Friday, September 11, 2015

The Accidental Pollinator

It seems we all know that pollinators (especially bees, who do most of the pollinating) are important, but did you know they aren't doing it on purpose?

The bright yellow pollen of a Mallow flower.
Pollination is the transfer of pollen from anthers (loosely, pollen is the male portion of a flower) to stigma (a part of the female portion of a flower).  Ideally, pollen from one flower ends up on the stigma of another flower, on a different plant of the same species.  Obviously, a flower can't uproot itself and stalk (puns intended) over to another plant to shake off a little excess pollen--pollinators are required.  Wind was the original pollinator, and many plants still rely on a light breeze to whisk away their tiny grains.  But somewhere along the lines, flowers "realized" the value of employing conscious couriers; wind can be whimsical.  If it doesn't blow (say, in the rain forest), a hopeful plant is out of luck.  If it blows the wrong way, pollen may be carried away, but to no good end. 

An insect rests on a California Poppy.
In contrast, insects seek out flowers for bite or a sip, for meeting up with the ladies, or as resting places.  Notice that not one of those reasons for visiting a flower is:  "to pollinate a flower".  Selfish creatures, insects are accidental pollinators.  As they muck about on flower heads, pollen grains adhere to their body parts (it is no accident that pollen grains are sticky).  Then, as they move from flower to flower, pollen grains are transferred.  Historically, this relationship has been referred to as a mutualism—one in which both partners benefit from the interaction.  While this is most certainly true, it is more complicated, especially when it comes to bees…

A Centris female, with full pollen loads (success!)
There are two points of view when it comes to this mutualism.  Consider the female bee:  her goal is to harvest as much pollen from a flower as possible, scraping,shaking, and squeezing it out of anthers and packing it tightly into her scopa—stiff leg hairs evolved specifically for holding pollen (they are even slightly electrostatically charged).  The more pollen she gathers, the more bee larvae she can provide for, and (therefore) the more successful she is as a parent.  To maximize her success, she sometimes limits herself to just one type of flower on foraging bouts—having to remember how to operate the various bits and pieces of, say, only cactus flowers is easier than trying to remember cactus flowers, penstemons, and also daisies.  She also moves quickly.  The faster she can gather the pollen, dump it at her nest, and get back out for more, the more babies she can provide for.  Any pollen ‘lost’ to a flower’s stigma as she moves about represents a failure, from her point of view. 

From the plant’s point of view, female bees are a bit of a conundrum.  They represent the best transporters of precious pollen grains—they are the most likely pollinators to visit consecutive flowers of the same species, and they move quickly, therefore shuffling a large number of pollen grains between flowers every day.  At the same time, they are thieves, and the majority of pollen handled by female bees is lost to the flowers.  As a result, flowers have had to evolve devious ways of manipulating bees—encouraging them to visit, but hurrying them on their way before they’ve done too much damage.

The many colors, shapes, and scents of
flowers are meant to draw in
potential pollinators.
On the one hand, flowers advertise; they lure bees in with the promise of sweet nectar, indicated by a heady scent detectable from great distances.  Like highway billboards, they promise all-you-can-eat buffets, unfurling petals in eye-popping shades of blue, and pink, and yellow, and orange, and even ultra-violet.  On the other hand they play hardball. Visitors to flowers with narrow corolla tubes are limited to those with slender waists.  Pollen grains are often coated with a toxic layer; a bee might enjoy a sip of nectar from a flower, but find the pollen unpalatable, and shed it at its earliest convenience.  Fruit trees frequently have ‘blank’ flowers; among the cornucopia of nectar-producing flowers on one tree are empty ones.  Disappointed bees often leave a tree and move on to another when they encounter two or more blank flowers in a row (thus guaranteeing at least some cross-pollination).  Some kinds of pollen grains also often have points and spines on them that prevent them from being packed too tightly onto bee legs (especially honey bee and bumble bee legs), making it easier for them to fall off when the bee lands on a new flower.

A Xylocopa steals nectar from a flower whose corolla is too
narrow for its burly body.
Not to be outdone, bees counter with their own maneuvers.  Narrow corolla tubes are bypassed by nipping a hole in the base of the flower, sipping out nectar without ever touching the anthers or the stigma.  Specialist bees have evolved hardy stomachs, capable of ingesting otherwise poisonous pollens.  Many bees are capable of smelling flowers before they land, assessing the quantity and quality of nectar and pollen before they waste their time,  Finally,some bees have extremely widely spaced pollen-collecting hairs (scopa), capable of holding even giant spiky pollen grains. 

Are bees important pollinators?  No doubt.  Perhaps THE most important, from the point of view of the majority of the world's flowers.  Nonetheless, their services come at a hefty price to the flower, seen most clearly in the amazing variety of shapes, colors, and scents on display in any flower patch.


  1. This relationship is so interesting. It has elements of an arms race, more typical of exploitative interactions. I guess the benefit to the plant is pretty essential, which shifts it toward mutualism. In reality it's a spectrum, right? So are there any bee species that are exclusively "cheaters"? Do those species rely on the fact that non-cheaters exist (so the plant doesn't lock it's pollen and nectar away all together)?

  2. First, sorry it took me two months to respond... didn't realize you'd commented. Oops--still figuring out how this blog works.

    Second, YES, I think that cheater-species totally rely on the fact that there are non-cheaters out there. Perfect example: Hylaeus. They ingest the pollen, and except for the few grains that stick to their scant body hair, they are hopeless pollinators. They are completely taking advantage of the flowers. Hylaeus also tend to visit flowers that are well-visited by other, more impressive, pollinators. From the plant's point of view its worth the loss of a little pollen to the pesky Hylaeus to continue attracting other bees.

  3. This is a wonderful post. It shows how the nature has created its own system of growing plants and trees on the face of teh earth. These little creatures carry with them the pollen to different places where plants grow.

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  5. Very interesting blogpost! Thanks for diving into some of the details and providing insight about the way the pollination process constantly changes. It always amazes me how species will coevolve to make sure they continue to benefit from their interaction with one another. Bees and flowers are so complex and fascinating, they deserve all the praise!! Please check out my blog (where I have also posted about pollinators) and become a follower if you please :