Wind & Animal Pollination

Biotic-Abiotic Pollination and the Problem with Specialisation

Bees are pollinators, and honey bees are the species commonly used as commercial pollinators. Almonds, apples, apricots, asparagus and all the way to zucchini – these fruits rely on the worker transporting pollen from one flower to the other. Apis mellifera have large colonies, a generalist foraging appetite and flower specificity, but they’re not the only animal that pollinates our foods.

Bumble bees are more effective at pollinating plants that require sonication (“buzzing”) – like tomatoes and blueberries and aubergines. Birds pollinate plants beyond the ones we eat (my blueberries know this lesson well). Bats, rodents, possums also pollinate. Wind and water spread seeds far and wide – rice and cereal grains depend on the former, seagrasses on the latter.

Biodiversity requires variety and “saving the bees” is not enough. This article describes the biotic and abiotic pollination, and the factors that determine whether a plant requires animals, wind, water or plain ol’ gravity¹ for pollination. Long story short: bright, showy & smelly flowers means animal pollination. Small, nondescript and hard-to-find flowers suggests wind pollination. If it’s underwater, it’s likely water pollinated, and of course- exceptions exist.

Animal Pollination is Determined by the Floral Organs

If pollination is about plant reproduction, then each part of the flower has developed to support the transfer of pollen from the male anthers to female stigma. With reference to the diagram below:

• The petals’ shape guides the pollinator to its reproductive organs. This can be through ultraviolet-patterns on flowers (for example: sunflowers), or by mimicking the shape of a pollinator, as in the case of bee orchids.
  • Brightly coloured petals like cosmos attract bees, whereas the large, long petals of trumpet vines draw hummingbirds who can reach the blossom’s depths.
• Sepals on the underside of the petals attract pollinators by acting as either landing platforms or adding to the petals’ allure.
• The anthers reside in the stamen, which sits on a slender stalk filament. When a plant has long, exposed stamens, they’re likely wind pollinated. Grasses and wheats fall in this category, after all – why spend energy producing pollen only to have it removed by a stiff westerly wind?
  • On the other hand, my blueberries and tomatoes flowers have small pores at the tip of their anthers. This adaptation restricts pollen release, unless a certain stimulus is produced, in this case – buzzing (“sonication”).
    • Blue-banded bees and bumble bees grasp the anthers and vibrate their flight muscles without flapping their wings, generating a vibrational frequency that shakes the pollen out of the anther pores. As the bee travels between flowers, the pollen is transferred to other stigmas, continuing reproduction.
• The stigma receives pollen. This pollen travels down the style and into the ovaries. In wind pollinated plants, the stigma is sticky to capture the airborne pollen. In animal-pollinated plants, they’re compact to increase the chance of receiving pollen from the body of a pollinator.
  • Self-pollinated plants have their stigma close to the stamen whereby the pollen reaches the stamen without intervention by a middleman (middle-being?). For example, tomatoes have their stigma encased in their anthers and can produce fruits without pollination. However, cross-pollination via a pollinator would result in bigger fruits (seen in strawberries – Dung et al., 2021).

Flower Shape, Scent and Symmetry

The plant’s flower arrangement determines how accessible it is to pollinators. Tulips have one solitary flower on the stem, whereas carrots produce multiple flowers from a single point. Flower clusters are more appealing to pollinators than single blooms, and having multiple blossoms on a single plant improves pollen transfer within the plant.

Flowers produce volatile organic compounds to advertise their attractiveness. There are sweet scents from lavenders and jasmines for generalist pollinators like bees and butterflies. Foul odours from corpse flowers for flies and beetles, or fermented, fruity scents from durian, to attract bats.

The most favorable flower arrangement is the symmetrical one. Viewing the flower parts relative to a central axis (e.g. the stem), gives you an idea of how its pollinators behave:

• “Radial” symmetry arises when flowers are divided into multiple identical parts along any plane. It’s like a circle, no matter how you slice it, it’ll look the same on both sides.
  • Two examples are roses and daisies. If you cut either flower in any direction, both sides will look the same; their petals are evenly distributed around the centre.
  • Radial flowers are accessible to more pollinators who can approach it from any direction, as opposed to from the stem only or in an arbitrarily compass direction.
• “Bilateral” symmetry is as the name implies, it can only be divided into two identical halves along one specific line. Think of your face – it only appears the same when divided into a left and right side².
  • Snapdragon flowers and orchids show bilateral symmetry. The latter has a single “landing platform” for pollinators, which drives their symmetry, whereas the former’s dragon mouth shape can only be split into two identical halves along one plane (hence “snapping-dragon”).
  • Bilateral symmetry is useful for plants with specialised pollinators – Advantageous as their pollen is more efficiently released, but disadvantages if their specialised pollinator disappears; the pollen stays locked up for longer and the plant may fail to reproduce. Many such cases.
• Asymmetrical flowers lack symmetry altogether, and the benefit of this is that they’re able to mimic something in nature, like another insect, or just nothing at all. Canna lilies are one such example.

Some flowers evolved specialized structures, like nectar spurs or traps or mimicries, these structures also assist with pollination by a specific animal, and are beyond the scope of this article.

Wind Pollination Uses Lightweight and Abundant Pollen Grains

Wind-pollinated plants are categorised by the absence of bright blooms. They do not need to attract pollinators, so their flowers are small, dull and nectarless. The stamen and stigmas are exposed, positioned towards the outer flower to maximise wind exposure.

The pollen produced is light, small and dry – easily carried by wind. Now plants can’t tell the Anemoi where to pick them up and drop them off, so large amounts of pollen are produced to increase the hence of successful pollination, if you’re going to gamble, gamble large and in little amounts, rather than gambling little and in large amounts (i.e. animal-pollinated plants).

Wind-pollinated plants do not produce sticky pollen. The plants’ stigma is sticky and adapted for capturing the drifting gamete; the silks of maize show good examples of feathery, branched structures for capturing pollen. This is unlike flowering plants, where you can see the pollinator covered in pollen because the pollen is innately sticky and adheres to her as she goes between flowers (the bee on the sunflower video below demonstrates this nicely).

The floral arrangement is simpler than insect-pollinated flowers, with unisex flowers containing either stamen or stigma. Some plants may have purely male flowers (stamen-only) or female flowers (stigma only)- for example. corn produces male-only flowers up the top of the stem (tassles) and female only flowers towards the centre stem (ears). The rate of pollen release and capture is increased by having flowers clustered on the stem.

Advantages and Disadvantages of Wind Pollination

Wind pollination arose in environments without access to biotic pollinators; it remains successful due to its energy efficiency and luck (isn’t all evolution luck?). Wind-pollinated plants do not need to produce nectar, fancy petals, or pleasing scents to attract animals. This spared energy is reallocated into pollen and seed production. Their independence from animals spares them from the shifts in pollinator populations and in areas with little biodiversity, i.e. the monocultures of industrial agriculture.

These same advantages can turn to disadvantages: Strong wind currents takes pollen far from any female stigma, hindering fertilization. Mass-producing pollen is energy intensive, and without favourable environmental conditions (windy, dry conditions) it becomes costly a wet year. Anything that can be brought about by luck can be taken away by luck; this aphorism is not about pollination.

Evolutionary Significance of Wind Pollination

How did wind pollination arise? In short, this adaptation arose in response to environmental conditions that lacked faunal biodiversity. It could be due to decline in pollinator abundance (many such cases throughout evolutionary history) or changes in the abiotic environment (for example, mass extinctions [five such cases throughout evolutionary history])(Friedman & Barrett, 2007).

Wind pollination is favoured in regions with:

• Higher latitude, as the colder weather means less animal life.
• Arid temperate environments, as the drier air make it easier to disperse pollen grains.
• Open vegetation, which reduces competition and physical interference from large trees during pollen dispersal.
  • Indeed, plants in closed forests within tropical regions use a mix of wind and insect pollination:
    • Chamaedorea pinnatifrons is a palm in the Peruvian Amazon that use beetles and thrips scatter their pollen on the flower. This pollen is then dispersed by the wind in tandem with the pollination action of the insects (Listabarth 1993).
    • The south east Asian flowering plant Mallotus japonicus exhibit floral traits adapted for both wind (exposed stamens) and insect pollination (well-developed nectaries). Both small-bodied insects and wind aid with pollen dispersal (Yamasaki & Sakai, 2013)
• Island floras: not all islands are inhabited by animals, nor can they be easily reached by animals.

Independent evolution in multiple plant lineages suggests that wind pollination is a favourable response to similar ecological pressures – grasses, grains and conifers are different plant families who independently developed wind-borne sperm to reproduce in environments where pollinators are not always present.

Evolution in Progress: From Insect to Wind Pollination

In the South African Conebushes Leucadendron, there were at least four shifts from insect to wind pollination (Welsford et al., 2016). This transition is marked by the decline of pronounced blooms to smaller wind-pollinated flowers, with a subsequent reduction in flower colouration and scent release. However, the structural traits like surface area and stem flower density remained unchanged.

Could further evolutionary changes reduce these structural traits to mirror those of grasses in the fynbos region, as such morphologies are more efficient for wind pollination? Let’s check in a few million years.

Sunflower pollen is spiky, allowing to adhere to the branched hairs on the worker bee’s body as she forages on the blossom.

Water Pollination in Aquatic Plants

A brief: Water pollination is confined to aquatic plants like seagrasses where the pollen is released and transported along water currents. Pollen can travel submerged or by floating on the surface until it makes contact with a compatible female flower. Examining these waterborne grains will show elongated, water-resistant cells coated with mucus-like substances to prevent water damage. Greatly advantageous and resilient unlike wind pollination, however it also requires large amounts of pollen (and luck!) to ensure fertilization.

Note: Spore Dispersal is not Pollination

You may think that plants like ferns also reproduce by wind pollination – this is not the case, as ferns disperse spores – these are mature “eggs” of the parent that travel on the wind currents and sprout into a new plant when it reaches the soil. Briefly:

1. Specialised reproductive structure on the underside of the fern leaves (“sporangia”) produce spores through meiosis. Clusters of sporangia (“sori”) are present on the leaf, generating the pollen required.
2. As the sporangia matures, it dries and hardens, creating tension within the cell walls that bursts upon stimulation (in this case, a stiff wind). The spores are cast into the air in a catapult motion, and since the pollen is lightweight, it rides the current in search of new areas to set seed.
3. The spore lands in a suitable location (usually moist, nutrient-rich soil – easy in a forest, difficult in suburbia) and germinates into the reproductive unit (“prothallus”). This structure produces both the male (“antheridia”) and female (“archegonia”) reproductive organs for sperm and ova reproduction, and so fertilisation begins in earnest.
4. Once fertilization occurs, the zygote develops into a new sporophyte which will grow into the fern and start the process anew.

The reader can appreciate how this differs from wind pollination, and how it is also energetically costly in terms of the gametes produced.

Distinguishing Between Flowers and their Pollinators

Flowers that are specific to a group of animals have traits to attract those animals. It’s hard to cover all aspects as the in-group preferences are varied (for example, sweet-smelling blooms for bees versus corpse-smelling for flies), and there can be overlap in flower types where there are introduced species (in Australia, nectar from mammal- or insect-pollinated plants is consumed by invasive birds without pollinating the flowers… look at you, Indian Myna). Some generalisations:

• Insect-attracting species can have UV-spectrum patterns, like sunflowers.
  • Given the diversity of insects, I wager that all flowers have an insect pollinator, even if they are not built for them.
• Bird-attracting flowers have large robust tubes to accommodate their hard beaks.
  • If you get up close, you’ll see that these flowers lack landing platforms, forcing the bird to hover or grasp a branch/stem to access the nectar.
• Bat-pollinated flowers are large and bowl-shaped, opening at night to coincide with their activity.
  • Flowers tend to have a strong scent with dull colours– white or cream, after all, who needs bright colours under moonlight?
• Rodent-pollinated flowers are low growing or grounded with a strong scent; rodents don’t see very well.
  • Reptile-pollinated flowers are a visual inverse: more explicit with poor scents for those lizards that see well and smell not-well.

At the end of the day, nectar is used to attract (or reward) an animal for visiting and taking its pollen. If a flower lacks a bird species, an insect may pollinate it instead and transfer the gametes to the stigma. This rewards generalist plants, who can survive in changing environments, at the expense of some nectar robbing by non-cooperative species.

The Dodo, or Animal Co-evolution with Plant Reproduction

The specificity of pollinator co-evolved plants means they can time their energy expenditure, but will suffer should the species die out. Consider the Tambalacoque tree that depended on the now-extinct Dodo bird to complete its reproduction. After the flower is pollinated, the tree produces seeds with thick lining that required passage through the dodo’s digestive tract to break dormancy and commence germination (Temple, 1977).

Following the bird’s extinction the late 1600s, natural germination of tambalacoque seeds ceased, and tree populations declined. It wasn’t until the seeds were exposed to other methods that mimicked the “damage” done by the dodo’s digestive tract (such as ingestion by tortoise; Iverson, 1987) that the seeds began to germinate and the tree population reestablish itself.

What started out as a mutually beneficial interaction (dodo receives nutrition from the seeds of the plant and excretes the now-germinating seed elsewhere) turned out disadvantageous with the dodo’s decline. Specialisation can reduce competition with generalists, but makes one more vulnerable to sudden rare events (black swans dodo events?)

A Heuristic for Distinguishing Between Wind & Insect-Pollinated Plants

To distinguish between a wind or insect pollinated plant, look for the flowers: bright, showy and smelly? That’s animal pollinated. Small or absent flowers? Probably wind pollinated. Underwater? Water pollinated.

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Footnotes

1. Some wheat species have their flowers positioned in a way where the male is located above the female. When the pollen falls out of the stamen, it is carried down by the gravity into the stigma, resulting in self-pollination. Wildly unreliable, and only works when the male is taller than the female counterpart (further reading: Garst et al., 2023). ↩︎
2. Assuming you’re blessed with symmetrical features. ↩︎