I’m sure you’ve all heard of ‘carnivorous plants’ like Venus flytraps. However, far from the man-eating blood thirsty Audrey II of “Little Shop of Horrors”, real carnivorous plants come in many varieties, partake in many degrees of carnivory, and are some of the most highly evolved and interesting members of the plant community.
In the animal world, a ‘carnivore’ is an animal that eats only meat (read: other animals) to obtain all of its nutrients and energy. Humans are omnivores and eat both plant and animal matter. Herbivores, like elephants, eat only plant matter. Plants, for the most part, are autotrophs (they can produce their own food), as opposed to heterotroph animals who must consume other organisms for food, nutrients, and energy. Plants photosynthesize – they use water, carbon dioxide in the atmosphere, and sunlight to conduct chemical reactions that produce glucose (sugar), oxygen, and energy for the plants to grow and thrive. In addition to these basics, plants require other nutrients like calcium, nitrogen, iron, and phosphate. These nutrients help plants to build cells and create proteins.
Most plants get these kind of nutrients from the soil. However, sometimes the soil in an area is nutrient-poor and normal plants can’t survive there. This is the reason that farmers usually give their fields some years off in between plantings, because the soils need time to regain enough nutrients to grow healthy plants. Carnivorous plants are evolution’s answer to nutrient-poor soils. Instead of just dying off when the soil is bad, or being limited to certain areas of the planet with good soils, carnivorous plants can grow in places (like swamps) that have difficult soils.
Carnivorous plants get their nutrients from insects, spiders, baby fish, crustaceans, frogs, lizards, mice, and even small birds. They don’t get energy from these animals – the plants still photosynthesize and use their prey only for nutrients, which means they are not true ‘carnivores’. The coolest part about plant carnivory is that scientists think it evolved at least six times independently across many different groups of plants. This means that the practice of trapping small animals to augment nutrients is a very effective one for plants – so effective that it evolved many different times and many different places for the same reasons. Being carnivorous also allows plants to grow where other plants can’t. This means that carnivorous plants are not forced to compete for space, sunlight, or water with other larger plants like trees, and they’re free to concentrate their energy on growing the apparatuses that allow them to catch and digest small animals.
There are many different ways these plants attract and trap their prey, and scientists have grouped them all into five basic trapping mechanisms. Venus flytraps, the most well-known carnivorous plants, are snap-trap carnivorous plants. Snap-traps are leaves that are brightly colored and coated with sugar water, and attract flies, bees, and other flying insects. When the insect touches the small, sensitive hairs on the leaf’s surface, the leaves snap shut and trap the bug inside, releasing digestive juices and slowing absorbing the insect’s nutrients.
Pitcher plants take a more passive stance. Pitcher plants are shaped like cups or cones, and can hold water. They fill their cup-like openings with water, sugars to attract prey, and digestive enzymes. The walls of the plants are usually very smooth, or the opening is hidden, and once a bug (or in some cases a small mouse, frog, or bird!) falls into the water they can’t find their way out. Eventually they starve, drown, or are digested alive by the plant.
Flypaper traps feed mostly on small insects, because the larger animals are usually strong enough to escape the plant’s grasp. Flypaper traps are leaves that are covered with sticky glue, mucus, or sap. Sometimes the leaves look completely normal, fooling prey into getting stuck to the plant and absorbed through the skin. Sometimes the leaves are covered in small sticky hairs and can actually roll up around captured insects, similar to the snap-trap.
Some aquatic carnivorous plants use a bladder trap to catch their prey. These plants can pump ions out of their inner chambers. The imbalance of ions causes water to travel out as well, through osmosis, leaving the plants with a vacuum inside. When prey triggers the entrance to the vacuum the ‘door’ hinges open and the prey is sucked up inside before the door closes and traps the small invertebrate or baby fish inside the plant to be digested.
The fifth and final type of carnivorous plant trap is called a lobster-pot trap. These traps are chambers that are easy to enter, but difficult to exit – either because the entrance becomes hidden or because it is blocked by bristles or spines. This trap is also common in aquatic plants. Once a prey animal finds its way inside it’s forced towards the plant’s ‘stomach’ as it struggles, and ends up trapped inside while it’s digested by the plant.
As morbid as this all sounds, I think it’s worth taking a minute to really appreciate the genius behind these plants. Not only are they a true feat of evolution, beating the problems of nutrient-poor soils and sunlight competition from other plants, but they’re plants. We never think of plants as having evolved, having dietary needs, and certainly I never think of plants when I talk about ‘stalking prey’, but these guys do just that. Since they’re so hardy, and used to harsh growing conditions, carnivorous plants are actually really easy to grow indoors as ornamentals. They make nice conversation pieces, and as long as you do provide them with good soil they don’t need to be fed bugs to survive (but they will keep your house gnat-free!).
Thanks to Jane Nearing and flickr’s creative commons for the lovely picture of snow!
And, as always, a special thank you to the masses of Wikipedia, for most of the background knowledge needed for this post.