the future. So understanding what sort of foods a carnivorous plant eats could be a useful way of finding out how they operate.

Pinguicula alpina
Image: Filip Dominec / Wikipedia

Klink and colleagues looked at two plants in the Lentibulariaceae, bladderworts, Utricularia, and butterworts, Pinguicula. They picked two species of each type to examine the ‘trophic level’ of prey. Trophic refers to what a thing eats. Plants are usually autotrophs, as most make their food from air, water and sunlight. A herbivorous animal is a trophic level higher because it eats something. A carnivorous animal is higher still because it eats something that eats something. An animal that only ate carnivores would be a trophic level higher still.

So what is the trophic level of prey for bladderworts and butterworts? Do they attract herbivores? Do they try to grab anything?

Klink’s team set out to find this. Working out what something has digested is a bit of a task – after it has been absorbed. So they used stable isotope analysis. Nitrogen usually weighs 14 atomic mass units, but some nitrogen has an extra neutron in it, making it one unit heavier. This nitrogen is called Nitrogen-15. Similarly, carbon weighs 12 atomic mass units, unless it has one extra neutron, then it’s a unit heavier, and that’s called Carbon-13.

The ratio of Nitrogen-14 to Nitrogen-15, δ15N varies between plants, herbivores and carnivores. Carbon gets used in making the traps and operating the leaves. Measuring δ13C, the carbon ratio, provided a way for Klink and colleagues to measure the efficiency of the plant.

What the authors found was that herbivorous animals made up most of butterwort’s diet. It rarely caught carnivorous animals. Also, it was able to incorporate some carbon from its prey. That means not all carbon in a butterwort comes from photosynthesis. Bladderworts ate proportionally more carnivores, but also a lot of phytoplankton, microscopic plants. Klink and colleagues think using phytoplankton as a reference will help better determine the details of a bladderwort’s diet in the future.

Further reading

Klink, S., Giesemann, P., & Gebauer, G. (2019). Picky carnivorous plants? Investigating preferences for preys’ trophic levels – a stable isotope natural abundance approach with two terrestrial and two aquatic Lentibulariaceae tested in Central Europe. Annals of Botany. https://doi.org/10.1093/aob/mcz022