To bee or not to bee: How plants are responding to insects vanishing

Some plants are cutting out the middleman and pollinating themselves!

Where did flowers come from?

Flowers have been around for at least 140 million years which is not long – if you consider that the Earth is 4.5 billion years old! If we compressed those 4.5 billion years into an hour, flowering plants would have come into existence just 1.5 minutes ago.

Flowering plants or angiosperms now outnumber other plants, such as conifers (cone-bearing trees) and ferns, that existed long before they did. More than 300,000 angiosperms are known, accounting for about 90% of all plant species scientists have identified.

The fossil record hints at what the earliest flowers looked like – they were small and simple with none of the flashy petals we associate with today’s flowers. Like the angiosperms of today, they had seeds encased in fruit, unlike conifers whose seeds are arranged in open cones. Some flowering plants were woody, others were herbaceous.

It is safe to say that angiosperms changed the world. They had an important advantage over the conifers – they grew and reproduced much faster, particularly the non-woody ones. Flowers were undoubtedly the secret to their success.

Parts of the flower infographics
The stamen is the male part of a flower while the pistil is the female part , Credit: depositphotos.com/inkoly

How do flowers help plants reproduce?

Flowers are how angiosperms reproduce. Most of them have both male and female parts, known as bisexual or complete flowers. During reproduction, pollen is produced by a part of the flower known as the stamen. Each pollen grain contains the flower’s genetic material, specifically two male gametes.

A flower gets pollinated when a tiny pollen grain lands on its pollen receptor – the stigma, which is attached to the top of a stalk in the center of the flower called the style. The pollen releases proteins to check if the flower it has landed on is genetically compatible.

If it is, then it will germinate and grow a tube down through the style, into the ovary (the female part of the flower), where it will fertilize the ovule. After fertilization, a seed will grow, which if it is lucky, will one day grow into a new plant.

A carpenter bee
A carpenter bee (Xylocopa sp.) pollinates a flower, Credit: depositphotos.com/har1224

How does the pollen get where it needs to go?

Many of the first flowers may have depended on the wind to carry pollen grains from their male parts to their female parts, which was very inefficient and dependent on chance. However, at some point, probably early on in the evolution of flowers, an insect visited a flower – and carried a grain of pollen to the style.

This method of ‘direct delivery’ was far more advantageous for the plants – but how could they encourage insect pollinators to visit them? Over time, they evolved colorful petals and became far more conspicuous – this led to an incredible explosion in the number of angiosperm species between 80 and 125 million years ago.

Since then, insects and flowering plants have changed each other forever, a phenomenon that scientists call coevolution. To attract pollinators, flowers developed brighter colors and tempting smells. Their petals evolved to become easier for their visitors to land on!

How do the insects benefit by pollinating flowers?

The biggest draw for insects, though, was nectar, which the plants used to reward them. As the ancestor of butterflies, bees, and wasps fed on nectar, the tiny grains of pollen would get stuck to their bodies, and get carried off to their next floral destination.

This relationship has existed since some of the earliest flowers; 86% of all angiosperm species in history are thought to have been pollinated by insects. Some flowers, like the common lantana, even change color to let pollinators know how much nectar they have.

A Common Mormon butterfly
A Common Mormon butterfly feeds on nectar from flowers of the common lantana (Lantana camara) in Kolkata, India, Credit; Priyanjana Pramanik

Are bees disappearing?

Unfortunately, this relationship is now at risk. Pollinators, particularly bees, are declining all over the world due to pesticide use habitat loss, and fragmentation, not to mention climate change. Over the last 50 years, scientists have observed declines not just in insect pollinators but in the plants that they pollinate.

How do plants get pollinated without insects?

Some plants, though, are responding to the loss of pollinator populations by finding new ways to reproduce. Scientists could understand how they do this by studying field pansies, which are common in many parts of Europe and come in purple, yellow, and white.

They used a method called ‘resurrection ecology’ where they grow dormant seeds that were collected many generations ago along with and under the same conditions as their descendants. This allows researchers to understand how the plant has evolved and diverged from its ancestors.

In this case, researchers grew field pansy seeds that had been collected twenty years ago near Paris and compared them with newly produced seeds. They tested whether the plants responded to the decrease in pollinator populations by evolving and changing their mating system. They also checked whether bumblebees prefer the ‘ancestral’ plants or the ‘descendant’ ones!

Once they had grown the plants, the team analyzed the genetic makeup of the plants and how they have changed over time. They also measured plant characteristics like the flowering date, floral characteristics, and the volume of nectar, as well as how successfully they reproduced when exposed to different pollination conditions.

The ‘descendant’ plants had smaller, less conspicuous flowers which produced less nectar. They were more likely to ‘self’ or pollinate themselves instead of relying on an insect pollinator. Unsurprisingly, the bumblebees seemed to prefer the ‘ancestor’ plants which could offer them more nectar!

This study shows how the field pansy seems to be moving towards self-fertilization as a reproductive strategy, thus becoming less attractive to the bumblebees who pollinated the flowers.

Will all plants start ‘selfing’?

Vidisha Bansal, a doctoral researcher with the Biodiversity Exploratories at the Technical University of Munich, Germany, studies the interactions between plants and pollinators, and says that there is a lot that we do not know about how these relationships are changing.

“Plants evolve much faster,” she says. “They’re more resilient, don’t have to move, and can adjust their dormancy periods. That’s why we see massive hive deaths – because the pollinators need time to catch up.”

Even if we see more selfing among plants, it is unlikely that the ancient relationships between plants and pollinators will disappear entirely.

As Vidisha puts it, “Inbreeding is not a good reproductive strategy!”

People also ask

What are the different ways in which flowers are pollinated?

Flowers can be self-pollinated, where the transfer of pollen grains from the anther to the stigma takes place within the same flower. For cross-pollination, pollen grains are transferred across flowers. This requires either abiotic agents such as wind and water or biotic agents like humans, insects, bats, birds, and sometimes even frogs!

How is pollination different from fertilization?

Pollination is the transfer of pollen grains while fertilization occurs after pollination, when the male and female gametes fuse and create a zygote. Pollination precedes fertilization and is necessary for the transfer of male gametes to the female reproductive structures, where fertilization occurs, leading to seed formation and plant reproduction.

Why is plant pollination important to humans?

Plant pollination is important to humans for food production, biodiversity conservation, economic value, medicinal resources, and the provision of essential ecosystem services. Protecting pollinators and their habitats is essential for ensuring the health and well-being of both human societies and natural ecosystems.

References

Acoca‐Pidolle, S., Gauthier, P., Devresse, L., Deverge Merdrignac, A., Pons, V., & Cheptou, P. O. (2023). Ongoing convergent evolution of a selfing syndrome threatens plant–pollinator interactions. New Phytologist.

Condamine, F. L., Silvestro, D., Koppelhus, E. B., & Antonelli, A. (2020). The rise of angiosperms pushed conifers to decline during global cooling. Proceedings of the National Academy of Sciences, 117(46), 28867-28875.

Crepet, W. L., & Niklas, K. J. (2009). Darwin’s second “abominable mystery”: Why are there so many angiosperm species? American journal of botany, 96(1), 366-381.

Li, H. T., Yi, T. S., Gao, L. M., Ma, P. F., Zhang, T., Yang, J. B., … & Li, D. Z. (2019). Origin of angiosperms and the puzzle of the Jurassic gap. Nature Plants, 5(5), 461-470.

Lima, M. A. P., Cutler, G. C., Mazzeo, G., & Hrncir, M. (2022). The decline of wild bees: Causes and consequences. Frontiers in Ecology and Evolution, 10, 1027169.

Maharaj, G., & Bourne, G. (2017). Honest signaling and the billboard effect: how Heliconiid pollinators respond to the trichromatic color changing Lantana camara L.(Verbenaceae). Journal of Pollination Ecology, 20, 40-50.

Raiol, R. L., Gastauer, M., Campbell, A. J., Borges, R. C., Awade, M., & Giannini, T. C. (2021). Specialist bee species are larger and less phylogenetically distinct than generalists in tropical plant–bee interaction networks. Frontiers in Ecology and Evolution, 9, 699649.

Stephens, R. E., Gallagher, R. V., Dun, L., Cornwell, W., & Sauquet, H. (2023). Insect pollination for most of angiosperm evolutionary history. New Phytologist, 240(2), 880-891.

Copyright @smorescience. All rights reserved. Do not copy, cite, publish, or distribute this content without permission.


Join 20,000+ parents and educators
To get the FREE science newsletter in your inbox!