A pollinator is the biotic agent (vector) that moves pollen from the male anthers of a flower to the female stigma of a flower to accomplish fertilization or syngamy of the female gamete in the ovule of the flower by the male gamete from the pollen grain. Though the terms are sometimes confused, a pollinator is different from a pollenizer, which is a plant that is a source of pollen for the pollination process.
Plants fall into pollination syndromes that reflect the type of pollinator being attracted. These are characteristics such as: overall flower size, the depth and width of the corolla, the color (including patterns called nectar guides that are visible only in ultraviolet light), the scent, amount of nectar, composition of nectar, etc. For example, birds visit red flowers with long narrow tubes and lots of nectar, but are not as strongly attracted to wide flowers with little nectar and copious pollen, which are more attractive to beetles. When these characteristics are experimentally modified (altering colour, size, orientation), pollinator visitation may decline.
Types of pollinators 
The most recognized pollinators are the various species of bees, which are plainly adapted to pollination. Bees typically are fuzzy and carry an electrostatic charge. Both features help pollen grains adhere to their bodies, but they also have specialized pollen-carrying structures; in most bees, this takes the form of a structure known as the scopa, which is on the hind legs of most bees, and/or the lower abdomen (e.g., of megachilid bees), made up of thick, plumose setae. Honey bees, bumblebees, and their relatives do not have a scopa, but the hind leg is modified into a structure called the corbicula (also known as the "pollen basket"). Most bees gather nectar, a concentrated energy source, and pollen, which is high protein food, to nurture their young, and inadvertently transfer some among the flowers as they are working. Euglossine bees pollinate orchids, but these are male bees collecting floral scents rather than females gathering nectar or pollen. Female orchid bees act as pollinators, but of flowers other than orchids. Eusocial bees such as honey bees need an abundant and steady source of pollen to multiply.
Honey bees 
Honey bees travel from flower to flower, collecting nectar (later converted to honey), and pollen grains. The bee collects the pollen by rubbing against the anthers. The pollen collects on the hind legs, in a structure referred to as a "pollen basket". As the bee flies from flower to flower, some of the pollen grains are transferred onto the stigma of other flowers.
Nectar provides the energy for bee nutrition; pollen provides the protein. When bees are rearing large quantities of brood (beekeepers say hives are "building"), bees deliberately gather pollen to meet the nutritional needs of the brood. A honey bee that is deliberately gathering pollen is up to ten times more efficient as a pollinator than one that is primarily gathering nectar and only unintentionally transferring pollen.
Good pollination management seeks to have bees in a "building" state during the bloom period of the crop, thus requiring them to gather pollen, and making them more efficient pollinators. Thus the management techniques of a beekeeper providing pollination services are different from, and to some extent in tension with, those of a beekeeper who is trying to produce honey.
Millions of hives of honey bees are contracted out as pollinators by beekeepers, and honey bees are by far the most important commercial pollinating agents, but many other kinds of pollinators, from bluebottle flies, to bumblebees, orchard mason bees, and leaf cutter bees are cultured and sold for managed pollination.
Other species of bees differ in various details of their behavior and pollen-gathering habits, and it should be remembered that honey bees are not native to the Western Hemisphere; all pollination of native plants in the Americas has been historically performed by various native bees.
Other insects 
Many insects other than bees accomplish pollination by visiting flowers for nectar or pollen, or commonly both. Many do so adventitiously, but the most important pollinators are specialists for at least parts of their life cycles for at least certain functions. For example, males of many species of Hymenoptera, including many hunting wasps, rely on freely flowering plants as sources of energy (in the form of nectar) and also as territories for meeting fertile females that visit the flowers. Prominent examples are predatory wasps (especially Sphecidae, Vespidae, and Pompilidae). The term "pollen wasps" in particular, is widely applied to the Masarinae, a subfamily of the Vespidae; they are remarkable among solitary wasps in that they specialise in gathering pollen for feeding their larvae, carried internally and regurgitated into a mud chamber prior to oviposition.
Syrphid flies, such as hoverflies and drone flies, are important non-selective pollinators.
Many bee flies, and some Tabanidae and Nemestrinidae are particularly adapted to pollinating fynbos and Karoo plants with narrow, deep corolla tubes, such as Lapeirousia species. Part of the adaptation takes the form of remarkably long proboscises.
Lepidoptera (butterflies and moths) also pollinate plants to various degrees. They are not major pollinators of food crops, but various moths are important pollinators of other commercial crops such as tobacco. Pollination by certain moths may be important however, even crucial, for some wildflowers mutually adapted to specialist pollinators. Spectacular examples include orchids such as Angraecum sesquipedale, dependant on a particular hawk moth, Morgan's Sphinx. Yucca species provide other examples, being fertilised in elaborate ecological interactions with particular species of yucca moths.
Beetles of species that specialise in eating pollen, nectar, or flowers themselves, are important cross-pollinators of some plants such as members of the Araceae and Zamiaceae, that produce prodigious amounts of pollen. Others, for example the Hopliini, specialise in free-flowering species of the Asteraceae and Aizoaceae.
Various midges, and thrips are comparatively minor opportunist pollinators. Ants also pollinate some kinds of flowers, but for the most part they are parasites, robbing nectar without conveying useful amounts of pollen to a stigma. Whole groups of plants, such as certain fynbos Moraeas and Ericas produce flowers on sticky peduncles or with sticky corolla tubes that only permit access to flying pollinators, whether bird, bat or insect.
Carrion flies and flesh flies in families such as Calliphoridae and Sarcophagidae are important for some species of plants whose flowers exude a fetid odor. The plants' ecological strategy varies; several species of Stapelia for example, attract carrion flies that futilely lay their eggs on the flower, where their larvae promptly starve for lack of carrion. Other species do decay rapidly after ripening, and offer the visiting insects large masses of food as well as pollen and sometimes seed to carry off when they leave.
A class of strategy of great biological interest is that of sexual deception, where plants, generally orchids, produce remarkably complex combinations of pheromonal attractants and physical mimicry that induce male bees or wasps to attempt to mate with them, conveying pollinia in the process. Examples are known from all continents apart from Antarctica, though Australia appears to be exceptionally rich in examples.
Bats are important pollinators of some tropical flowers. Birds, particularly hummingbirds, honeyeaters and sunbirds also accomplish much pollination, especially of deep-throated flowers. Other vertebrates, such as monkeys, lemurs, possums, rodents and lizards have been recorded pollinating some plants.
Humans can be pollinators, as many gardeners have discovered that they must hand pollinate garden vegetables, whether because of pollinator decline (as has been occurring in parts of the U.S. since the mid-20th century) or simply to keep a strain genetically pure. This can involve using a small brush or cotton swab to move pollen, or to simply tap or shake tomato blossoms to release the pollen for the self pollinating flowers. Tomato blossoms are self-fertile, but (with the exception of potato-leaf varieties) have the pollen inside the anther, and the flower requires shaking to release the pollen through pores. This can be done by wind, by humans, or by a sonicating bee (one that vibrates its wing muscles while perched on the flower), such as a bumblebee. Sonicating bees are extremely efficient pollinators of tomatoes, and colonies of bumblebees are quickly replacing humans as the primary pollinators for greenhouse tomatoes.
Pollinator population declines and conservation 
In 1999 the Convention on Biological Diversity issued the São Paulo Declaration on Pollinators, recognizing the critical role that these species play in supporting and maintaining terrestrial productivity as well as the survival challenges they face due to anthropogenic change. Today pollinators are considered to be in a state of decline; some species, such as Franklin’s bumble bee (Bombus franklini) have been red-listed and are in danger of extinction.
Declines in the health and population of pollinators pose what could be a significant threat to the integrity of biodiversity, to global food webs, and to human health. At least 80% of our world's crop species require pollination to set seed. An estimated one out of every three bites of food comes to us through the work of animal pollinators.
See also 
- List of crop plants pollinated by bees
- Pollination syndromes
- Pollinator decline
- Pollinator Partnership
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- New "Pollinator Garden Wheel" from the National Academies provides information on pollination and tips on building a pollinator-friendly garden.
- Pollinator & Visitor Image Database
- Pollinator Partnership
- Nature's Pollinators an article written by Elizah Leigh
- News on Insect Pollinators Initiative