Fearsome Flora: Plants that Bite Back

Picture this - you’re walking through the woods, minding your own business, perhaps thinking about the delicious dinner waiting for you at home, when suddenly, the forest floor erupts beneath you, giant leaves sealing you into a botanical tomb. In the woods no one can hear you scream…

Thankfully, there are no plants that eat humans, however much Little Shop of Horrors and The Day of the Triffids may try to tell you otherwise. Yet we remain fascinated by plants that fight back, the predator rather than the prey. We tell stories about them, watch revealing documentaries, and even keep them in our homes.

But why have some plants evolved these strategies?

Carnivorous plants

There are around 630 species of carnivorous plants, and they are found in tropical and temperate areas around the world. In fact, they are found on every continent except Antarctica. These plants are known for chowing down on insects, but they will eat microbes and larger vertebrates like small fish, reptiles, and even small mammals. Meat supplements the plant’s more regular diet of sugars produced via sunlight. But why? Most carnivorous plants are found in wet areas with low nutrients where the soil is deficient in nitrates, such as bogs and swamps. Eating other species allows them to absorb nitrogen from elsewhere, and fulfil all their nutritional needs.

So how do they do it? Step one is attracting prey. Some produce delicious nectar that tempt in their prey, like pitcher plants. Insects are attracted to the smell, and lose their grip on the slippery rim, plunging into the pool of digestive juices at the base of the pitcher. Others camouflage themselves so animals unknowingly walk into them. Others, like sundews (also called Drosera), are sticky, holding insects fast with a type of secreted glue.

The infamous Venus flytrap (Dionaea muscipula) has trigger hairs that are an evolutionary masterpiece. To ensure it doesn’t waste energy, prey must trigger three hairs within 20 seconds. If triggered, the trap snaps shut at about the speed of a human blink, or a 10th of a second. It then secretes enzymes which digest its prey alive. There are even aquatic carnivorous plants. The waterwheel (Aldrovanda vesiculosa) traps swimming insects and even tadpoles. Each movement made by the prey while they struggle draws the unfortunate victim deeper into the plant.

Carnivorous plants do work in tandem with animals and other species too. Their flowers are high above their deadly traps to protect their pollinators. Some plants like Darlingtonia species don’t produce their own digestive juices and rely on organisms like bacteria to do it for them, providing a home for them in return. Several frog species even lay their eggs in pitcher plants, and Hardwicke’s woolly bat even sleeps inside the Nepenthes hemsleyana pitcher. Some larger species of pitchers will produce nectar to attract rats and shrews, which in return poop into the plant and provide it with manure.

The Roridula plant has a special relationship with the assassin bug. Unlike other ‘sticky’ carnivorous plant species, Roridula produce a resin rather than mucus. And here is the drawback—plants can easily digest the nutrients of their prey through mucus, but resin is tougher (think of the sticky resin of a tree). So how to solve this problem? The assassin bug is immune to the sticky coating on the plant’s leaves, but other insects are not, so once trapped, the assassin bug sucks out the prey insect’s insides and then poops on the leaves. Roridula can then digest the prey it traps through the assassin bug’s poop!

Meat-eating plants in science…

Once plants were confirmed to be carnivorous in the 1800s, the public perception of them shifted overnight. People referred to them as “vegetable wickedness”, and Reginald Ferrer calls sundews “evil little things” that must have committed some great crime to be punished with a life of “murder and fraud”¹.

Carnivorous plants challenged the ideas of hierarchies of nature with plants as passive and at the bottom of the pecking order. When John Ellis (1710 – 1776) an English naturalist, wrote to botanist and taxonomist Carl Linnaeus (1707 – 1778) about the Venus flytrap and how he believed it to be carnivorous, Linnaeus was not a fan. He thought this did not fit with the 18th-century view of the natural world, calling it “against the order of nature as willed by God”¹. It is worth remembering that at this time many people believed in nature being an act of divine creation, with humans at the top of the tree of life. The work of Charles Darwin (1809 – 1882) and Alfred Russel Wallace (1823 – 1913) on evolution would later shake up this accepted mode of thinking.

Darwin was a carnivorous plant enthusiast, calling the Venus flytrap one of the most wonderful plants in the world. He was one of the first to describe the meat-eating strategies of plants, alongside Mary Treat (1830 – 1923) in America. Treat was a keen entomologist and botanist, and initiated contact with Darwin because she was interested in sharing her observations of carnivorous plants. He even kept some in his personal greenhouse in Down House and studied native UK carnivorous plants like the sundew Drosera, publishing his work in 1875 in the book Insectivorous Plants.