The premise seems simple. Plants need their genes deposited onto other plants to reproduce, thus plants offer rewards (nectar and/or pollen) to pollinators. Pollinators collect the rewards while picking up and dropping pollen from one plant to another . However, the implementation is not so straight forward: not only do flowers come in many colors, sizes and shapes, but there are also many different pollinators.
Fig. 1 This solitary leafcutter bee is collecting pollen on its hairy abdomen and thorax. Image via Wikimedia Commons.
Now that I have planted the seeds of confusion, I will try to explain why there is so much diversity and why pollination is beautifully complex. An insect hovers over a bright yellow sunflower, and an instant later it lands. Is this insect a pollinator? Well, not necessarily. The word “pollinator” implies that the insect is helping to spread pollen from one flower to another, but this insect may actually not help at all. A lot of visitors, probably most, indeed are pollinators: they take the rewards offered by the flower and move the plant’s pollen onto others. But there are a few flower visitors that don’t do much for the plant. Some take the rewards without performing the pollinator job.
The difference between a pollinator and a flower visitor can be subtle or quite large. A flower visitor is just a visitor if it does not transport pollen. Some visitors can’t carry pollen, they don’t have hairs at all, or they steal the nectar. For example, some wasps are known to drink nectar but since they are not hairy they don’t carry pollen with them. There are also some bumblebees that perforate the flower, drink the nectar and never touch the pollen. So, they are not actually pollinators. I call these guys herbivores; they eat part of the plant but don’t give anything in return. Ok, so some visitors just take the rewards and others do pollinate. Simple enough, right? Well…
Big differences exist between the perfect pollinators and the pollen- and nectar-eating herbivores, and there’s a gradient when it comes to how effective flower visitors are at moving pollen from plant to plant. There are a couple of ways in which pollinators can be ineffective pollinators. Some pollinators are a lot more efficient at collecting pollen for themselves than they are at moving it around. For example, some social bees collect pollen on pollen baskets. Some of them are very good at cleaning the pollen off their bodies. And that means that there is a smaller chance of pollen being moved to the female parts of another flowering plant.
Fig. 2 A bumble bee collecting pollen. Notice the its pollen baskets and its relative cleanliness. Good for the colony, not so much for the flower. Image via Wikimedia Commons.
Even pollinators that are good at getting pollen to stick to them, and don’t necessarily clean themselves well, can be ineffective. For a plant to be pollinated, it must receive pollen from the same species. But most pollinators don’t visit only one single plant species. Most pollinators are thought to be generalists, getting their nectar and pollen from several species. When a generalist pollinator lands on a plant, it carries pollen from several different species. The probability of depositing the right type of pollen decreases as the number of species visited increases. If enough pollen from the wrong species is placed on the flower stigma (female receptacle), this can actually prevent the right pollen from making contact and fertilizing the ovules.
It now seems that finding the perfect pollinator is next to impossible. So what would make the perfect pollination partner? There are a couple of things that makes the perfect partner. It must be hairy, so that it always carries pollen on its body, and it must only carry the pollen of the receiving plant species. This combination is actually quite rare–but yucca moths and fig wasps are some examples that come to mind. In reality, most pollination partnerships are quite imperfect, yet it all works out. Animal mediated pollination is still the most common way for plants to reproduce. And these are the kind of things that make pollination so fascinating!
 Willmer, Pat. Pollination and floral ecology. Princeton University Press, 2011.
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