We’ve all heard of the Venus flytrap, but the plant kingdom contains many more wonderful examples of meat-eating species. Some plants have been known to feed on insects, frogs, and even birds and small mammals. But if plants are so good at getting energy from the sun, why would these eating habits ever have evolved in the first place?
Meat eaters are relatively rare in the plant world, yet there is a stunning variety of carnivorous plants inhabiting almost all of the world’s continents. Out of the 300,000 or so species of flowering plants, we know of about 630 truly carnivorous ones.
All carnivorous plants rely on some kind of clever trap. Unsuspecting animals (usually insects) are lured with the promise of a nutritious reward like nectar. In some cases the animal will slip in to a pool of digestive enzymes and drown, in others the animal will trigger the release of a special trapping structure on the plant (think Venus flytrap). Some species, such as the sundews, glue their prey to the spot, only to wrap them up and slowly digest them.
In addition to nectar, some species attract their prey with fluorescent rings on their leaves that are invisible to the human eye. These rings shine with ultraviolet light, which insects are highly sensitive to.
The incredible adaptations of carnivorous plants suggest a long history of evolutionary pressure. But if plants can get their energy from the sun through photosynthesis, why would the need to feed on animals ever have arisen?
To understand why carnivorous tendencies might have evolved in plants, lets look at one of the most famous groups of carnivorous plants: the pitcher plants. They are recognised by their jug-shaped body, the “pitcher”. With an ultra-slippery rim, the pitcher traps and drowns animals in a soup of digestive enzymes. The largest known species is probably Nepenthes rajah, which can have pitchers up to 20 cm wide. In addition to its usual diet of insects, N. rajah has been known to drown lizards, birds, and rats.
Pitcher plants actually belong to many different plant families, rather than being from the same branch of the evolutionary tree. The pitcher body plan has evolved independently in different plant lineages from around the world — a good example of convergent evolution.
But what do all pitcher plants have in common, besides their appearance? It turns out that they all live exclusively in boggy environments that are acidic and poor in nutrients. The consumption of animals appears to be a way of supplementing a diet that would otherwise be poor in nutrients such as nitrogen and phosphorous.
This turns out to be true for all carnivorous plants. In addition to water and sunlight, all plants require a regular source of nutrients. This usually comes from the soil, but carnivorous plants have simply found a way to get around that.
Carving a niche
You might be thinking “why don’t they just live in better soil and get their nutrients that way, like normal plants do?” The answer is that by adapting to a harsh environment that other plants can’t tolerate, carnivorous plants have much less competition. They have created a niche. This is a common theme in evolution, which explains, for example, why mangroves live in salty water and even why the first land dwelling animals left the oceans all those millions of years ago.
As we continue to learn from carnivorous plants and even identify new varieties, isn’t it a shame that some species might never be studied and appreciated before they disappear for good? Of the 12 new species of Nepenthes that were discovered last year, all native to the Philippines, some are feared to already be extinct from loss of habitat.