Nepenthes is a genus of tropical pitcher plants, ranging from Southeast Asia, its center of diversity, westward into the Seychelles and eastern Madagascar and south to Australia. Nepenthes is the most diverse group of carnivorous plants to have evolved sophisticated pitcher traps and the only genus in the family Nepenthaceae. As in other carnivorous plants, the adaptive value of carnivory is thought to lie in the acquisition of nitrogen in nitrogen-poor environments.
Pitchers grow on tendrils extending from the midribs of leaves and trap prey passively, collecting pools of water into which prey are lured (with bright colors and nectar secreted on the pitcher rim), drowned, and digested with no energetic active movement on the part of the plant. In some species a single plant may grow some pitchers that lie recumbent on the substrate and others that hang suspended in mid-air; this results in leaf dimorphism, in which ground pitchers are different in shape, size, and appearance from aerial pitchers. Though most prey are small nectarivorous insects, the largest Nepenthes species may produce pitchers capable of trapping small vertebrates such as lizards, rodents, and birds. A few species have evolved modifications of the prototypical pitcher morphology and behavior to collect leaf litter (N. ampullaria) or vertebrate droppings (N. lorii, N. rajah, N. macrophylla, and N. rafflesiana).
Phytotelmata, the pools collected in Nepenthes pitchers, provide unique habitats that can support not only opportunistic species but entire faunal communities. These unusual and specialized communities are analogous to the phytotelmata collected by tree holes and New World bromeliads. Not surprisingly, Nepenthes species potentially form many mutualistic and commensalistic interactions with animals in their native ranges. The small organisms associated with Nepenthes traps are known as nepenthephiles, and may consist of both opportunistic and specialized inhabitants; the nature of their relationships with other nepenthephiles and with the plant, and the costs and benefits for all participants is often unclear. The water itself contains a community of protozoa, invertebrates, and even tadpoles that feed on excess prey, undigested prey remains, or each other, and may aid the plant in digestion.
The small mutualistic ant Camponotus (Colobopsis) schmitzi lives in the hollowed-out leaf tendrils of N. calcarata, enhancing the plant's capture ability by protecting developing traps from herbivores, maintaining the slippery interior of the pitchers, and even swimming into the pools to subdue large prey. In exchange, the ants are provided with living space, prey, and nectar. Coprophagous (dung-eating) species lure small arboreal mammals with copious nectar or shelter; while feeding or roosting, the animals defecate or urinate into the pitcher. Small frogs, land crabs, and spiders may also take advantage of the insects and shelter afforded by the pitcher. Some Nepenthes species are known as 'monkey cups' because monkeys are known to drink the water in the pitchers.
Human interest in Nepenthes ranges from the utilitarian to the aesthetic. Its unique carnivorous habit and unusual and varied forms made the genus an object of fascination by early naturalists and a fashionable but difficult plant to rear in captivity, and the culture and study of Nepenthes by an active community of enthusiasts continues. The highly slippery wax surfaces of the pitcher interior have also inspired modern attempts at engineering similar materials. Today many species, some of which are already scarce and occur only in a few localities, are threatened with extinction due to habitat loss and deterioration.
