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An epiphyte is an organism that grows on the surface of a plant and derives its moisture and nutrients from the air, rain, water (in marine environments) or from debris accumulating around it. In terrestrial ecosystems only plant species can be considered epiphytes, but in marine systems any sessile organism growing on the surface of a plant is considered an epiphyte. Epiphytes take part in nutrient cycles and add to both the diversity and biomass of the ecosystem in which they occur like any other organism. The are an important source of food for many species. Typically, the older parts of a plant will have more epiphytes growing on them. Epiphytes differ from parasites in that epiphytes grow on other plants for physical support and do not necessarily negatively affect the host. An epiphytic organism that is not a plant is called an epibiont. Epiphytes are usually found in the temperate zone (e.g., many mosses, liverworts, lichens, and algae) or in the tropics (e.g., many ferns, cacti, orchids, and bromeliads). Epiphyte species make good houseplants due to their minimal water and soil requirements. Epiphytes provide a rich and diverse habitat for other organisms including animals, fungi, bacteria, and myxomycetes.

Terrestrial Epiphytes
The best-known epiphytic plants include mosses, orchids, and bromeliads such as Spanish moss (of the genus Tillandsia), but epiphytes may be found in every major group of the plant kingdom. 89% of terrestrial epiphyte species (about 24,000) are flowering plants. The second largest group are the leptosporangiate ferns, with about 2800 species (10% of epiphytes). In fact, about one third of all ferns are epiphytes. The third largest group is clubmosses, with 190 species, followed by a handful of species in each of the spikemosses, other ferns, Gnetales, and cycads. The first important monograph on epiphytic plant ecology was written by A.F.W. Schimper (Die epiphytische Vegetation Amerikas, 1888). Assemblages of large epiphytes occur most abundantly in moist tropical forests, but mosses and lichens occur as epiphytes in almost all biomes. In Europe there are no dedicated epiphytic plants using roots, but rich assemblages of mosses and lichens grow on trees in damp areas (mainly the western coastal fringe), and the common polypody fern grows epiphytically along branches. Rarely, grass, small bushes or small trees may grow in suspended soils up trees (typically in a rot-hole).

Epiphytic plants attached to their hosts high in the canopy have an advantage over herbs restricted to the ground where there is less light and herbivores may be more active. Epiphytic plants are also important to certain animals that may live in their water reservoirs, such as some types of frogs and arthropods.

Epiphytes can have a significant effect on the microenvironment of their host, and of ecosystems where they are abundant, as they hold water in the canopy and decrease water input to the soil. The epiphytes create a significantly cooler and moister environment in the host plant canopy, potentially greatly reducing water loss by the host through transpiration.

Marine Epiphytes
The ecology of epiphytes in marine environments differs from those in terrestrial ecosystems. Epiphytes in marine systems are species of algae, bacteria, fungi, sponges, bryozoans, ascidians, protozoa, crustaceans, molluscs and any other sessile organism that grows on the surface of a plant, typically seagrasses or other algae. Settlement of epiphytic species is influenced by a number of factors including light, temperature, currents, nutrients, and trophic interactions. Algae are the most common group of epiphytes in marine systems. Photosynthetic epiphytes account for a large amount of the photosynthesis in systems in which they occur. This is typically between 20 and 60% of the total primary production of the ecosystem. Epiphytes are a general group of organisms and are highly diverse, providing food for a great number of fauna. Snail and nudibranch species are two common grazers of epiphytes. Epiphyte species composition and the amount of epiphytes can be indicative of changes in the environment. Recent increases in epiphyte abundance have been linked to excessive nitrogen put into the environment from farm runoff and storm water. High abundance of epiphytes are considered detrimental to the plants that they grow on often causing damage or death, particularly in seagrasses. This is because too many epiphytes can block access to sunlight or nutrients. Epiphytes in marine systems are known to grow quickly with very fast generation times.