Talk Report: Dr David Roberts – Carnivorous Plants

Dr David Roberts from the Durrell Institute of Conservation and Ecology (DICE) at the University of Kent gave us an enthusiastic talk on these fascinating plants. His interest started as a small boy, when his father built him a large tank to grow his collection of orchids and carnivorous plants. His collection was transferred to Kew where he worked for several years.
After Charles Darwin had published his magnum opuses (or magna opera), On The Origin of Species and the Descent of Man, he turned his attention to Insectivorous plants, which were defined as ‘those plants which by some means or other capture living insects, and then profit by their digestion or decay, in aiding their own growth’.
They are now known as carnivorous plants; insects are their major prey but they also trap and digest other arthropods (such as beetles), lizards, small mammals and birds, but not humans as claimed for the Madagascar Tree in the New York World in 1847.
Carnivorous plants are found throughout the world (except in the Antarctic) usually found growing in nutrient-poor (especially nitrogen-poor) locations, often in acidic bogs, but also on bare rock. They still rely on photosynthesis to supply their energy, but rely on their prey for other nutrients. There are over 500 known species, with more being discovered on jungle expeditions by enthusiastic botanists.
In the UK we have the Sundew (Drosera rotundifolia) which catches its prey with sticky droplets at the end of tendrils on their leaves. The leaf curls up and releases enzymes to digest the prey. The Common Butterwort (Pinguicula vulgaris) has pretty violet-like flowers, but uses sticky fluid on its leaves to catch its prey. These are examples of flypaper traps. Also in the UK, the Lesser Bladderwort (Utricularia minor) floats in pools in peat bogs using bladder traps. On its stems it has small bladders which are maintained at a lower pressure. When a small invertebrate touches the hairs on the bladder, a trap door opens within milliseconds and the prey is sucked in to meet its fate.
The most well-known carnivorous plant is the Venus flytrap, so beloved of small boys. It is an endangered native of the wetlands of Carolinas in the east of the USA, but is widely available in cultivation. It is an example of the Snap Trap. The trap is an adapted leaf, hinged in the middle, with sensitive trigger hairs on their inner surfaces which stimulate the open leaves to clamp shut when touched, known as thigmonasty. (Alas, too many letters for Scrabble).
The most glamorous of the carnivorous plants are the many and various Pitcher plants. These use pitfall traps, adapted leaves which form a deep vat into which the prey falls attracted by nectar and bright colouring and patterning. The lip of the pitcher is waxy, and slippery. Downward pointing hairs prevent prey from climbing out. At the bottom, the prey is drowned and digested.
The Trumpet pitchers (about a dozen Sarracenia species) are native to wetlands in the United States, where they are under threat, although enthusiasts have introduced them to the Lake District, Germany and Ireland. Trumpet pitchers can reach several feet tall. They hybridise freely, varying from pale yellow, and green, to pink, maroon and purple, all with deeply veined pitchers – some cultivars have even been awarded AGM status.
The other major carnivorous plants are the tropical pitcher plants or monkey cups, with over 150 species of Nepenthes in the tropics, especially in Indonesia and Borneo. Some hang on tendrils, other larger pitchers sit on the ground looking rather like lavatory bowls- which in some cases is appropriate. The largest pitcher, N rajah, can grow up to 41cm high and 20cm wide, holding 2.5 litres of digestive fluid. N. attenboroughii, recently discovered in the mountains of the Phillipines, is only slightly smaller.
Many species, or their remains, have been found in larger pitcher plants, including lizards, frogs and rodents. However, a wide variety of species have formed a beneficial association with pitcher plants. Some frogs breed in the pitchers and crab spiders eat prey from them. The diving ant eats prey and keeps the lips of the pitcher slippery.
Several species of Nepenthes produce a sweet, fruity secretion on the underside of the pitcher lid. This attracts tree shrews which perch on the rim of the pitcher to reach the lid. As they balance on the rim, they poo into the pitcher. The plant gets its nutrients, not from digesting the shrews, but from their droppings. Another species of Nepenthes provides a roost for small bat colonies of Hardwicke’s woolly bat which hang from the lid and obligingly provide their droppings for their sanctuary. It has recently been proposed that one species, N. ampullaria has moved away from carnivory. The tightly packed pitchers sit on the jungle floor beneath the forest canopy, trapping leaf litter which provides required nutrients. Vegetarian carnivores!