For the Pollinators!
In lieu of pollinator week, a few notes on pollinators and mutualistic dynamics.
Yes, that’s right: it’s pollinators week! Didn’t know that was a thing did you?
Honestly, I didn’t either. I only looked it up because a reader (I believe SL?) asked if I could cover pollinators. I sourced a few papers, but unfortunately fell behind on actually sitting down and parsing what I read, so I shelved that and looked for any reason to hold off on making a post.
So here we are, in the middle of pollinator week. Pollinator week appears to be a week typically set in early-mid June to talk all things pollinator. You can even get a pollinator week shirt- pretty spiffy!
This year, pollinator week is taking place between June 19- June 25, and it appears the topic is based on climate change’s influence on pollinators.
It’s hard to look at such a topic and not consider the deeply political aspects of these discussions, although it’s a rather apparent fact that the destruction of pollinator habitats, as well as the widespread use of insecticides, have clearly harmed pollinator populations.
Here, I’ll cover a few topics of pollinators that I have come across. Hopefully some of the information provides a broad examination, with some more closer information provided. This article will likely be presented in parts if people are interested in various pollinator-related topics, so please let me know!
To bee a pollinator
Pollinators refer to both biotic and abiotic groups that are involved with the pollination of angiosperms, or flower-producing plants. In general, pollen (male gametes) is taken from the anther of flowers and transported to other angiosperms. The pollen may then land on the stigma of an angiosperm, eventually fertilizing the flower and producing a fruiting body with seeds that hopefully spread and produce other plants.
When we think of pollinators, we many generally think of bees, moths, wasps, butterflies, beetles, birds, bats, and other animals.
It’s true that animals, especially insects, are the primary pollinators. However, wind itself can also be considered a pollinator by way of dispersing pollen and carrying the male gametes across large regions, hence the abiotic aspect.
With that being said, the main pollinator we think of are bees, especially honey bees.
It’s suspected that there are over 20,000 species of bees in the world, with over 4,000 being native to the US.1
For bees, the act of pollen scavenging is critical to bee survival, and for our survival as well. Honey bees collect nectar and pollen from flowers as a source of nutrients for larvae and other bees in the colony. In doing so, the bees inherently cross-pollinate flower species as they scavenge around carrying their little pollen sacks.
I must admit that I never knew that bees carried their pollen in pollen sacks until someone pointed it out to me. However, it does make sense as a method of collecting pollen. Interestingly, an article in Science briefly notes that the pollen stays rather firmly in place on the leg hairs of bees, with some researchers suggesting that the force needed to remove the pollen being up to 20 times greater than the force a bee exerts while flying.
The method of pollen collection deployed by bees is rather interesting. It seems that some species of bees utilize a method called floral sonication, in which bees latch onto the anthers of flowers (possibly by biting), contract their flight muscles, and wiggle the pollen loose by way of vibration caused by the flight muscles. Some articles have referred to this method as “buzz pollination” as well.
Although fascinating many bee species, including honey bees, appear to not engage in floral sonication. The assumption here seems to be based on the energy needed to buzz, in that the energy cost of buzzing is not properly compensated by way of scavenging.2 This concept is one critical to evolutionary theory, and explains some of the differences in pollinator behavior across species.
Depending on pollinators
As a species, we are heavily reliant on pollinators for most of our food. Many of the fruits and vegetables we eat rely on pollinators. In fact, the USDA suggests that over 100 of the crops grown in the US rely on pollinators.
Unfortunately, destruction of habitats, as well as environmental toxins, may be reducing pollinator populations.
A 2021 article from Our World in Data suggests that up to 5-8% of the world’s crop production may decline with the decline of pollinators. They appear to draw this information from studies, and because I have not looked at this data for myself I should remind readers that skepticism should be provided to these types of reports.
The degree to which crops rely on pollinators vary. Our World in Data provides a list of some of these crops and the degree of dependency they have on pollinator (insects in particular it appears).
Mutualism at its finest
The relationship between organisms are generally categorized by the give/take that is seen. In the case of pollinators and angiosperms, these groups serve as some of the best examples of mutualistic behavior, in which both parties benefit from one another.
It is believed that Darwin first posited the relationship between mutualistic behavior and the notion of coevolution in which two organisms may evolve together.
Bronstein, et al.3 notes this idea of Darwin [context included]:
To develop this idea [mutualism] further, Darwin analysed the evolution of fruits and flowers, showing how traits that benefit animals function first and foremost to increase plants’ own reproductive success. In showing that mutualisms could arise strictly through selfish actions, limited by costs and driven by conflicts of interest between partners, Darwin laid the foundation for our current view of mutualism. He pointed out cases in which the interests of mutualists could come into conflict, discussing how cheating behaviors such as nectar-robbing might become favored. It was also in the context of mutualism that he explicitly pointed to the possibility of coevolution (reciprocal evolutionary modification of traits) (Darwin, 1859):
‘Thus I can understand how a flower and a bee might slowly become, either simultaneously or one after the other, modified and adapted to each other in the most perfect manner, by the continued preservation of all the individuals which presented slight deviations of structure mutually favourable to each other.’
Covering the degree of mutualism that occurs between angiosperms and insects are far above what can be explained here, although I suggest people refer to the Bronstein, et al. review.
In short, the relationship between flowers and pollinators are so deeply entwined that one, in most cases, cannot exist without the other.
However, this relationship should be recognized as not being inherently static. Rather, there’s an ebb and flow to the relationship, in which one must recognize that the mutualism comes with a large degree of selfishness.
Consider an insect that feasts on the nectar provided by a flower. By consuming the nectar, the insect may come into close proximity to the anthers of the flower and may have some pollen rub onto the insect. As the insect travels around to other flowers, the constant rubbing and depositing would allow for pollination across different flowering species.
However, some insects may engage in a method of nectar thievery, in which an insect may consume the nectar of a flower without providing the flower any benefit through pollination. For example, an insect may extend the distance between itself and the nectar, and so the insect does not rub against the anthers and collect pollen. Some insects may also find ways around the flower’s opening, likely chewing through the underside of the flower in order to access the nectary.
In this regard, both insects and angiosperms evolve in a way that benefits themselves. However, the constant interaction between pollinators and flowers makes it so that the selfish behaviors result in the maintenance of mutualism through coevolution.
A clear example of this interaction was noticed by Darwin, and later validated by scientists by way of the Malagasy star orchid which has a strangely long nectar spur.
It appears that, in Darwin’s observation, he assumed that the Malagasy star orchid may be associated with a moth with a very long proboscis that allows it to reach the nectar. Essentially, there is some form of coevolution occurring between the flower and a suspected moth.
As noted by Johnson, S.D. & Anderson, B.4:
In hypothesizing how the Malagasy star orchid (Angraecum sesquipedale) might have evolved its extraordinarily long (c. 30 centimeters) nectar spur, Darwin (1862) proposed the first mechanistic model of the coevolutionary process. Although he never used the term coevolution, he did refer to “a race in gaining length between the nectary of Angraecum and the proboscis of certain moths” (Darwin 1862). He noticed that the Angraecum, like the local British moth-pollinated Platanthera orchids, had nectar at the very bottom of the long spur and that moths would require a long proboscis in order to claim this reward. The fittest moths in a population would then be those with long tongues that could access the nectar in even the deepest flowers, whereas the shorter-tongued moths would access less nectar (Fig. 1). Thus, moths would be expected to be under strong directional selection and should evolve greater tongue lengths (Fig. 1). By inserting rods of different diameter into flowers, Darwin ascertained that pollen would only be transferred between moths and Angraecum orchids if the moths push the thickened base of their proboscis right up against the reproductive parts of the flower when they try to drain the last drops of nectar from the bottom of the nectar spur.
The dynamics here outline a form of mutualism by way of coevolution, in that a moth that has a too-long proboscis can engage in nectar thievery without pollinating the orchid. Thus, an orchid must evolve with a spur long enough to force a moth within close proximity in order to allow pollination.
As outlined by Johnson, S.D. & Anderson, B, a study detailing just that was eventually conducted, in which researchers artificially shortened the spur of European orchids and examined pollen exchange (by way of deposition and removal), with the assumption that artificially shortened spurs would show far lower levels of pollen exchange.
The research, conducted by Swedish pollination biologist Anders Nilsson, and later corroborated by other scientists, appear to suggest just that.
Essentially, a goldilocks match between the tongue/proboscis length of pollinators and the length of flower spurs have coevolved in such a way that both groups benefit one another.
There are plenty more mutual relationships between pollinators and flowers, including the dynamics between figs and fig wasps.
I’ll save some of this information for later, but for now remember that the relationship between pollinators and flowers runs deep, and this mutualism is what is critical to the pollination of many of the foods we eat.
When examining our own lives, taken into account the work that pollinators do in order for us to have the lives we have. Without them, we’d be left to eating potatoes and bananas.
I’m not sure how long this series will go for, but if there are any other bits of information one would like answered please let me know. I will likely cover figs and fig wasps as a form of obligate mutualism later on, as well as something called nectar guides.
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Patel, V., Pauli, N., Biggs, E. et al. Why bees are critical for achieving sustainable development. Ambio 50, 49–59 (2021). https://doi.org/10.1007/s13280-020-01333-9
Sophie Cardinal and others, The evolution of floral sonication, a pollen foraging behavior used by bees (Anthophila), Evolution, Volume 72, Issue 3, 1 March 2018, Pages 590–600, https://doi.org/10.1111/evo.13446
Bronstein, J.L., Alarcón, R. and Geber, M. (2006), The evolution of plant–insect mutualisms. New Phytologist, 172: 412-428. https://doi.org/10.1111/j.1469-8137.2006.01864.x
Johnson, S.D., Anderson, B. Coevolution Between Food-Rewarding Flowers and Their Pollinators. Evo Edu Outreach 3, 32–39 (2010). https://doi.org/10.1007/s12052-009-0192-6
Ah, you did it! Thank you for a very interesting read. I might be a mutualist. I reluctantly admit to needing the services of many people, but for selfish reasons entirely. Did you know elephants are also pollinators? Humming birds and elephants. What magic.
do you know about human pollinators for date farms? Since I consume dates regularly I was curious about them and some videos for Southern California farms talked about how in the wild (i.e. middle east) the date palms would rely mostly on wind for pollination, if I recall correctly, but on the date farms, using humans to pollinate them yields fruit orders of magnitude greater. So that also makes me wonder about greenhouses, indoor farming, hydroponics. I saw a documentary about such agriculture in Russia to raise plants that normally can't grow there and they at the time used human pollinators but I've also read about efforts to automate it.