The Diversity and Ecology of Carnivorous Plants
What are Carnivorous Plants?
Carnivorous plants can be defined as any plant that derives part or most of their nutrients (but not energy, which they derive from sunlight, like all other plants) from trapping and consuming protozoans and animals such as insects. These plants are well adapted to live in nutrient poor areas, such as rocky areas and acidic bogs. Carnivory in plants has evolved independently in five different orders, yielding over 500 species. In addition to this, some 300 species of protocarnivorous plants have been recognised by science (more on these later). The carnivorous plants can be grouped based on the structure and mechanisms of their traps.
- Flypaper traps
- Pitfall traps
- Lobster-pot traps
- Bladder traps
- Snap traps
- Protocarnivorous Plants
Where are Carnivorous Plants found?
Carnivorous plants have a widespread distribution, from Nepenthes pitcher plants in Southeast Asia, trigger plants in Australia and the Great Sundew (Drosera anglica) which is found in temperate regions such as Alaska and Great Britain. Carnivorous plants are commonly found in boggy, nutrient poor soils, which are low in Nitrates and Phosphates. Like all plants, they need these elements to survive, and so by catching and digesting animal prey they can utilise these from the bodies.
Flypaper Traps: Sundews and Butterworts
Flypaper traps utilise mucilage-secreting glands in the leaf which make the surface of the leaf incredibly sticky. The leaves of some species such as Sundews (Drosera spp.) appear shiny as though laden with nectar, attracting insects which become stuck to the leaf. Flypaper traps have evolved independently at least five times and are thought to be derived from more complex traps such as those of pitcher plants.
Probably the best known flypaper traps are those of the sundew (Drosera). With some 194 species, the sundews are widely distributed, from as north as Alaska to New Zealand in the south, though it's main hotspots are Australia, South America and South Africa. Growing in seasonally moist (or sometimes wet) habitats with acidic soils and lots of sunlight, they trap insect prey such as mosquitoes and gnats using tentacles topped with a sweet, sticky secretion which attracts insects onto the leaf. This secretion prevents the insect from escaping and it dies usually within around 15 minutes due to exhaustion or suffocation (as the secretion blocks it's airways). Digestive enzymes such as proteases and phosphatases are secreted which enable the plant to digest it's prey. The tentacles are incredibly sensitive to touch and will move towards the prey, rolling the leaf inwards.
The same trapping mechanism is observed in the closely related Portuguese Sundew (Drosophyllum lusitanicum), the main difference being that the leaves are incapable of rapid movement. Drosophyllum is one of the only species of carnivorous plant which grows in a semi-desert environment. The australian Rainbow Plant (Byblis sp.) and the liana Triphyophyllum peltatum also have a similar trap structure with mucilaginous tentacle, however they are incapable of rapid movement in the tentacles nor the leaf itself. Triphyophyllum has a three-stage lifecycle, with carnivorous leaves present in only one of these stages.
The other main type of flypaper trap is that of the Butterworts (genus Pinguicula). The tentacles are much smaller and closer to the surface of the leaf, giving the leaf a shiny appearance. Much like Drosera, butterwort leaves are able to roll inwards in response to prey trapped on the leaf (thigmotropism), to prevent rainwater washing prey away. These plants are found in Europe, northern Asia and the Americas and the insects caught are fungus gnats.
Pitfall Traps: Pitcher Plants and Bromeliads
Pitfall traps have evolved eight times within the pitcher plants and bromeliads. Characterised by an internal chamber, pitfall traps are passive, attracting insects with nectar bribes which is secreted by the peristome ('mouth' of the pitcher) and anthocyanin pigments found in the pitcher. Many pitchers are lined with downward pointing hairs and waxy scales which cause prey to slip into the bottom of the pitcher. Prey are eventually broken down by digestive enzymes (such as proteases) which are secreted from the wall of the pitcher. Rainfall can accumulate in the bottom of the pitcher, creating a habitat for aquatic life, such as protozoa, mosquito larvae and even frogs (called a phytotelma).
The simplest pitfall traps are found in the Marsh Pitchers (Heliamphora sp.). Living in high rainfall areas in South America such as Mount Roraima. The trap is formed from the transverse folding of the leaf to form a simple tube. Heliamphora is vulnerable to overflowing of pitchers due to the environment that it lives in, and the plant has small gaps in the leaf margins so that excess water can flow out of the pitcher. The North American pitchers Sarracenia sp. and Darlingtonia californica are closely related to the marsh pitchers, have a cover, or operculum, over the rim of the pitcher to prevent overflowing. This also allows the secretion of enzymes used in the digestion of prey (Heliamphora relies on bacterial digestion). The operculum of D. californica is balloon-like and has many chlorophyll-free areas through which light penetrates to form 'false exits'. When insects such as ants enter these compartments, they tire themselves trying to escape and eventually fall into the pitcher.
The Tropical Pitcher Plants of the genus Nepenthes possess the most advanced pitchers of all. The pitchers are borne at the tip of leaf-forming tendrils, and extension of the midrib of the leaf. Some 150 species of Nepenthes are known, occurring mainly in southeast Asia with a few species in Australia, Madagascar and India. Most species catch insects, though some species such as Nepenthes rajah have been observed to feed on lizards and small rodents such as shrews.
The Albany Pitcher Plant Cephalotus follicularis is found in western Australia and possesses small pitchers with a pronounced peristome with spines which allow insects to enter the pitcher but hinder escape.
Carnivory is known in two genera of bromeliad: Catopsis and Brocchinia. The tightly packed and waxy leaves form a pitted depression at the base of the plant, which can become filled with rainwater (much like the pitcher plants). This pool provides a micro-habitat for insects, amphibians (particularly poison dart frogs whose tadpoles live within these pools) and microbes, including diazotrophic (nitrogen-fixing) bacteria which are very important for the plant.
Lobster-Pot Traps: Corkscrew Plants
A lobster-pot trap can be defined as a chamber which is easy to enter, but difficult to escape from, either because the exit is difficult to find or is obstructed by downward pointing hairs or bristles. These traps are known in the corkscrew plant (Genlisea), a group of small wet terrestrial and semi-aquatic species which are found in Africa and South and Central America. The traps of these species are underground and derived from leaves which are highly modified into a spiral shape. They feed mainly on aquatic protozoa which enter the traps and are unable to exit due to downward-pointing hairs in the trap. Trapped within the y-shaped structure, they are then digested by the plant.
Bladder Traps: Bladderworts
The bladder traps of the bladderworts (Utricularia) are the fastest acting traps of all, which suck in prey in less than a millisecond. The bladderworts are a diverse group of plants, with over 200 species, and can be terrestrial or aquatic, found on every continent except Antarctica.The traps, which give the plants their English name, are small bladder-like structures. The opening or 'mouth' is lined with several trigger hairs, much like that of a Venus flytrap, and when these are touched, the door is deformed, releasing a vacuum and sucking prey into the bladder. The bladders of aquatic species are usually much larger than those of terrestrial species, and have been known to trap water fleas, mosquito larvae and even fish fry. The traps are known to secrete mucus containing a sugary substance which is thought to attract prey. Like the traps of the corkscrew plant, water flow plays a part in the vacuum establishment in bladderwort traps.
Snap Traps: Venus' Fly Trap and Waterwheel Plant
Snap traps are only seen in two species which still exist today, Venus' fly trap (Dionaea muscipula) and the waterwheel plant (Aldrovanda vesiculosa). Like the bladderworts the snap traps have trigger hairs, which cause the traps to shut. Venus' fly trap lives in boggy wetlands in North and South Carolina where it feeds on arthropods such as insects and spiders. The waterwheel plant can be found in aquatic environments and has smaller traps; it feeds on aquatic invertebrates and fish fry.
The mechanism by which these plants capture their prey is a complex one and is still not fully understood.The trap is a modified tip of the leaf, which is split into two lobes. Each lobe has several trigger hairs (3 in D. muscipula and numerous longer hairs in A. vesiculosa) which are sensitive to touch. The bending of the hairs causes ion channels in the hair cells to open and create an action potential similar to that of a neuron in animals.This travels to cells in the midrib, changing the shape of these cells, thus resulting in the closing of the trap. As the prey struggles within the trap, it causes the trap to shut tighter, and digestive enzymes are secreted from the walls of the to which break down the body. The process of shutting the trap requires lots of energy, and so the it is only triggered if two or more hairs are touched within 30 seconds of each other.
In order for a plant to be carnivorous, it must be able to trap, kill and consume it's prey. There are many species of plant which are able to trap and kill prey such as insects but the inability to digest the bodies makes their classification as carnivores as contentious.
Roridula is a typical example of a protocarnivorous plant. It is found in South Africa and has dewy tentacles on the leaves similar to that of sundews and the dewy pine with which it catches flying insects such as gnats and flies. However it is unable to digest it's prey so it has developed a symbiotic relationship with the capsid bug Pameridea roridulae which feeds on these insects. The plant relies on the droppings of the bug for nutrition.
Some plants have carnivorous traps at certain stages, such as the passionflower Passiflora foetida. The bracts surrounding the flowers have sticky tentacles similar in design to Drosera leaves and appear to catch and kill insects, however they are not classified as carnivorous due to the inability ton digest 'prey'.
The Australian trigger plants (Stylidium) which are known for their aggressive pollen delivery mechanisms, possess glandular trichomes on the leaves, sepals and flower parts which cause small insects to suffocate and die. Originally thought to protect against damage to flowers, recent evidence of digestive enzymes in the trichomes could suggest that these plants could actually be truly carnivorous.
- Darnowski, D. W., Carroll, D. M., Plachno, B., Kabanoff, E. and Cinnamon, E., 2006. Evidence of Protocarnivory in Triggerplants (Stylidium spp.; Stylidiaceae). Plant Biology, 8 (6), 805-812.
- Schaefer, H. M. and Ruxton, D. G., 2008. Fatal attraction: carnivorous plants roll out the red carpet to lure insects. Biology Letters, 4 (2).
- Ellison, A. M. and Gotelli, N. J., 2001. Evolutionary ecology of carnivorous plants. Trends in Ecology and Evolution, 16 (11), 623-629.
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© 2017 Jack Dazley