Pollination represents a unique partnership between plants and pollinators. These are wild creatures working toward their own self-interest, so both the plants and the pollinators will do everything in their power to get the most bang for their buck out of the arrangement.
In our previous post giving a rough overview of pollinators, we described what plants get out of pollination; the ability to produce seeds and reproduce. However, pollinator-assisted reproduction is only one of many ways that plants can reproduce. Many plants have “perfect flowers” that include both male and female parts, and any slightly jostling of the flower (as may occur from a breeze or rainfall) can be sufficient to dislodge pollen onto the female parts and fertilize the flower. Other plants, especially grasses (including major food crops such as corn, wheat, rice, sorghum, etc.) are wind-pollinated. Still other plants reproduce vegetatively, sending rhizomes or suckers along the ground that sprout new stems. However, each of these reproductive methods has downsides. Self-fertilization with perfect flowers limits gene flow between plants, making whole populations slower to evolve to changing conditions. Vegetative reproduction expands that problem further; it produces clonal plants that are identical to the original. Commercially produced bananas are exclusively propagated by vegetative cloning, meaning that all grocery store bananas are genetically identical and equally susceptible to the looming catastrophic fungal pathogen, Panama disease Tropical Race 4. Wind pollination allows for genetic mixing between plants, but pollen sent to drift in the wind has a low chance of finding its target in a female flower. Therefore, wind pollinated plants must produce enormous quantities of pollen so that some of the pollen grains will strike their mark. Producing that much pollen is costly; pollen is rich in protein, which contains nitrogen – and nitrogen is very valuable to a plant. Pollinator-assisted reproduction allows a plant to mix genetically with distant plants without producing huge quantities of costly pollen.
Plants have a good reason to rely on pollinators, but why do pollinators go to all the work of pollinating? It’s not a free service; plants produce rewards, usually in the form of sugary nectar. Unlike pollen, which is rich in protein and therefore costly nitrogen, the sugars in nectar are made of carbon. The carbon in that sugar ultimately comes from the carbon dioxide in the air – a resource that plants are unlikely to deplete any time soon – so sugar is a much cheaper investment for a plant than protein. For some pollinators, sugary nectar is reward enough; many wasps, flies, moths, butterflies, bats, and birds pollinate flowers mostly or exclusively for the sugar rewards. In fact, many plants even produce nectar outside of flowers to draw in wasps and other omnivorous insects to help with pest control! However, sugar alone does not make a balanced diet, and any pollinator needs a protein source of some sort. Bees, which are probably the most specialized of all pollinators, mostly rely entirely on their pollinating lifestyle for food for all life stages. Foraging bees gather both nectar and pollen; the sugary nectar is mostly fuel to keep them flying and working, while the protein-rich pollen is an excellent food source to feed their larvae. The closest relatives of bees are predatory wasps that feed their larvae paralyzed insects or spiders, suggesting that the pollen is roughly as rich a food source as meaty prey. Even with bees eating some of the pollen they collect, it is often more efficient for a plant to feed a bee and have pollen delivered directly where it needs to go than to scatter pollen to the wind and hope it lands somewhere useful.
Photo credit for title image (Macroglossum stellatarum): IronChris, via Wikipedia