They don’t look like much, do they?
But for a few weeks in February and early March, the lives of bees, trees and quite a few humans revolve around nothing but this: tiny grains of pollen, a few hundredths of millimetres in size, little more than a bit of yellow dust to the human eye.
This is almond pollen. It’s one of the plainer types (especially if compared to these), but that doesn’t stop it from being the currency of choice in an existential bargain.
A pollen grain is a germ cell that holds a complete single set of chromosomes. Pollen grains are generally referred to as male, because they are the smaller, mobile units of reproduction of the plant; its „sperm“ if you like (although, unlike in humans and many other animals, they don’t differ genetically from the females).
Every spring every almond tree produces large amounts of pollen, but all this pollen is of no use whatsoever for pollinating their own flowers. For almond trees depend on cross-pollination. They need pollen from another tree, and of a different variety, to reproduce. A complication, yes, but also a failsafe way to avoid inbreeding. But if it can’t use the pollen for its own flowers, why does the individual tree produce any pollen at all? Especially when the pollen has the same genetic setup as the female parts? Dispersal? There will be seeds to take care of that later. What else then? It seems that trees that produce lots of pollen have an advantage over their “lazy” neighbours. But how is it that their investment pays off?
Enter the bee.
Bees love pollen.
It’s their prime source of protein and never do they crave it more than in early spring. After a long and taxing winter, the adult bees are old and ragged and hell-bent on raising new sisters to replace them. Some of these sisters will, eventually, be swarming (and hence propagate a new generation), while others will work all summer to build sufficient stores to survive the following winter that will inevitably come.
For the old bees, at this time, nectar is a welcome bonus, but not essential (which is really good for almond trees, because they don’t have much to offer in terms of sweet juices, neither in quantity nor taste…). What the bees want is pollen.
So if it is spring and you have pollen, you can assume that bees will find you rather attractive.
That, of course, couldn’t be of less concern to the almond tree, if it weren’t for another twist in its own reproductive biology. Because not only does it need pollen from a different tree, it also needs it to be actively rubbed onto its pistils. Which is best done by an insect.
You can see where this is going.
If you are a tree, and you need bees to visit you in spring to assist with your very vital reproduction, it might help to produce something they want.
Even if you can’t use it for yourself.
As long as it attracts bees and makes them crawl around on your flowers, it is well worth the effort. Because chances are that any given bee has been crawling around on another tree before you, rummaging around in its flowers, stuffing her pollen baskets and scattering the precious dust all over her hairy body; not caring the least for you or your reproduction, but so what? There she comes, loaded to the brim with pollen to rub onto your pistils, every single grain holding the potential of a new tree with within its striated walls. It’s all about give and take, right? And who knows, she might even carry some of your pollen into the world and onto yet another tree. Or she drags it back into her lair and has it baked into gluey yellow bee-cake. Which would be a terrible waste of your genetic potential, of course. But hey, you win some, you lose some. It’s almost as if the trees had planned it that way. Which, of course, they haven’t. It’s just that those that happened to do this were the ones more likely to leave descendants.
That’s how it is.
Or could be, anyway.
There might be a more prosaic answer to the question of why almond trees still “bother” to produce pollen.
Imagine a tree that is very productive when it comes to pollen. Chances are that this would lead to a spread of the trait “high pollen productivity” in a population, simply because when you produce a lot of pollen you increase the probability of reproductive success. But maybe, with all that energy that goes into pollen production, these productive trees wouldn’t grow as old as others or would need more water or what have you, and so, over time, pollen production will settle down at some middle ground.
And conversely: Imagine “lazy” trees that have stopped producing pollen alltogether. They invest in flowers/pistils and try to catch every passing pollen they can. However, many of these pollen grains will come from “pollen-productive” trees. So there is a limit to how “lazy” a population will become. Unless all trees become “lazy” at the same time. Then there probably will be a point when there isn’t enough pollen around anymore and the population will die out.
It’s probably a bit of both. Those bees and trees have, after all, a long history of co-evolution. But one could go on speculating all day. If anybody knows of some proper research into these things, I would be delighted!
[post modified April 6, 2011]