Pollination syndromes are suites of
flowerA flower, sometimes known as a bloom or blossom, is the reproductive structure found in flowering plants . The biological function of a flower is to mediate the union of male sperm with female ovum in order to produce seeds...
traits that have evolved in response to
natural selectionNatural selection is the process by which heritable traits that make it more likely for an organism to survive and successfully reproduce become more common in a population over successive generations...
imposed by different pollen vectors, which can be abiotic (wind and water) or biotic, such as birds, bees, flies, and so forth. These traits include flower shape, size, colour, odour, reward type and amount, nectar composition, timing of flowering, etc. For example, tubular red flowers with copious nectar often attract birds; foul smelling flowers attract carrion flies or beetles, etc. Pollination syndromes are excellent examples of
convergent evolutionConvergent evolution describes the acquisition of the same biological trait in unrelated lineages.The wing is a classic example of convergent evolution in action. Although their last common ancestor did not have wings, birds and bats do, and are capable of powered flight. The wings are similar in...
.
The "classical" pollination syndromes as they are currently defined (see below) were developed in the 19th Century by the Italian botanist
Federico DelpinoFrederico Delpino was an Italian botanist who made very important observations on the pollination of flowers by insects...
. Although they have been useful in developing our understanding of plant-pollinator interactions, an uncritical acceptance of pollination syndromes as providing a framework for classifying these relationships is rather out of date.
Abiotic pollination syndromes
These do not attract animal pollinators. Nevertheless, they often have suites of shared traits.
Wind pollination (anemophilyAnemophily or wind pollination is a form of pollination whereby pollen is distributed by wind. Anemophilous plants may be either gymnosperms or angiosperms ....
)
Flowers may be small and inconspicuous, green and not showy. They produce enormous numbers of relatively small
pollenPollen is a fine to coarse powder containing the microgametophytes of seed plants, which produce the male gametes . Pollen grains have a hard coat that protects the sperm cells during the process of their movement between the stamens to the pistil of flowering plants or from the male cone to the...
grains (hence wind-pollinated plants may be
allergenAn allergen is a nonparasitic antigen capable of stimulating a type-I hypersensitivity reaction in atopic individuals.Most humans mount significant Immunoglobulin E responses only as a defense against parasitic infections. However, some individuals mount an IgE response against common...
s, but seldom are animal-pollinated plants allergenic). Their stigmas may be large and feathery to catch the pollen grains. Insects may visit them to collect pollen; in some cases these are ineffective pollinators and exert little
natural selectionNatural selection is the process by which heritable traits that make it more likely for an organism to survive and successfully reproduce become more common in a population over successive generations...
on the flowers, but there are also examples of ambophilous flowers which are both wind and insect pollinated.
Water pollination (hydrophilyHydrophily is a fairly uncommon form of pollination whereby pollen is distributed by the flow of waters, particularly in rivers and streams.Hydrophilous species fall into two categories: those that distribute their pollen to the surface of water, and those that distribute it beneath the surface...
)
Water-pollinated plants are
aquaticAquatic plants — also called hydrophytic plants or hydrophytes — are plants that have adapted to living in or on aquatic environments. Because living on or under water surface requires numerous special adaptations, aquatic plants can only grow in water or permanently saturated soil...
and pollen is released into the water. Water currents therefore act as a pollen vector in a similar way to wind currents. Their flowers tend to be small and inconspicuous with lots of pollen grains and large, feathery stigmas to catch the pollen. However, this is relatively uncommon and most aquatic plants are insect-pollinated, with flowers that emerge into the air.
Biotic pollination syndromes
Bee pollination (melittophily)
BeeBees are flying insects closely related to wasps and ants, and are known for their role in pollination and for producing honey and beeswax. Bees are a monophyletic lineage within the superfamily Apoidea, presently classified by the unranked taxon name Anthophila...
-pollinated flowers can be very variable in their size, shape and colouration. They can be open and bowl-shaped (radially symmetrical) or more complex and non-radially symmetric ("zygomorphic") such as many
peaA pea is most commonly the small spherical seed or the seed-pod of the legume Pisum sativum. Each pod contains several peas. Although it is botanically a fruit, it is treated as a vegetable in cooking...
s, foxgloves, etc.
Some bee flowers tend to be yellow or blue, often with ultraviolet
nectar guideNectar guides are patterns seen in some flowers that guide pollinators to the nectar and pollen.These patterns are sometimes visible to humans; for instance, the Dalmatian toadflax has yellow flowers with orange nectar guides. However, in some plants, such as sunflowers, they are visible only...
s and scent. Nectar, pollen, or both are offered as rewards in varying amounts. The sugar in the nectar tends to be
sucroseSucrose, commonly called table sugar, is a moosaccharide of glucose and fructose with the molecular formula C12H22O11. This white, odorless, crystalline powder has a pleasing, sweet taste. It is best known for its role in human nutrition...
-dominated.
There are diverse types of bees, however. Honeybees,
bumblebeeA bumblebee is any member of the bee genus Bombus, in the family Apidae. There are over 250 known species, existing primarily in the Northern Hemisphere....
s, orchid bees, etc are large groups that are quite distinctive in size,
tongueThe tongue is a muscle on the floor of the mouth that manipulates food for chewing and swallowing . It is the primary organ of taste, as much of the upper surface of the tongue is covered in papillae and taste buds. A secondary function of the tongue is speech, in which the organ assists...
length and behaviour (some solitary, some colonial). Thus generalization about bees is difficult. Some plants can only be pollinated by bees because their anthers release pollen internally, and it must be shaken out by
buzz pollinationSonication or buzz pollination is a technique used by some bees to release pollen which is more or less firmly held by the anthers, which makes pollination more efficient. The anther of buzz pollinated species of plants is typically tubular, with only an opening at one end, and the pollen is inside...
. Bees are the only animals that perform this behaviour.
Bee pollination from mobile beehives is of great economic value for orchards such as
appleThe apple is the pomaceous fruit of the apple tree, species Malus domestica in the rose family Rosaceae. It is one of the most widely cultivated tree fruits...
or
almondThe Almond is a species of tree native to the Middle East. Almond is also the name of the edible and widely cultivated nut of this tree...
.
Butterfly pollination (psychophily)
ButterflyA butterfly is an insect of the order Lepidoptera. Like all Lepidoptera, butterflies are notable for their unusual life cycle with a larval caterpillar stage, an inactive pupal stage, and a spectacular metamorphosis into a familiar and colourful winged adult form. Most species are day-flying so...
-pollinated flowers tend to be large and showy, pink or lavender in colour, frequently have a landing area, and are usually scented. Since butterflies do not
digestDigestion is the mechanical and chemical breaking down of food into smaller components, to a form that can be absorbed, for instance, by a blood stream. Digestion is a form of catabolism.that means break down of macro food molecules to smaller one....
pollen (with one exception), more nectar is offered than pollen. The flowers have simple nectar guides with the nectaries usually hidden in narrow tubes or spurs, reached by the long tongue of the butterflies.
Moth pollination (phalaenophily)
Among the more important
mothA moth is an insect closely related to the butterfly, both being of the order Lepidoptera. The differences between butterflies and moths are more than just taxonomy. Sometimes the names "Rhopalocera" and "Heterocera" are used to formalize the popular distinction...
pollinators are the hawk moths (
SphingidaeSphingidae is a family of moths , commonly known as hawk moths, sphinx moths and hornworms, that includes about 1,200 species . It is best represented in the tropics but there are species in every region . They are moderate to large in size and are distinguished among moths for their rapid,...
). Their behaviour is similar to
hummingbirdHummingbirds are among the smallest of birds, and include the smallest extant bird species, the Bee Hummingbirds. They can hover in mid-air by rapidly flapping their wings 12-90 times per second . They can also fly backwards, and are the only group of birds able to do so. Their English name derives...
s: they hover in front of flowers with rapid wingbeats. Most are nocturnal or
crepuscularCrepuscular is a term used to describe some animals that are primarily active during twilight, that is at dawn and at dusk. The word is derived from the Latin word crepusculum, meaning "twilight." Crepuscular is thus in contrast with diurnal and nocturnal behavior. Crepuscular animals may also be...
. So moth-pollinated flowers tend to be white, night-opening, large and showy with tubular
corollaA petal is one member or part of the corolla of a flower. The corolla is the name for all of the petals of a flower; the inner perianth whorl, term used when this is not the same in appearance as the outermost whorl and is used to attract pollinators based on its bright color...
s and a strong, sweet scent produced in the evening, night or early morning. A lot of nectar is produced to fuel the high metabolic rates needed to power their flight.
Other moths (Noctuids, Geometrids, Pyralids, for example) fly slowly and settle on the flower. They do not require as much nectar as the fast-flying hawk moths, and the flowers tend to be small (though they may be aggregated in heads).
Fly pollination (myophily and sapromyophily)
The classical syndromes recognise two types of fly pollination: myophily and sapromyophily. Some flies feed on nectar and pollen as adults (particularly bee flies (Bombyliidae),
hoverfliesFlies in the family Syrphidae are commonly known as hoverflies, flower flies, or syrphid flies.As their common names suggest, they are often seen hovering or nectaring at flowers; the adults of many species feed mainly on nectar and pollen, while the larvae eat a wide range of foods...
(Syrphidae), etc.). Those regularly visit flowers. On the other hand, male
fruit fliesTephritidae is one of two fly families referred to as "fruit flies". Tephritidae does not include the biological model organisms of the genus Drosophila, which is often called the "common fruit fly". Drosophila is, instead, the type genus of the second "fruit fly" family, Drosophilidae...
(Tephritidae) are attracted to and feed on specific floral attractant, which acts as fly's sex pheromone precursor or booster, of some wild orchids (Bulbophyllum species - with highly moveable lip) that do not produce nectar. These are the myophiles. Sapromyophiles, on the other hand, normally visit dead animals or
dungDung may refer to:* Dung, animal feces* Dung, Doubs, a commune in the Doubs department in France* Mundungus "Dung" Fletcher, a character in Harry Potter* Dung beetle...
. They are attracted to flowers that mimic these odoriferous items. They obtain no reward and would quickly leave, but the plant may have traps to slow them down. These plants have a strong, unpleasant odor, and are brown or orange in color. They are not as common as myophilous plants. Myophilous plants do not tend to have a strong scent, and tend to be purple, violet, blue, and white, open dishes, or tubes. Although they have traditionally been considered to be ineffective and unreliable pollinators, their sheer numbers and the presence of some flies throughout the year make them important pollinators for many plants.
Flies tend to be important pollinators in high-
altitudeAltitude is defined based on the context in which it is used . As a general definition, altitude is a distance measurement, usually in the vertical or "up" direction, between a reference datum and a point or object...
and high-
latitudeLatitude, usually denoted by the Greek letter phi gives the location of a place on Earth north or south of the equator. Lines of Latitude are the imaginary horizontal lines shown running east-to-west on maps that run either north or south of the equator...
systems, where they are numerous and other insect groups may be lacking.
Bird pollination (ornithophilyOrnithophily is the pollination of flowering plants by birds. This coevolutionary association is derived from insect pollination and is particularly well developed in some parts of the world, especially in the tropics and on some island chains. The association involves several distinctive plant...
)
Although
hummingbirdHummingbirds are among the smallest of birds, and include the smallest extant bird species, the Bee Hummingbirds. They can hover in mid-air by rapidly flapping their wings 12-90 times per second . They can also fly backwards, and are the only group of birds able to do so. Their English name derives...
s are the most familiar nectar-feeding
birdBirds are winged, bipedal, endothermic , vertebrate animals that lay eggs. There are around 10,000 living species, making them the most numerous tetrapod vertebrates. They inhabit ecosystems across the globe, from the Arctic to the Antarctic. Birds range in size from the Bee Hummingbird to the ...
s for
North AmericaNorth America is the northern continent of the Americas, situated in the Earth's northern hemisphere and in the western hemisphere. It is bordered on the north by the Arctic Ocean, on the east by the North Atlantic Ocean, on the southeast by the Caribbean Sea, and on the west by the North Pacific...
ns, there are analogous species in other parts of the world:
sunbirdThe sunbirds and spiderhunters are a family, Nectariniidae, of very small passerine birds. There are 132 species in 15 genera. The family is distributed throughout Africa, southern Asia and just reaches northern Australia. Most sunbirds feed largely on nectar, although they will also take insects,...
s,
honeyeaterThe honeyeaters are a large and diverse family of small to medium sized birds most common in Australia and New Guinea, but also found in New Zealand, the Pacific islands as far east as Samoa and Tonga, and the islands to the north and west of New Guinea known as Wallacea...
s,
flowerpeckerThe flowerpeckers are a family, Dicaeidae , of passerine birds. The family comprises two genera, Prionochilus and Dicaeum, with 44 species in total. The family has sometimes been included in an enlarged sunbird family Nectariniidae. The berrypeckers of the family Melanocharitidae and the painted...
s,
honeycreeperThe typical honeycreepers are small birds in the tanager family. They are found in the tropical New World from Mexico south to Brazil.They occur in the forest canopy, and, as the name implies, they are specialist nectar feeders with long curved bills....
s,
bananaquitThe Bananaquit, Coereba flaveola, is a passerine bird first described by Linnaeus in his Systema naturae in 1758 as Certhia flaveola.It was reclassified as the only member of the genus Coereba by Louis Jean Pierre Vieillot in 1809....
s,
flowerpiercerDiglossa is a genus of flowerpiercers in the family Thraupidae. Sometimes classified in the bunting and American sparrow family Emberizidae, more recent studies have shown it to belong in the Thraupidae...
s, lories and lorikeets. Hummingbirds are the oldest group, with the greatest degree of specialization on nectar. Flowers attractive to hummingbirds that can hover in front of the flower tend to be large red or orange tubes with a lot of dilute nectar, secreted during the day. Since birds do not have a strong response to scent, they tend to be odorless. Perching birds need a substantial landing platform, so sunbirds, honeyeaters, and the like are less associated with tubular flowers.
Bat pollination (chiropterophily)
BatBats are flying mammals in the order Chiroptera . The forelimbs of bats are webbed and developed as wings, making them the only mammals naturally capable of true and sustained flight. By contrast, other mammals said to fly, such as flying squirrels, gliding possums and colugos, glide rather than...
-pollinated flowers tend to be large and showy, white or light coloured, open at night and have strong odours. They are often large and
bellA bell is a simple sound-making device. The bell is a percussion instrument and an idiophone. Its form is usually a hollow, cup-shaped object, which resonates upon being struck...
-shaped. Bats drink the nectar, and these plants typically offer nectar for extended periods of time. Sight, smell, and
echo-locationEcholocation, also called biosonar, is the biological sonar used by several animals such as dolphins, shrews, most bats, and most whales. The term was coined by Donald Griffin, who was the first to conclusively demonstrate its existence in bats...
are used to initially find the flowers, and excellent spatial memory is used to visit them repeatedly. In fact, bats can identify nectar-producing flowers using echolocation, a talent that was only recently discovered. In the
New WorldThe New World is one of the names used for the non-Afro-Eurasian parts of the Earth, specifically the Americas and possibly Australia. When the term originated in the late 15th century, the Americas were new to the Europeans, who previously thought of the world as consisting only of Europe, Asia,...
, bat pollinated flowers often have
sulfurSulfur or sulphur is the chemical element that has the atomic number 16. It is denoted with the symbol S. It is an abundant, multivalent non-metal. Sulfur, in its native form, is a yellow crystalline solid. In nature, it can be found as the pure element and as sulfide and sulfate minerals...
-scented compounds, but this does not carry to other parts of the world. Bat-pollinated plants have bigger pollen than their relatives.
Beetle pollination (cantharophily)
BeetleBeetles are the group of insects with the largest number of known species. They are classified in the order Coleoptera , which contains more described species than in any other order in the animal kingdom, constituting about 25% of all known life-forms...
-pollinated flowers are usually large, greenish or off-white in color and heavily scented. Scents may be spicy, fruity, or similar to decaying organic material. Most beetle-pollinated flowers are flattened or dish shaped, with pollen easily accessible, although they may include traps to keep the beetle longer. The plant's ovaries are usually well protected from the biting mouthparts of their pollinators. Beetles may be particularly important in some parts of the world such as semi-arid areas of
southern AfricaSouthern Africa is the southernmost region of the African continent, variably defined by geography or geopolitics. Within the region are numerous territories, including the Republic of South Africa ; nowadays, the simpler term South Africa is generally reserved for the country in English.-UN...
and
southern CaliforniaSouthern California, or SoCal, is defined as the southern portion of the U.S. state of California. Its population centers around three major metropolitan areas, each of which have over 3 million people; the Los Angeles Metropolitan Area with over 12 million inhabitants, the San Bernardino-Riverside...
and the montane grasslands of
KwaZulu-NatalKwaZulu-Natal is a province of South Africa. Prior to 1994 the territory now known as KwaZulu-Natal was made up of the province of Natal and all pieces of territory that made up the homeland of KwaZulu....
in
South AfricaThe Republic of South Africa is a country located at the southern tip of Africa, with a coastline on the Atlantic and Indian Oceans. To the north lie Namibia, Botswana and Zimbabwe, to the east are Mozambique and Swaziland, while Lesotho is an independent country surrounded by South Africa.Modern...
.
Biology
Pollination syndromes reflect
convergent evolutionConvergent evolution describes the acquisition of the same biological trait in unrelated lineages.The wing is a classic example of convergent evolution in action. Although their last common ancestor did not have wings, birds and bats do, and are capable of powered flight. The wings are similar in...
towards forms (
phenotypeA phenotype is any observable characteristic or trait of an organism: such as its morphology, development, biochemical or physiological properties, or behavior. Phenotypes result from the expression of an organism's genes as well as the influence of environmental factors and possible interactions...
s) that limit the number of species of
pollinatorA pollinator is the biotic agent 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...
s visiting the plant. They increase the functional specialization of the plant with regard to pollination, though this may not affect the ecological specialization (i.e. the number of species of pollinators within that functional group). They are responses to common selection pressures exerted by shared pollinators or abiotic pollen vectors, which generate correlations among traits. That is, if two distantly related plant species are both pollinated by nocturnal moths, for example, their flowers will converge on a form which is recognised by the moths (e.g. pale colour, sweet scent, nectar released at the base of a long tube, night-flowering).
Advantages of specialization
- Efficiency of pollination: the rewards given to pollinators (commonly nectar or pollen or both, but sometimes oil, scents, resins, or wax) may be costly to produce. Nectar can be cheap, but pollen is generally expensive as it is relatively high in nitrogen compounds. Plants have evolved to obtain the maximum pollen transfer for the minimum reward. Different pollinators, because of their size, shape, or behaviour, have different efficiencies of transfer of pollen. And the floral traits affect efficiency of transfer: columbine
Aquilegia is a genus of about 60-70 species of columbines, herbaceous perennial plants that are found in meadows, woodlands, and at higher altitudes throughout the Northern Hemisphere. They are known for their distinctive flowers, generally bell-shaped, with each petal modified into an elongated...
flowers were experimentally altered and presented to hawkmothSphingidae is a family of moths , commonly known as hawk moths, sphinx moths and hornworms, that includes about 1,200 species . It is best represented in the tropics but there are species in every region . They are moderate to large in size and are distinguished among moths for their rapid,...
s, and flower orientation, shape, and colour were found to affect visitation rates or pollen removal.
- Pollinator constancy: to efficiently transfer pollen, it is best for the plant if the pollinator focuses on one species of plant, ignoring other species. Otherwise, pollen may be dropped uselessly on the stigmas of other species. Animals, of course, do not aim to pollinate, they aim to collect food as fast as they can. However, many pollinator species exhibit constancy, passing up available flowers to focus on one plant species. Why should animals specialize on a plant species, rather than move to the next flower of any species? Although pollinator constancy was recognized by Aristotle
Aristotle was a Greek philosopher, a student of Plato and teacher of Alexander the Great. He wrote on many subjects, including physics, metaphysics, poetry, theater, music, logic, rhetoric, politics, government, ethics, biology, and zoology.Together with Plato and Socrates , Aristotle is one of...
, the benefits to animals are not yet fully understood. The most common hypothesis is that pollinators must learn to handle particular types of flowers, and they have limited capacity to learn different types. They can only efficiently gather rewards from one type of flower.
Pollinator constancy: these honeybees selectively visit flowers from only one species, as can be seen by the colour of the pollen in their baskets:
Advantages of generalization
Pollinators fluctuate in abundance and activity independently of their plants, and any one species may fail to pollinate a plant in a particular year. Thus a plant may be at an advantage if it attracts several species or types of pollinators, ensuring pollen transfer every year. Plants do, in many species, have the back-up option of self-pollination, if they are not self-incompatible.
Criticisms of the syndromes
Whilst it is clear that pollination syndromes can be observed in nature, there has been much debate amongst scientists as to how frequent they are and to what extent we can use the classical syndromes to classify plant-pollinator interactions. Although some species of plants are visited only by one type of animal (i.e. they are functionally specialized), many plant species are visited by very different
pollinatorA pollinator is the biotic agent 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...
s. For example, a flower may be pollinated by bees, butterflies, and birds. Strict specialization of plants relying on one species of pollinator is relatively rare, probably because it can result in variable reproductive success across years as pollinator populations vary significantly. In such cases, plants should generalize on a wide range of pollinators, and such ecological generalization is frequently found in nature. A study in Tasmania found the syndromes did not usefully predict the pollinators.
This debate has led to a critical re-evaluation of the syndromes, which suggests that on average about one third of the flowering plants can be classified into the classical syndromes. This reflects the fact that nature is much less predictable and straightforward than 19th Century biologists originally thought. Pollination syndromes can be thought of as extremes of a continuum of greater or lesser specialization or generalization onto particular functional groups of pollinators that exert similar selective pressures” and the frequency with which flowers conform to the expectations of the pollination syndromes is relatively rare. In addition, new types of plant-pollinator interaction, involving "unusual" pollinating animals are regularly being discovered, such as specialized pollination by spider hunting wasps (Pompilidae) and fruit chafers (Cetoniidae) in the eastern grasslands of South Africa. These plants do not fit into the classical syndromes, though they may show evidence of convergent evolution in their own right.
An analysis of flower traits and visitation in 49 species in the plant genus
PenstemonPenstemon , Beard-tongue, is a large genus of North American and East Asian plants traditionally placed in the Scrophulariaceae family. Due to new genetic research, it has now been placed in the vastly expanded family Plantaginaceae....
found that it was possible to separate bird- and bee- pollinated species quite well, but only by using floral traits which were not considered in the classical accounts of syndromes, such as the details of anther opening. Although a recent review concluded that there is “overwhelming evidence that functional groups exert different selection pressures on floral traits”, the sheer complexity and subtlety of plant-pollinator interactions (and the growing recognition that non-pollinating organisms such as seed predators can affect the evolution of flower traits) means that this debate is likely to continue for some time.
See also
- Pollinator decline
The term pollinator decline refers to the reduction in abundance of pollinators in many ecosystems worldwide during the end of the twentieth century....
- Pollinator
A pollinator is the biotic agent 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...
- Entomophily
Entomophily is a form of pollination whereby pollen is distributed by insects, particularly bees, Lepidoptera , flies and beetles. Entomophilous species frequently evolve mechanisms to make themselves more appealing to insects, e.g., brightly-colored or scented flowers, nectar, or appealing shapes...
- Zoophily
Zoophily is a form of pollination whereby pollen is transferred by vertebrates, particularly by hummingbirds and other birds, and bats, but also by monkeys, marsupials, lemurs, bears, rabbits, deer, rodents, lizards and other animals...
- Mutualism
Mutualism is a biological interaction between two organisms, where each individual derives a fitness benefit . Similar interactions within a species are known as co-operation...