What is pollination?

Close-up of Poppy flower stamens and stigma

Pollination is the fertilisation process that is required by plants to reproduce. Only when a plant has been pollinated can it produce seeds and fruit.

Pollination occurs when pollen, a plant’s male gamete, is transferred from the male part of the flower (anther) to the female parts (stigma) of another plant of the same species. The pollen begins to grow and forms a pollen tube on the surface of the stigma, which navigates its way through the tissue of the flower until it reaches an ovule inside the ovary. The nucleus of the pollen grain then passes through the pollen tube and fuses with the nucleus of the ovule. After this process has occurred the female parts of the plant are fertilised and the ovules develop into seeds. The seeds contain all of the genetic information that is required to create a new plant.

A small amount of plants are self-pollinating and therefore do not require any assistance from external sources, although most are cross-pollinators and rely on other organisms or natural processes to reproduce. The majority of plants are wholly reliant on the mutualistic symbiotic relationship they have with pollinators to be able to reproduce, and its pollinators are reliant on the plants as a food source. A much smaller percentage of plants rely on the Earth’s physical processes to transport their pollen and have evolved many weird and wonderful ways to ensure that their pollen reaches the right destination.

Types of pollination

All plant species have a main method of reproduction, relying on a specific evolutionary adaptation that helps pollination. Despite this, a large number of plants will also have the capacity to reproduce using another pollination method, using it as an ‘insurance policy’.

Cross-pollination

Cross-pollinating plants are self-incompatible, so they need to spread their pollen to a different plant of the same species to reproduce. There are three external factors which allow plants to spread their own pollen away from themselves – pollinators, wind and water.

Pollinator

Bee

Pollinators are responsible for reproduction in the majority of plant species, both naturally occurring and cultivated, and play a critical role in maintaining plant communities around the world. Plants produce nectar to attract and feed pollinators and this generosity is returned by the visitor spreading the pollen that it has unknowingly acquired while in contact with the plant. This mutualistic symbiotic relationship is the reproduction strategy for over 80 percent of the world’s plant species.

Pollinators come in all shapes and sizes, from insects so small that they are invisible to the naked eye to large primates and bats. Some pollinators are generalists and will visit numerous different plant species for nectar, whereas others are wholly reliant on a specific species and may have developed adaptations that allow them to feed on plants that others cannot, such as the thin, elongated bill of the sunbirds which enables them to feed from the long, tubular flowers of aloes.

The pollen of these plants is usually sticky and has barbs attached to its surface to allow easy attachment to the body of a visiting organism to ensure that it will remain there until it reaches the next plant. Many pollinators have morphological adaptations which make them better at collecting pollen from plants, such as the fine hair around the mouths of nectar-eating bats.

Protein-rich pollen and energy-rich nectar are an attractive prospect for most pollinators, although some plants use flowers to entice more passers-by, producing elaborate inflorescences of all colours and shapes and countless different scents. Some plant species even use mimicry to attract attention from pollinators.

Bees are the best-known and most significant pollinators in the world and are responsible for the majority of pollination in both natural and cultivated plant communities. Perfectly designed for pollination, bees have an electrostatic body which attracts pollen grains, stiff hairs on their legs which allow them to groom the pollen into pockets on their body to be carried back to their nest and a lifestyle that is seemingly solely dedicated to collecting pollen. Bee species usually focus on one flower type, improving the chance that the pollen from one plant will be transferred to another of the same species. Despite bees being the world’s number one pollinator, birds, bats, primates and other insects play very important roles in generalist and specialist pollination, and even the wildly unpopular wasps and mosquitoes assist in the pollination of certain species.

Wind

Grass flowers with pollen blowing in the wind

Around 12 percent of the world’s flowering plants are wind-pollinated. Naturally occurring species such as conifers and grasses, as well as cultivated crop plants such as rice and wheat, are reliant on the wind’s currents to disperse and deliver their pollen. Most of these plants do not have the impressive flowers produced by insect-pollinated plants and usually have exposed stigmas and stamens to ensure that contact with floating pollen grains occurs as often as possible. The stamen of many of these species is arranged in a catkin, a long structure with many small, grouped inflorescences, which produce large amounts of pollen.

As wind-pollinated plants do not need to attract pollinators, they do not need to produce nectar or showy flowers and can invest more of their energy into producing pollen, which they produce in vast quantities. The pollen of these species is extremely small and lightweight so that can be easily dispersed within wind currents. These types of plants are responsible for producing most of the pollen that causes hay fever in humans.

Water

Eel Grass (Zostera) beds

A very small amount of plants are pollinated by water, a process known as ‘hydrophily’, and all are aquatic species such as seagrass and waterweeds. Their pollen is released directly into the water, and floats on the surface or within the water column until it reaches the stigma of another plant of the same species, starting the fertilisation process.

Self-pollination

Pendulous male and erect young female catkins of Silver birch tree (Betula pendula)

Self-pollinating plants can produce flowers containing both the male and female parts of the plant, allowing pollen to easily reach the ovary. This asexual pollination can be advantageous due to the plant not requiring any external factors to reproduce and having the ability inhabit areas where pollinators do not exist, such as at high elevations. As these plants do not need to attract pollinators, the inflorescences are very basic and do not have the colourful, elaborate flowers of pollinator-reliant species, or nectaries to feed visitors. Self-pollinating plants are mostly found in temperate regions, and include oaks and birches, as well as cultivated plants such as corn.

Self-pollination is relatively rare as asexual reproduction carries many disadvantages due the low genetic variation that it creates within a population. Offspring produced by the parent plant are practically identical and therefore are all equally as vulnerable to external stressors, meaning that a single event, such as adverse weather or disease, could destroy an entire population.

Why is pollination important?

Rainforest River or Stream aerial Amazon, Peru

Pollination is one of the world’s most important ecosystem services and is responsible for the construction and maintenance of natural plant communities as well as the production of the majority of the food eaten by humans. Plants are the basis of most of the world’s ecosystems and support the rest of the food web, from the herbivores that eat them to the top predator.

Despite all methods being important, animal-induced pollination is responsible for the majority of the world’s plants reproducing. Worldwide, around 1,000 plants are grown for human consumption, of which 75 percent are dependent on pollinators. The services provided by wild insect pollinators have been valued at around USD$200 billion annually. Without wild insect pollinators, there would be no passionfruit, cocoa to make chocolate, pumpkin, aubergine or cucumber and they also play an extremely important role in the lifecycle of mango, avocado, peaches, almonds, vanilla and coffee. As the world population grows and food security decreases, it is becoming more important to conserve our pollinators. Some areas now have so few pollinators that the farmers have to pollinate their crops themselves, which is an extremely long and laborious process. Natural habitats are also suffering, and the floral diversity of many areas has been affected by the decreasing pollinator populations.

Over time, many plant species have evolved to rely on a specific pollinator to reproduce and many pollinators have developed physical adaptations that are extremely advantageous for feeding on one type of plant, but not others. Having a dependent relationship means that both pollinator and plant rely on each other, therefore the decline of one population would mean the same for the other, and decline could eventually lead to extinction.

What threats do pollinators face?

Sumatran landscape showing habitat destruction

The activities of humans have reduced pollinator populations around the world, and the negative impacts can be seen in both natural habitats and agricultural land.

Around the world, widespread habitat destruction is reducing the amount of suitable habitat for most of the world’s species, and pollinators are no exception. Land conversion and deforestation have destroyed vast areas that were previously inhabited by bees and other pollinators, and many of the areas that remain have very limited resources and can only support very small populations, which will eventually disappear due to inbreeding depression or other genetic issues, or could be wiped out due to disease and poor weather. Habitat loss is believed to be the main factor for the worldwide pollinator population declines.

Pesticides used in the agricultural industry are extremely harmful to pollinators, especially bees, and the worldwide population has declined as more chemicals started being used on crop plants. Despite the chemicals being intended for pest insects which feed on crop plants, useful pollinating insects are also harmed or killed by their usage. It has been estimated that the United States alone has lost around 50 percent of its honeybee population over the past decade due to pesticide use. Pesticides make bees and other pollinating insects vulnerable to disease, although they often cause almost instant mortality to pollinators that come into contact with them.

Agricultural chemicals often leach into the waterways and natural habitats that surround farmland, poisoning wildlife and killing naturally occurring plants. The use of herbicides and fertilisers in agriculture decreases the diversity of floral communities in areas surrounding farmland, reducing food resources for pollinators, and fungicides are known to interfere with microbes within the guts of bees that metabolise pollen grains.

Farmland is usually managed as a monoculture, with the crops only flowering at certain times of year; therefore the crops can only provide food for pollinators occasionally. In comparison, natural habitats have a diverse assemblage of plant species which all flower at different times of the year, ensuring that pollinators always have food available. Due to large swathes of land being converted into monocultures, many pollinator populations have died due to a lack of food availability and nesting sites, which usually disappear as a consequence of land conversion.

Urban areas can be both positive and negative for pollinator populations. A natural landscape is always more beneficial for local pollinator populations, but the presence of gardens within urban areas can support large populations if they mirror the floral community which once inhabited the area, or are very similar. If an urban garden has native species and is managed correctly, it can provide a perfect habitat for pollinators. Unfortunately, the most popular plants in horticulture are those with large, impressive flowers which are often not native to the area, and this is extremely detrimental to the native plant population and has been responsible for the decline and even the extinction of many species around the world. These plants often become invasive, spreading to areas that they were not destined for and outcompeting native plants for the resources that they need to survive. In the United Kingdom, the Himalayan balsam became a very popular garden plant, but before long began to attract more pollinators than native plant species due to the large amount of nectar it produces and is now a widespread invasive plant species which is extremely hard to keep under control. Invasive plant species which produce vast amount of nectar can be positive for some generalist pollinators, but those who rely on a specific plant species are suffering as their once reliable food source is in decline. A non-native parasite, Varroa destructor, which was accidentally introduced in the United Kingdom, Canada and the United States via has caused the collapse of many honeybee populations. Attempts at introducing non-native pollinators into certain areas has also led to hybridisation with native species, resulting in the production of offspring which outcompete with the native species.

Climate change is impacting pollinator populations around the world, causing shifts in their distribution and altering the floral communities and environment within their native habitats. Some areas which were previously inhabited by pollinators are now unsuitable due to desiccation and altered weather patterns. Climate change is also causing extreme weather events which can wipe out pollinator populations and can also facilitate the spread of introduced pests and disease-causing pathogens.

What conservation measures can we take to help pollinators?

Close-up of insect nest box occupied by Red mason bees (Osmia bicornis). The bees can be seen at work with some holes already sealed with mud and others containing yellow pollen stores to feed hatching larva.

The main cause of pollinator decline is human activity, but our actions can help towards reversing the damage that has been done.

The agriculture industry is beginning to implement various methods to protect bees and other pollinators and re-establish the populations surrounding their land. Farmers are being encouraged to look at the way they use chemicals and adapt their behaviour to ensure that pollinators are not harmed, including not using pesticides when their crops are in flower and implementing Integrated Pest Management which allows biological pest control.

Urban gardeners are being encouraged by conservation organisations to plant ‘bee-friendly’ gardens which include native plant species, limited chemical use and provision of nesting areas for bees and other insects. Although urban areas will never be as good for pollinators as their original habitat, making them as natural as possible will provide them with suitable habitat and encourage them to disperse to other areas, spreading pollen on their journey. The general public have also been asked to join citizen science programmes which assist scientists in monitoring bee populations around the world by reporting sightings, helping them to determine where populations exist, what species there are and whether they are in need of help.

Many conservation organisations around the world are working to regenerate habitats and create pollinator-friendly, diverse areas such as hedgerows, wildflower meadows and forests, as well as protecting those that remain. Legally protected areas have been designated to conserve some of the remaining pollinator populations, as well as other species that share their habitat, although these areas are often still at risk from forestry activities and chemical leaching from nearby agricultural land. More research is needed into how areas can be properly protected and optimised to allow pollinators to thrive.

How you can help pollinators

  • Grow native plants in your garden and encourage people in your local area to do the same
  • Buy organic food which has been grown without agricultural chemical or, better still, grow your own!
  • Supply water in your garden for wildlife
  • Do not use chemicals in your garden
  • Create a nesting area for your local bees in your garden to encourage them to breed
  • Conserve hedgerows and encourage their growth
  • Join a citizen science scheme in your area and help a local conservation organisation monitor pollinator populations
  • Walk, cycle or use public transport to reduce pollution in your local area and decrease your carbon footprint

Glossary

Anther
Part of the stamen (the male reproductive organ of a flower) that produces pollen.
Asexual
A type of reproduction that does not involve the formation of sex cells (‘gametes’). In many species, asexual reproduction can occur by fission (or in plants ‘vegetative reproduction’); part of the organism breaks away and develops into a separate individual. Some animals, including vertebrates can develop from unfertilised eggs, this process, known as parthenogenesis gives rise to offspring that are genetically identical to the parent.
Asexual reproduction
Reproduction that does not involve the formation of sex cells (‘gametes’). In many species, asexual reproduction can occur by existing cells splitting into two, or part of the organism breaking away and developing into a separate individual. Some animals, including vertebrates, can also develop from unfertilised eggs; this process, known as parthenogenesis, gives rise to offspring that are genetically identical to the parent.
Catkin
A pendulous, hanging flower cluster, usually belonging to a wind-pollinated plant such as willow or hazel.
Fertilisation
The fusion of gametes (male and female reproductive cells) to produce an embryo, which grows into a new individual.
Gamete
A reproductive cell which carries the genetic information from an individual, and is capable of fusing with a gamete of the opposite sex to produce a fertilised egg. In animals, male gametes are called sperm and female gametes are called ova.
Hybridisation
Cross-breeding between two different species or subspecies.
Inbreeding depression
The reduction in viability, birth weight, and fertility that occurs in a population after one or more generations of inbreeding (interbreeding among close relatives).
Inflorescences
The reproductive shoots of a plant, which bear groups or clusters of flowers.
Mimicry
A phenomenon in which a species gains an advantage by closely resembling another species in appearance or behaviour.
Monoculture
The cultivation of a single plant species over a given area.
Mutualistic
A mutualism is an interaction between two different species that benefits both.
Nectaries
Nectar-secreting glands, typically located at the base of insect-pollinated flowers. They usually attract insects to flowers, but can also attract seed-dispersing insects.
Ovary
The female reproductive organ that produces ova, or eggs.
Ovule
A structure within the female reproductive organs of plants that contains eggs and when fertilized by pollen, develops into seeds.
Pollination
The transfer of pollen grains from the stamen (male part of a flower) to the stigma (female part of a flower) of a flowering plant. This usually leads to fertilisation, the development of seeds and, eventually, a new plant.
Pollinators
Animals that in the act of visiting a plant’s flowers transfer pollen grains from the stamen (male part of a flower) to the stigma (female part of a flower) of a flowering plant. This usually leads to fertilisation, the development of seeds and, eventually, a new plant.
Self-pollinating
A plant which can reproduce by transferring pollen from the stamen (male part) of a flower, to the stigma (female part) of the same flower, or a different flower on the same plant.
Stamens
The male reproductive organs of a flower. Each stamen is comprised of an anther (the pollen-producing organ) and a filament (stalk).
Stigma
The part of the female reproductive organ of a flower which receives the pollen, and on which the pollen germinates (starts growing).
Symbiotic relationship
Relationship in which two organisms form a close association. The term is now usually used only for associations that benefit both organisms (a mutualism).