Worldwide about 24 000 species or 10% of vascular plants are epiphytes, with the proportion up to 50% in tropical forests. Similar figures are unavailable for nonvascular epiphytes. The green biomass of epiphytes may match that of their host trees, though epiphyte productivity is likely to be lower due to resource limitation (see above). In moist tropical forests the habitat complexity, water and food provided by epiphytes is a major contributor to the high diversity of insects and other animals.
The productivity of aquatic epiphytes often exceeds that of their host macrophytes, though in some habitats (e.g., seagrass beds) the dominance of epiphytes may be a recent artefact of anthropogenic eutrophication or the overharvesting of vertebrate herbivores (sirenia, turtles) that once cropped macrophytes before they could be fouled by epiphytes. In aquatic systems, epiflora are usually more suited to direct consumption by herbivores than their vascular plant hosts, which tend to be tougher, and contain less nitrogen than epiflora but more indigestible fiber and secondary metabolites like phenolics.
It is difficult to compare epifaunal diversities among habitats because of variation among studies in the size range of organisms examined and in the number of hosts or area of seafloor sampled. However, some generalizations are possible. For instance, marine epifauna are far more taxonomically diverse than freshwater epifauna, and complex structures host more species than simple ones. In the oceans, species of macrophytes and sessile animals host up to several hundred epifaunal species.
Small organisms are more metabolically active than larger ones (they have higher mass-specific metabolic rates and shorter generation times), and will likely play increasingly greater roles in aquatic ecosystems as larger organisms are progressively removed by fishing. On New Zealand rocky reefs, small (0.5-10 mm) mobile epifauna contribute about 80% of total secondary (animal) production, and probably make similar contributions to total food consumption and nutrient regeneration by animals.
Small mobile epifauna are important trophic links between fishes and primary producers, while suspensionfeeding epifauna link pelagic and benthic ecosystems by accelerating the flow of organic matter from the water column to the seafloor.
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