The Venus flytrap is only one among hundreds of flesh-eating plant species. Kirsty Strawbridge looks at the many ways they catch and eat their prey
There are about 600 known species of carnivorous plants in the world. They attract and trap prey, which they then digest to absorb beneficial nutrients.
But beyond those similarities, there are some striking differences. These 600 species come from more than a dozen genera in five orders. The different types evolved independently – they do not have a common ancestor.
Some plants are out-and-out carnivores, like the Venus flytrap and the common sundew Drosera rotundifolia. They have all three of the characteristics of a carnivorous plant: attracting prey, trapping it, digesting it for nutrients.
Some plants are ‘murderous’. Plants like blue plumbago (Plumbago auriculata) kill insects but don’t use their nutrients afterwards.
In practice, what makes a plant carnivorous isn’t always clear. The eastern purple bladderwort (Utricularia purpurea) can trap and kill prey, but it doesn’t do it often. Unlike other Utricularia species, it gets most of its nutrition from algae and zooplankton.
When plants fulfil only some of the criteria for a carnivorous plant, they’re sometimes called semi-carnivorous or para-carnivorous. Plants like birthwort (Aristolochia) temporarily trap insects in their flowers. The insects escape unharmed, but the trap makes pollination more effective.
Carrion flowers – plants that smell like rotting flesh – attract pollinators, which are sometimes trapped for pollination but never killed.
What they eat and how they catch it
Despite what science fiction says, we don’t have to worry about man-eating plants yet (see ‘The plants rooted in science fiction’ for a rundown of the best plants invented by writers for books and film). Carnivorous plants eat insects (especially flies, moths, wasps, butterflies, beetles and ants), spiders, crustaceans, and small vertebrates like frogs and mice.
The biggest carnivorous plant is Borneo’s rat-trapping pitcher plant (Nepenthes rajah). As its name suggests, it can trap rats and other small mammals.
Carnivorous plants have one of five types of trap:
- Plants like the tropical pitcher plants (Nepenthes) use pitfall traps, or pitcher traps. The pits contain liquid, and they often have steep or slippery walls. Pitcher plants lure their prey in with nectar and bright colours.
- Flypaper traps, or sticky traps, probably started out as a defence mechanism. Plants like sundews (Drosera) produce a glue-like substance called mucilage that traps their prey.
- Two plants have snap traps: the Venus flytrap (Dionaea muscipula) and the waterwheel plant (Aldrovanda vesiculosa). These traps close around the prey when it touches one of the trigger hairs inside the trap.
- Bladder traps are only seen in one genus. The bladderworts (Utricularia) have small underwater pouches, or bladders, with a partial vacuum inside. When the prey touches a trigger hair, a ‘door’ in the bladder opens – sucking in the prey and surrounding water.
- The aquatic corkscrew plants (Genlisea) use corkscrew traps. So do terrestrial plants like the cobra lily (Darlingtonia californica). They are lined with hairs pointing one way, which make the traps easy to get into but hard to escape.
Like animals, carnivorous plants use enzymes for digestion. The enzymes dissolve soft tissue but can’t completely dissolve skeletons or insects’ exoskeletons.
Most carnivorous plants make their own digestive enzymes. Some, like cobra lilies, have a symbiotic relationship with bacteria that break down their food. Others, like corkscrew plants, use both approaches at once.
Where you’ll find them
Non-carnivorous plants use light energy, carbon dioxide and water to produce glucose through photosynthesis and take nutrients from the soil. Carnivorous plants also produce glucose this way, but they get nutrients from their prey.
They do best in aquatic areas (such as rivers and peat bogs), coastlines and rainforests. They grow in places that have:
- waterlogged, acidic soil
- low levels of nitrogen and phosphorus in the soil
- lots of light.
Non-carnivorous plants struggle to survive in those places, but carnivorous plants have adapted to them. Taking nitrogen and other nutrients from prey is how they survive such extreme conditions.
But being carnivorous comes at a cost: traps can’t photosynthesise as well as ordinary leaves can. That’s why these plants need lots of sunlight and are outcompeted by other plants in less acidic soil.Lead image:
- Carnivory in the bromeliad Brocchinia reducta (1984)
- International Carnivorous Plant Society: Carnivorous plant digestion and nutrient assimilation
- Murderous plants: Victorian Gothic, Darwin and modern insights into vegetable carnivory (2009)
- National Geographic: What’s the biggest killer plant?
- Structure and biomechanics of trapping flower trichomes and their role in the pollination biology of Aristolochia plants (2009)
- The Carnivorous Plant FAQ
Questions for discussion
- If someone has a Venus flytrap, should they feed it bits of burger? Why/why not?
- With over 600 species of carnivorous plants, why is the Venus flytrap the most famous?
- Are there carnivorous plants in the wild near you? Do some research online to find ones native to your area.
- Why do plants like the eastern purple bladderwort get less carnivorous over time?