For some reason, whenever I say “Pollination Ecology,” in relation to explaining what I am studying in graduate school, people often seem to think I am saying “Polynesian Ecology.” I don’t know why this is, maybe I can’t enunciate, but I know pretty much nothing about Polynesian Ecology (though I certainly wouldn’t mind learning if anyone is going…). Polynesia is a group of islands in the central and south Pacific, technically including Hawaii, New Zealand, American Samoa, and Fiji (as a Melanesian outlier). I’ve heard these islands offer fantastic scenery, alluring trails, and cultural enchantments, and I have personally designated New Zealand as my emergency escape route from graduate school (don’t tell anyone), but that’s essentially all I think you need to know about Polynesia at the moment. I do, however, believe everyone should know a thing or two about Pollination Ecology. And since I recently was shocked to find out that my own grandmother did not have a clear understanding of why pollination might be interesting enough to study for six years, I am compelled to answer that question for any other soft-spoken doubters right here, right now. Pollination is beautiful, lyrical, complicated, fragile, powerful, and essential. So let me tell you about the birds and the bees, the flowers and the trees.
Pollination is beautiful. A labyrinthine aerial dance choreographed by millennia of evolution, pollination unites sessile plants, entrenched in the earth but stretching skyward to offer decorative petals and nutritious rewards, with free-roaming, but ever-searching solicitors of floral recompense. The elaborate corolla displays and scents we love are loved also by these flying insects and countless other pollinating animals. The entire radiation of angiosperm floral diversity is thought to exist for the sole purpose of enticing these visitors ever closer, even drawing them into the flowers’ own inner chambers. Once a bee twists and contorts its body to access the often hidden, but sometimes freely offered, pool of nectar, it emerges handily dusted in pollen, a precious package it will deliver unknowingly to the next outstretched flower down the line. Enacted over and over with different mobile players and new varieties of floral baits, this cross-species courtship dance intertwines and perpetuates the existence of landscape communities. Lest they chance scattering their seed to wind and water, angiosperm plants must somehow tacitly enlist passing pollinators to deliver miniscule male sex cells (pollen) to receptive female flowers of the correct species, at the correct time. They cannot deliver this message nor court their mates themselves, they must stay rooted in place and trust in their peripatetic neighbors to facilitate this imperative cargo transfer. Pollinators, in turn, often cannot survive without their proverbial giving trees. Many animal pollinators, including some flies, beetles, butterflies, hummingbirds, bats and lemurs, rely on sugary nectar as an energy source. Most bees cannot reproduce without concocting a specific blend of protein and carbohydrates from floral pollen and nectar, respectively. Some bees are even developmentally dependent on a narrow range of floral pollens and must have access to those species to feed their young. Stitched together by their divergent talents for motion or food production from only the sun’s light, and refined over generations of evolution, plants and pollinators perform daily rituals full of vibrant color, pleasing fragrance, and aerial acrobatics more stunning than perhaps any other show on the planet. For a visual reminder of the wonders of something you’ve walked by a thousand times, but maybe never stopped to observe, watch Louie Schwartzberg’s TED talk video on the “Hidden Beauty of Pollination.”
Pollination is lyrical. Corolla. Perianth. Calyx. Petiole. Androecium. Apiary. Arolia. Ocelli. Basitarsus. Brevystylous. Papilionaceous. Hyaline. Subrugose. Mesopleura. Melittophily. Chiropterophily. Psychophily. Palynology. Pollex. Pollinia. Petal. Sepal. Tepal. Striae. Stigma. Style. Sterna. Terga. Tegula. Tessellate. Testacious. Tristylous. Tomentose. Vespertine. Violaceous. What’s not to love in that list, or to study for years on end? One of my favorite authors, Barbara Kingsolver, adds her lyrical style and also her weight to the importance of pollination in the following excerpt from her excellent book, The Bean Trees:
I tracked down my father, who had wandered a little distance from the garden and was sitting against a tree trunk. In his fingers he carefully stretched out something that looked like a wasp, still alive. It was as broad as my hand, and had a yellow “8” on each clear wing, as plain as if some careful school child or God had painted it there. My father looked like he’d just had a look down Main Street, Heaven. He told me, “There aren’t any pollinators…no insects here to pollinate the garden. Look at this thing. How would it know what to do with a Kentucky Wonder bean?” I couldn’t know if he was right or wrong. I only faintly understood about pollination. I did know that the industrious bees did the most of it. “I guess we should have brought some bees over in our pockets too.” He looked at me like I was his spanking newborn baby; as if he loved me terribly but the world would never be what any of us had hoped for. “Rae Ann, honey,” he said, “you can’t bring the bees. You might as well bring the whole world over here with you, and there’s no room for it.”
Pollination is complicated. Beetles may be useful pollinators to some plants, but voracious herbivores to others. Bees with broad floral predilections (we call them generalists) and little floral constancy may take the pollen from one plant and carelessly deliver it to an unrelated plant, squandering that chance for a fruitful encounter and possibly clogging up the stigma (female landing pad for male pollen) of the second plant with the wrong kind of pollen, hindering its opportunity to produce viable seeds also. Specialist bees, on the other hand, may be so adept at grooming off pollen collected in their specialized hairs to take home to their nest that they bear little for delivery to the next flower. Nectar robbers are pollinators that have learned to bypass the arduous work of climbing into a flower past the anthers full of pollen dust, and instead just slit the throat, so to speak, of the flower from underneath to drain its nectar without making contact with the plants’ reproductive parts. Rewardless flowers, in turn, employ false floral advertising to draw in pollinators to where they can transfer their pollen package, but neglect to expend the energy to fill their nectar reservoirs for them. I may have painted you a pretty picture above of pollination as a harmonious partnership between disparate organisms in a harsh world, but, like most every thing else in cruel nature, it is a constant “arms race” to take more and give less. This, of course, makes it all the more interesting, as we are now talking about an elaborate game of intrigue, deception, and carefully balanced manipulation, all for the chance of a sexual encounter between two plants across a field from each other. Hearten up, though, there are exceptions. True mutualisms do exist. The yucca moth (Prodoxidae), for example, actively collects yucca pollen grains using its modified elongated mouthparts, then climbs into another yucca flower and forces them into a receptive female stigma. She then uses her ovipositor to pierce the flowers’ ovary and deposit her eggs, which upon hatching rely exclusively upon fertilized yucca seeds to fuel their development. This mutualism is so highly developed that some yucca plants cannot reproduce without the purposeful labors of their particular yucca moth, and the yucca moth will rear no young in the absence of reproducing yucca plants. Yucca flowers even offer protective refuge to these small, white moths, as you will find if you pull the petals apart and peak in some summer day. Even still, there is a balancing act going on between these two players. If the moth lays too many eggs so that too high a proportion of yucca seeds will be consumed by them, the plant will abort that flower and cease to invest resources in it. So the checks are in place and the partnership continues.
Pollination is fragile. As you might imagine, obligate mutualisms like that of the yucca and yucca moth described above are risky. If either suffers a sudden parasite or habitat loss, it spells certain doom for the other, as no substitute will do in this highly specialized relationship. Pollinator limitation is another risk plants face when contracting out the transfer of their reproductive goods. Successful pollination is not simply a matter of a pollen grain or two hitting its mark and smooth sailing to next year’s baby plants (seedlings) from there. Quality pollination often requires substantial deposition of exactly the right kind of pollen–not too similar (self), but not too different (heterospecific)–for plant populations to maintain healthy genetic transfer and achieve high seed set. Since these two communities of plants and pollinators support each others’ growth and reproduction year after year, shaving down or otherwise impeding the abundance of one may whittle down the other as well. The term ‘Allee effect‘ describes a concept of positive density dependence in populations, where individuals fare better when local abundance of their species is high (as opposed to competitive interactions leading to negative density dependence). When habitat fragmentation, due most often to land degradation and development, leads to plant populations becoming isolated in patches too small to nourish and sustain their pollinator populations, the whole house of cards can collapse. Sometimes one population of plants is actually propped up by the existence of an unrelated group of plants that supports the same pollinators earlier in the season, as is the case with Delphinium flowers nourishing alpine hummingbirds, whose retention in the same meadows is required by later-blooming Ipomopsis flowers. Pesticides and exotic species may also introduce enough stress into a pollination network that the delicate balance of resource transfer and production between original mutualistic members topples to its end. Migrating pollinators, such as bats, depend on the persistence of rich corridors of nectar to sustain them along their entire journeys. A field of corn where they expected a stand of nocturnally blooming cacti may halt their progress in its tracks and leave the cacti further down the line equally wanting. Not only in space, but unexpected shifts in time can be catastrophic to the fluidity of pollen transfer as well. Among its many other endearing qualities, climate change has been shown to be inducing phenological shifts in bloom times of alpine plants in relation to the flight times of their pollinators. When solitary bees emerge from their underground overwintering nests to find that the flowers expecting their services, and upon which they depend, have already withered or are yet to bud, an emergency state comes into play for both parties, the outcome of which we have yet to discern.
Still, pollination is powerful. Charles Darwin called the origin of angiosperms “an abominable mystery.” From what we now understand, the near infinite diversity of flowering plants is thanks largely to the behavioral bents of their pollinators. Perpetually seeking to attract more pollinators than its neighbors, plants are constantly evolving new floral colors, shapes, fragrances, and other attributes to help them stand out among the competition. Because pollinators’ refined floral search images allow them to focus on a particular group of plants and transfer conspecific pollen appropriately, a higher diversity of plants can coexist than is feasible among plants banking on highly density-dependent wind pollen transfer. The geographic distribution of many plants is also directly connected to the wanderings of their pollinators, who may bring pollen to the outer reaches of a plant population and allow it to extend its range by radiating seedlings in new directions. If, on the other hand, pollinators neglect to connect two detached populations of a plant species, the reduced gene flow between them may induce allopatric speciation. But the circle of influence does not stop there. Herbivores have preferences too, as do the carnivores that eat them and structure our landscapes in many other ways. The existence of such a variety of blooming plants has allowed for specialization and diversification all the way up the food chain. Bees and their pollinating brethren, you could say, lay the groundwork for the much of the macroscopic biodiversity on earth, supporting plants that prevent habitat erosion while they shelter and support herbivores, who feed and drive the movements of carnivores. The camel, for example, before it was fed grass by its domesticators, relied on sparse and thorny xeric plants likely pollinated by desert-loving bees. Now thanks in part to bees, you can ride a camel to a beautiful sandy vista and enjoy a camel lasagna and camel ice cream picnic on a camel hair blanket. Pretty impressive, huh?
Pollination is essential. In addition to providing fodder for delightful creatures such as the camel above, pollinators are also key players in the production of many primary agricultural crops and cultivated livestock feed…meaning that pollinators fuel your delightful existence too, and that understanding the ways in which diverse pollinators interact with their environments is connected to the security of our food system. The squash bee, Peponapis pruinosa, is an important pollinator of wild and cultivated squash and pumpkins. Alfalfa, a major livestock feed crop with distinctively shaped pea-family flowers, receives efficient pollination services from alfalfa leafcutter bees, Megachile rotundata, uniquely effective at “tripping” alfalfa flowers in such a way that the keel petal pops up and deposits pollen onto the bee’s head before allowing access to nectar. (The common honey bee declines to play this game and will often cut into the flower from below to rob nectar stores, skipping pollen transfer all together.) Infamous honey bees are credited with providing valuable pollination services to such crops as almond, blueberry, apple, asparagus, avocado, broccoli, watermelon, canola, cherry, beets, and many other products you would miss. It has been estimated that one in three mouthfuls are thanks to pollinators (elsewhere this is cited as thanks specifically to bees), and certainly the variety and color on our plates would suffer in the absence of pollinators. Einstein has been (probably mis-) quoted as saying that if all the bees died tomorrow, humans would have only four years to live. And they would be four years of bland, grain-based diets, anemia, maybe scurvy, not to mention angry, constipated neighbors and possibly zombies. The US Department of Agriculture deems pollination ecology important enough to fund a dozen scientists to hang out in Logan, Utah and look at dead bees under a microscope all day, and to support the maintenance of a collection of over one million bees from all over the world, carefully curated in glass drawers so heavy I worry about it falling through our building floor onto the water lab people below one day. The idea behind this use of your tax dollars is that, since pollination is essential, the more we can unravel about the mysterious life histories, distributions, and foraging habits of the vastly diverse pollinators, the better equipped we are to avoid coming home one day to find our scurvy-ridden friends raiding our pantries for fruit preserves. Pollinators, and specifically bees, are the shoulders (technically terga) upon which our entire food system stands. If we are unable to resurrect the honey bee to its former pollinating glory, it will be critical to understand ways in which we might enlist special-ops pollinators like the alfalfa leafcutting bee and the squash bee. So aside from the beauty, lyricism, complexity, fragility and power of pollination, its essential connection to food production might be the most obvious reason to study and revere pollinators. Perhaps I should apply for funding from the US Department of Defense. Knowledge is power. And, clearly, bees are the anti-Apocalypse.
Bartomeus, I., Ascher, J.S., Wagner, D., Danforth, B.N., Colla, S., Kornbluth, S., and Winfree, R. (2011). Climate-associated phenological advances in bee pollinators and bee-pollinated plants. Proc. Natl. Acad. Sci. 108, 20645–20649.
Buchmann, S.L., and Nabhan, G.P. (1996). The forgotten pollinators (Island Press).
Chittka, L., and Thomson, J.D. (2001). Cognitive Ecology of Pollination: Animal Behaviour and Floral Evolution (Cambridge University Press).
Delaplane, K.S., and Mayer, D.F. (2000). Crop Pollination by Bees (CABI).
Waser, N.M., and Ollerton, J. (2006). Plant-Pollinator Interactions: From Specialization to Generalization (University of Chicago Press).
Willmer, P. (2011). Pollination and Floral Ecology (Princeton University Press).