Mistletoe: a unique constituent of canopies worldwide.

Mistletoes are a polyphyletic group of approximately 1,400 species of obligate hemiparasites, distributed worldwide. While they obtain all of their minerals and water from their host (typically a tree or shrub), they are green plants that generally manufacture their own carbohydrates by photosynthesis. They maintain low water potentials relative to their hosts by concentrating soluble cations and organic compounds in their tissues, and typically have higher rates of transpiration than their hosts. They are pollinated primarily by birds, and birds are also the main vector for fruit dispersal. In addition to these mutualistic interactions, mistletoe is a popular food source for a wide variety of animals, often providing nutritional resources when little else is available. They are also highly favored roost and nest sites for a wide range of animals and are considered one of the main causes of hollow formation in many regions. Mistletoe can therefore be considered a keystone resource, having a disproportionate influence on the ecosystem as a whole, affecting distribution patterns of organisms both directly and indirectly. While some mistletoe species can have deleterious effects on their host, most mistletoes are best regarded as water-parasites, having a negligible effect on host survivorship. Given the range of interactions mistletoes have with pollinators, dispersers, herbivores, and hosts, they are a sensitive assay of environmental integrity and have been used as indicators of overall habitat health.