pollination biology

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Year of Pollination: Most Effective Pollinator Principle and Beyond, part two

“The most effective pollinator principle implies that floral characteristics often reflect adaptation to the pollinator that transfers the most pollen, through a combination of high rate of visitation to flowers and effective deposition of pollen during each visit.” – Mayfield, et al., Annals of Botany (2001) 88 (4): 591-596

In part one, I reviewed a chapter by Jose M. Gomez and Regino Zamora in the book Plant-Pollinator Interactions: From Specialization to Generalization that argues that the most effective pollinator principle (MEPP) “represents just one of multiple evolutionary solutions.” In part two, I summarize a chapter by Paul A. Aigner in the same book that further explains how floral characteristics can evolve without strictly adhering to the MEPP.

maximilian sunflower
Aigner is interested in how specialization develops in different environments and whether or not flowering plants, having adapted to interact with a limited number of pollinators, experience trade-offs. A trade-off occurs when a species or population adapts to a specific environmental state and, in the process, loses adaptation to another state. Or in other words, a beneficial change in one trait results in the deterioration of another. Trade-offs and specialization are often seen as going hand in hand, but Aigner argues that trade-offs are not always necessary for an organism to evolve towards specialization. Plant-pollinator interactions provide an excellent opportunity to test this.

“Flowers demand study of specialization and diversification,” Aigner writes, not only due to their ubiquity, “but because much of the remarkable diversity seen in these organisms is thought to have evolved in response to a single and conspicuous element of the environment – pollination by animals.” If pollinators have such a strong influence on shaping the appearance of flowers, pollination studies should be rife with evidence for trade-offs, but they are not. Apart from not being well-studied, Aigner has other ideas about why trade-offs are not often observed in this scenario.

Aigner is particularly interested in specialization occuring in fine-grained environments. A course-grained environment is “one in which an organism experiences a single environmental state for all of its life.” Specialization is well understood in this type of environment. A fine-grained environment is “one in which an organism experiences all environmental states within its lifetime,” such as “a flowering plant [being] visited by a succession of animal pollinators.” For specialization to develop in a fine-grained environment, a flowering plant must “evolve adaptations to a particular type of pollinator while other types of pollinators are also present.”

It’s important to note that the specialization that Aigner mainly refers to is phenotypic specialization. That is, a flower’s phenotype [observable features derived from genes + environment] appears to be adapted for pollination by a specific type of pollinator, but in fact may be pollinated by various types of pollinators. In other words, it is phenotypically specialized but ecologically generalized. Aigner uses a theoretical model to show that specialization can develop in a fine-grained environment with and without trade-offs. He also uses his model to demonstrates that a flower’s phenotype does not necessarily result from its most effective pollinator acting as the most important selection agent. Instead, specialization can evolve in response to a less-effective pollinator “when performance gains from adapting to the less-effective pollinator can be had with little loss in the performance contribution of the more effective pollinator.”

Essentially, Aigner’s argument is that the agents that are the most influential in shaping a particular organism are not necessarily the same agents that offer the greatest contribution to that organism’s overall fitness. This statement flies in the face of the MEPP, and Aigner backs up his argument with (among other examples) his studies involving the genus Dudleya.

Dudleya saxosa (panamint liveforever) - photo credit: wikimedia commons

Dudleya saxosa (panamint liveforever) – photo credit: wikimedia commons

Dudleya is ecologically generalized. Pollinators include hummingbirds, bumblebees, solitary bees, bee flies, hover flies, and butterflies. “Some Dudleya species and populations are visited by all of these taxa, whereas others seem to be visited by only a subset.” Aigner was curious to see if certain species or populations were experiencing trade-offs by adapting to a particular category of pollinators. Aigner found variations in flower characteristics among species and populations as well as differences in pollinator assemblages that visited the various groups of flowers over time but could not conclude that there were trade-offs “in pollination performance.”

In one study, he looked at pollination services provided by hummingbirds vs. bumblebees as corolla flare changed in size. In male flowers, bumblebees were efficient at removing pollen regardless of corolla flare size, while hummingbirds removed pollen more effectively as corolla flare decreased. Both groups deposited pollen more effectively as corolla flare decreased, but hummingbirds more strongly so. Ultimately, Aigner concluded that “the interactions did not take the form of trade-offs,” or, as stated in the abstract of the study, ” phenotypic specialization [for pollination by hummingbirds] might evolve without trading-off the effectiveness of bumblebees.”

Aigner goes on to explain why floral adaptations may occur without obvious trade-offs. One reason is that different groups of pollinators are acting as selective agents for different floral traits, “so that few functional trade-offs exist with respect to individual traits.” Pollinators have different reasons for visiting flowers and flowers use the pollination services of visitors differently. Another reason involves the “genetic architecture” of the traits being selected for. Results can differ depending on whether or not the genes being influenced are linked to other genes, and genetically based fitness trade-offs may not be observable phenotypically. Further studies involving the genetic architecure of specialized phenotypes are necessary.

And finally, as indicated in part one, pollinators are not the only floral visitors. In the words of Aigner, “if floral larcenists and herbivores select for floral traits in different directions than do pollinators, plants may face direct trade-offs in improving pollination service versus defending against enemies.” These “floral enemies” can have an effect on the visitation rates and per-visit effectiveness of pollinators, which can drastically alter their influence as selective agents.

Like pollination syndromes, the most effective pollinator principle seems to have encouraged and directed a huge amount of research in the field of pollination biology, despite not holding entirely true in the real world. As research continues, a more complete picture will develop. It doesn’t appear that it will conform to an easily digestible principle, but there is no question that, even in its complexity, it will be fascinating.

I will end as I began, with an excerpt from Thor Hanson’s book, The Triumph of Seeds: “The notion of coevolution implies that change in one organism can lead to change in another – if antelope run faster, then cheetahs must run faster still to catch them. Traditional definitions describe the process as a tango between familiar partners, where each step is met by an equal and elegant counter-step. In reality, the dance floor of evolution is usually a lot more crowded. Relationships like those between rodents and seeds [or pollinators and flowers] develop in the midst of something more like a square dance, with couples constantly switching partners in a whir of spins, promenades, and do-si-dos. The end result may appear like quid pro quo, but chances are a lot of other dancers influenced the outcome – leading, following, and stepping on toes along the way.”

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Year of Pollination: Most Effective Pollinator Principle and Beyond, part one

Have you ever considered the diversity of flowers? Why do they come in so many different shapes, sizes, and colors? And why do they produce so many different odors – or none at all? Flowering plants evolved around 140 million years ago, a fairly recent emergence evolutionarily speaking. Along with them evolved numerous species of insects, birds, and mammals. In his book, The Triumph of Seeds, Thor Hanson describes the event this way: “In nature, the flowering plants put sex, seeds, and dispersal on full display, spurring not only their own evolution but also that of the animals and insects with which they became so entwined. In most cases, the diversity of dispersers, consumers, parasites – and, most especially, pollinators – rose right alongside that of the plants they depended upon.”

Speaking of dependence, most flowering plants depend upon pollinators for successful reproduction – it is, for the most part, a mutually beneficial relationship. Even the casual observer of flowers will note that a large portion of the creatures that visit them appear to be pollinators. Thus, it is no wonder that pollination biologists have given pollinators so much credit in shaping the flowers that we see today.

Consider G. Ledyard Stebbins and his Most Effective Pollinator Principle which he defined in a paper published in 1970: “the characteristics of the flower will be molded by those pollinators that visit it most frequently and effectively in the region where it is evolving.” He then goes on to reference pollination syndromes, a phenomenon that describes how the traits of flowers are best suited for their “predominant and most effective vector[s].” In my post about pollination syndromes a few months ago, I discussed how a strict adherence to this concept has waned. In the next two posts, I discuss how the Most Effective Pollinator Principle (MEPP) may not be the best way to explain why flowers look the way they do.

 

To make this argument I am drawing mainly from two chapters in the book Plant-Pollinator Interactions: From Specialization to Generalization. The first is “Ecological Factors That Promote the Evolution of Generalization in Pollination Systems” by Jose M. Gomez and Regino Zamora, and the second is “The Evolution of Specialized Floral Phenotypes in a Fine-grained Pollination Environment” by Paul A. Aigner.

According to Aigner the MEPP “states that a plant should evolve specializations to its most effective pollinators at the expense of less effective ones.” And according to Gomez and Zamora it “states that natural selection should modify plant phenotypes [observable characteristics derived from interactions between a plant’s genes and its surrounding environment] to increase the frequency of interaction [between] plants and the pollinators that confer the best services,” and so “we would expect the flowers of most plants to be visited predominantly by a reduced group of highly effective pollinators.” This is otherwise known as adaptive specialization.

Specialization is something that, in theory, plants are generally expected to evolve towards, particularly in regards to plant-pollinator relationships. Observations, on the other hand, demonstrate the opposite – that specialization is rare and most flowering plants are generalists. However, the authors of both chapters advise that specialization and generalization are extreme ends to a continuum, and that they are comparative terms. One species may be more specialized than another simply because it is visited by a smaller “assemblage” of pollinators. The diversity of pollinators in that assemblage and the pollinator availability in the environment should also be taken into consideration when deciding whether a relationship is specialized or generalized.

That pollinators can be agents in shaping floral forms and that flowering plant species can become specialized in their interactions with pollinators is not the question. There is evidence enough to say that it occurs. However, that the most abundant and/or effective pollinators are the main agents of selection and that specialization is a sort of climax state in the evolutionary process (as the MEPP seems to suggest) is up for debate. Generalization is more common than specialization, despite observations demonstrating that pollinators are drawn to certain floral phenotypes. So, could generalization be seen as an adaptive strategy?

In exploring this question, Gomez and Zamora first consider what it takes for pollinators to act as selective agents. They determine that “pollinators must first benefit plant fitness,” and that when calculating this benefit, the entire life cycle of the plant should be considered, including seed germination rate, seedling survival, fecundity, etc. The ability of a pollinator species to benefit plant fitness depends on its visitation rate and its per-visit effectiveness (how efficiently pollen is transferred) – put simply, a pollinator’s quantity and quality during pollination. There should also be “among-pollinator differences in the evolutionary effect on the plant,” meaning that one species or group of pollinators – through being more abundant, effective, or both – contributes more to plant fitness compared to others. “Natural selection will favor those plant traits that attract the most efficient or abundant pollinators and will also favor the evolution of the phenotypes that cause the most abundant pollinators to also be the most effective.” This process implies possible “trade-offs,” which will be discussed in part two.

When pollinators act as selective agents in this way, the MEPP is supported; however, Gomez and Zamora argue that this scenario “only takes place when some restrictive ecological conditions are met” and that while specialization can be seen as the “outcome of strong pollinator-mediated selection,” generalization can also be “mediated by selection exerted by pollinators…in some ecological scenarios.” This is termed adaptive generalization. In situations where ecological forces constrain the development of specialization and pollinators are not seen as active selection agents, nonadaptive generalization may be occurring.

Gomez and Zamora spend much of their chapter exploring “several causes that would fuel the evolution of generalization” both adaptive and nonadaptive, which are outlined briefly below.

  • Spatiotemporal Variability: Temporal variability describes differences in pollinator assemblages over time, both throughout a single year and over several years. Spatial variability describes differences in pollinator assemblages both among populations where gene flow occurs and within populations. Taken together, such variability can have a measurable effect on the ability of a particular pollinator or group of pollinators to act as a selective agent.
  • Similarity among Pollinators: Different pollinator species can have “equivalent abundance and above all comparable effectiveness” making them “functional equivalents from the plant perspective.” This may be the case with both closely and distantly related species. Additionally, a highly effective pollinator can select for floral traits that attract less effective pollinators.
  • The Real Effects on Plant Fitness: An abundant and efficient pollinator may select for one “fitness component” of a plant, but may “lead to a low overall effect on total fitness.” An example being that “a pollinator may benefit seed production by fertilizing many ovules but reduce seedling survival because it causes the ripening of many low-quality seeds.” This is why “as much of the life cycle as possible” should be considered “in assessing pollinator effectiveness.”
  • Other Flower Visitors: Pollinators are not the only visitors of flowers. Herbivores, nectar robbers, seed predators, etc. may be drawn in by the same floral traits as pollinators, and pollinators may be less attracted to flowers that have been visited by such creatures. “Several plant traits are currently thought to be the evolutionary result of conflicting selection exerted by these two kinds of organisms,” and “adaptations to avoid herbivory can constrain the evolution of plant-pollinator interactions.”

This, of course, only scratches the surface of the argument laid out by Gomez and Zamora. If this sort of thing interests you, I highly encourage you to read their chapter. Next week I will summarize Aigner’s chapter. If you have thoughts on this subject or arguments to make please don’t hesitate to comment or contact me directly. This is a dialogue, dudes.

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Year of Pollination: Mosquitoes as Pollinators

It is difficult to have positive feelings about mosquitoes, especially during summer months when they are out in droves and our exposed skin – soft, supple, and largely hair-free – is irresistible to them. We are viewed as walking blood meals by female mosquitoes who are simply trying to produce young – to perpetuate their species just like any other species endeavors to do. Unfortunately, we are left with small, annoying bumps in our skin – red, itchy, and painful – risking the possibility that the mosquitoes that just drew our blood may have passed along any number of mosquito-borne diseases, some (such as malaria) that potentially kill millions of people every year. For this, it is okay to hate mosquitoes and to long for the day of their complete eradication from the planet. However, their ecological roles (and yes, they do have some) are also worth considering.

There are more than 3,500 species of mosquito. Luckily, only 200 or so consume human blood. Mosquitoes go back at least 100 million years and have co-evolved with species of plants and animals found in diverse habitats around the world. Adult mosquitoes and their larvae (which live in standing water) provide food for a wide variety of creatures including birds, bats, insects, spiders, fish, frogs, lizards, and salamanders. Mosquito larvae also help break down organic matter in the bodies of water they inhabit. They even play an important role in the food webs found inside the pitchers of northern pitcher plants (Sarracenia spp.). Interestingly enough, Arctic mosquitoes influence the migration patterns of caribou. They emerge in swarms so big and so voracious that they have been said to kill caribou through either blood loss or asphyxiation.

However, blood is not the main food source of mosquitoes; flower nectar is. Males don’t consume blood at all, and females only consume it when they are producing eggs. Any insect that visits flowers for nectar has the potential to unwittingly collect pollen and transfer it to a nearby flower, thereby aiding in pollination. Mosquitoes are no exception. They have been observed acting as pollinators for a handful of species, and could be acting as pollinators for many more.

Bluntleaved orchid (Platanthera obtusata) is pollinated by mosquitoes. phot credit: wikimedia commons

Bluntleaved orchid (Platanthera obtusata) is pollinated by mosquitoes. photo credit: wikimedia commons

The scientific literature describes the pollination by mosquitoes of at least two plant species: Platanthera obtusata (syn. Habenaria obtusata) and Silene otites. P. obtusata – bluntleaved orchid – is found in cold, wet regions in North America and northern Eurasia. It is pollinated by mosquitoes from multiple genera including several species in the genus Aedes. Mosquitoes visit the flowers to feed on the nectar and, subsequently, pollinia (clusters of pollen) become attached to their eyes and are moved from flower to flower. This scenario likely plays out in other species of Arctic orchids as well*.

S. otites – Spanish catchfly – is a European species that is pollinated by mosquitoes and moths. Researches have been studying the floral odors of S. otites that attract mosquitoes, suggesting that determining the compounds involved in these odors “might lead to the development of new means of pest control and mosquito attractants and repellents.”

Northern House Mosquito (Culex pipiens) - one of the species of mosquitoes that has been observed pollinating Silene otitis. photo credit: www.eol.org

Northern House Mosquito (Culex pipiens) – one of the species of mosquitoes that has been observed pollinating Silene otites. photo credit: www.eol.org

Despite the list of functions that mosquitoes serve in their varied habitats, an article that appeared in Nature back in 2010 argues for wiping mosquitoes off the Earth, stating that “the ecological scar left by a missing mosquito would heal quickly as the niche was filled by other organisms.” And even though “thousands of plant species would lose a group of pollinators,” mosquitoes are not important pollinators of the “crops on which humans depend,” nor do they appear to be the sole pollinator of any single plant species [the species mentioned above are pollinated by other insects as well]. Eliminating mosquitoes, however, is more of a pipe dream than a realistic possibility as our “best efforts can’t seriously threaten an insect with few redeeming features.”

*Speaking of orchids and pollination, endless posts could be written about this incredibly fascinating and diverse group of plants and their equally fascinating and complex mechanisms surrounding pollination. There will be more to come on such topics. Meanwhile, it should be noted that orchids are also a notoriously threatened group of plants. To learn more about orchids and orchid conservation in North America, visit North American Orchid Conservation Center.

Read more about mosquito pollination here.

And now for your listening pleasure:

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Year of Pollination: An Argentinian Cactus and Its Unlikely Pollinator

A few weeks ago I wrote about pollination syndromes – sets of floral triats that are said to attract specific groups of pollinators. In that post I discussed how pollination syndromes have largely fallen out of favor as a reliable method of predicting the pollinators that will visit particular flowers. In this post I review a recent study involving a species of cactus in Argentina that, as the authors state in their abstract, “adds another example to the growing body of mismatches between floral syndrome and observed pollinator.”

Denmoza rhodacantha is one of many species of cacti found in Argentina. It is the only species in its genus, and it is widely distributed across the east slopes and foothills of the Andes. It is a slow growing cactus, maintaining a globulous (globe-shaped) form through its juvenile phase and developing a columnar form as it reaches maturity. D. rhodacantha can reach up to 4 meters tall and can live beyond 100 years of age. Individual plants can begin flowering in their juvenile stage. Flowers are red, nectar rich, scentless, and tubular. The stigma is lobed and is surrounded by a dense grouping of stamens. Both male and female reproductive organs are extended above the corolla. The flowers have been described by multiple sources as being hummingbird pollinated, not based on direct observation of hummingbirds visiting the flowers, but rather due to the floral traits of the species.

Denmoza rhodacantha illustration - image credit: www.eol.org

Denmoza rhodacantha illustration  (image credit: www.eol.org)

In a paper entitled, Flowering phenology and observations on the pollination biology of South American cacti – Denmoza rhodacantha, which was published in volume 20 of Haseltonia (the yearbook of the Cactus and Succulent Society of America), Urs Eggli and Mario Giorgetta discuss their findings after making detailed observations of a population of D. rhodacantha in early 2013 and late 2013 – early 2014. The population consisted of about 30 individuals (both juveniles and adults) located in the Calchaqui Valley near the village of Angastaco, Argentina. At least three other species with “hummingbird-syndrome flowers” were noted in the area, and three species of hummingbirds were observed during the study periods. Over 100 observation hours were logged, and during that time “the studied plants, their flowering phenology, and flower and fruit visitors were documented by means of photographs and video.”

The flowers of D. rhodacantha only persist for a few short days, and in that time their sexual organs are only receptive for about 24 hours. The flowers are self-sterile and so require a pollinator to cross pollinate them. Despite their red, tubular shape and abundant nectar, no hummingbirds were observed visiting the flowers. One individual hummingbird approached but quickly turned away. Hummingbirds were, however, observed visiting the flowers of an associated species, Tecoma fulva ssp. garrocha. Instead, a species of halictid bee (possibly in the genus Dialictus) was regularly observed visiting the flowers of D. rhodacantha. The bees collected pollen on their hind legs and abdomen and were seen crawling across the lobes of the stigma. None of them were found feeding on the nectar. In one observation, a flower was visited by a bee that was “already heavily loaded with the typical violet-coloured pollen of Denmoza,” suggesting that this particular bee species was seeking out these flowers for their pollen. Small, unidentified beetles and ants were seen entering the flowers to consume nectar, however they didn’t appear to be capable of offering a pollination service.

D. rhodacantha populations have been observed in many cases to produce few fruits, suggesting that pollination success is minimal. The authors witnessed “very low fruit set” in the population that they were studying, which was “in marked contrast to the almost 100% fruit set rates of the sympatric cactus species at the study site.” This observation wasn’t of great concern to the authors though, because juvenile plants are present in observed populations, so recruitment appears to be occurring. In this study, dehisced fruits were “rapidly visited by several unidentified species of ants of different sizes.” The “scant pulp” was harvested by smaller ants, and larger ants carried away the seeds after “cleaning them from adhering pulp.”

The authors propose at least two reasons why hummingbirds avoid the flowers of D. rhodacantha. The first being that the native hummingbirds have bills that are too short to reach the nectar inside the long tubular flowers, and often the flowers barely extend beyond the spines of the cactus which may deter the hummingbirds from approaching. The second reason is that other plants in the area flower during the same period and have nectar that is easier to gather. The authors acknowledge that this is just speculation, but it could help explain why the flowers are pollinated instead by an insect (the opportunist, generalist halictid bee species) for whom the flowers “could be considered to be ill adapted.” The authors go on to say, “it should be kept in mind, however, that adaptions do not have to be perfect, as long as they work sufficiently well.”

Patagona gigas (giant hummingbird) was observed approaching the flower of a Denmoza rhodacantha but quickly turned away (photo credit: www.eol.org)

Patagona gigas (giant hummingbird) was observed approaching the flower of a Denmoza rhodacantha but quickly turned away (photo credit: www.eol.org)

More Year of Pollination posts on Awkward Botany:

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Year of Pollination: Pollination Syndromes and Beyond

A discussion of pollination syndromes should begin with the caveat that they are a largely outdated way to categorize plant-pollinator interactions. Still, they are important to be aware of because they have informed so much of our understanding about pollination biology, and they continue to be an impetus for ongoing research. The concept of pollination syndromes exists in part because we are a pattern seeking species, endeavoring to place things in neat little boxes in order to make sense of them. This is relatively easy to do in a hypothetical or controlled environment where the parameters are selected and closely monitored and efforts are made to eliminate noise. However, the real world is considerably more dynamic than a controlled experiment and does not conform to black and white ways of thinking. Patterns are harder to unveil, and it takes great effort to ensure that observed patterns are genuine and not simply imposed by our pattern seeking brains.

That being said, what are pollination syndromes?  Pollination syndromes are sets of floral traits that are thought to attract specific types of pollinators. The floral traits are considered to have evolved in order to appeal to a particular group of pollinators – or in other words, selective pressures led to adaptations resulting in mutualistic relationships between plants and pollinators. Pollination syndromes are examples of convergent evolution because distantly related plant species have developed similar floral traits, presumably due to similar selection pressures. Pollination syndromes were first described by Italian botanist, Federico Delpino, in the last half of the 19th century. Over several decades his rudimentary ideas were fleshed out by other botanists, resulting in the method of categorization described (albeit briefly) below.

Honey bee on bee's friend (Phacelia tanacetifolia)

A honey bee getting friendly with bee’s friend (Phacelia tanacetifolia)

Pollination by bees (melittophily) – Flowers are blue, purple, yellow, or white and usually have nectar guides. Flowers are open and shallow with a landing platform. Some are non-symmetrical and tubular like pea flowers. Nectar is present, and flowers give off a mild (sometimes strong) sweet scent.

Pollination by butterflies (psychophily) – Flowers are pink, purple, red, blue, yellow, or white and often have nectar guides. They are typically large with a wide landing pad. Nectar is inside a long, narrow tube (or spur), and flowers have a sweet scent.

Pollination by hawkmoths and moths (sphingophily and phalaenophily) – Moth pollinated flowers open at night, have no nectar guides, and emit a strong, sweet scent. Flowers pollinated by hawkmoths are often white, cream, or dull violet and are large and tubular with lots of nectar. Those pollinated by other moths are smaller, not as nectar rich, and are white or pale shades of green, yellow, red, purple, or pink.

Pollination by flies (myophily or sapromyophily) – Flowers are shaped like a basin, saucer, or kettle and are brown, brown-red, purple, green, yellow, white, or blue.  Some have patterns of dots and stripes. If nectar is available, it is easily accessible. Their scent is usually putrid. A sapromyophile is an organism that is attracted to carcasses and dung. Flies that fall into this category visit flowers that are very foul smelling, offer no nectar reward, and essentially trick the fly into performing a pollination service.

Pollination by birds (ornithophily) –  Flowers are usually large, tubular, and red, orange, white, blue, or yellow. They are typically without nectar guides and are odorless since birds don’t respond to scent. Nectar is abundant and found at various depths within the flower.

Pollination by bats (chiropterophily) – Flowers are large, tubular or bell shaped, and white or cream colored with no nectar guides. They open at night, have abundant nectar and pollen, and have scents that vary from musty to fruity to foul.

Pollination by beetles (cantharophily) – Flowers are large and bowl shaped and green or white. There are no nectar guides and usually no nectar. The scent is strong and can be fruity, spicy, or putrid. Like flies, some beetles are sapromyophiles.

Locust borer meets rubber rabbitbrush (Ericameria nauseosa)

A locust borer meets rubber rabbitbrush (Ericameria nauseosa)

In addition to biotic pollination syndromes, there are two abiotic pollination syndromes:

Pollination by wind (anemophily) – Flowers are miniscule and brown or green. They produce abundant pollen but no nectar or odor. The pollen grains are very small, and the stigmas protrude from the flower in order to capture the windborne pollen.

Pollination by water (hydrophily) –  Most aquatic plants are insect-pollinated, but some have tiny flowers that release their pollen into the water, which is picked up by the stigmas of flowers in a similar manner to plants with windborne pollen.

This is, of course, a quick look at the major pollination syndromes. More complete descriptions can be found elsewhere, and they will differ slightly depending on the source. It’s probably obvious just by reading a brief overview that there is some overlap in the floral traits and that, for example, a flower being visited by a bee could also be visited by a butterfly or a bird. Such an observation explains, in part, why this method of categorizing plant-pollinator interactions has fallen out of favor. Studies have been demonstrating that this is not a reliable method of predicting which species of pollinators will pollinate certain flowers. A close observation of floral visitors also reveals insects that visit flowers to obtain nectar, pollen, and other items, but do not assist in pollination. These are called robbers. On the other hand, a plant species may receive some floral visitors that are considerably more effective and reliable pollinators than others. What is a plant to do?

Pollination syndromes imply specialization, however field observations reveal that specialization is quite rare, and that most flowering plants are generalists, employing all available pollinators in assisting them in their reproduction efforts. This is smart, considering that populations of pollinators fluctuate from year to year, so if a plant species is relying on a particular pollinator (or taxonomic group of pollinators) to aid in its reproduction, it may find itself out of luck. Considering that a flower may receive many types of visitors on even a semi-regular basis suggests that the selective pressures on floral traits may not solely include the most efficient pollinators, but could also include all other pollinating visitors and, yes, even robbers. This is an area where much more research is needed, and questions like this are a reason why pollination biology is a vibrant and robust field of research.

A bumble bee hugs Mojave sage (Salvia pachyphylla)

A bumble bee hugs the flower of a blue sage (Salvia pachyphylla)

Interactions between plants and pollinators is something that interests me greatly. Questions regarding specialization and generalization are an important part of these interactions. To help satiate my curiosity, I will be reading through a book put out a few years ago by the University of Chicago Press entitled, Plant-Pollinator Interactions: From Specialization to Generalization, edited by Nickolas M. Waser and Jeff Ollerton. You can expect future posts on this subject as I read through the book. To pique your interest, here is a short excerpt from Waser’s introductory chapter:

Much of pollination biology over the past few centuries logically focused on a single plant or pollinator species and its mutualistic partners, whereas a focus at the level of entire communities was uncommon. Recently we see a revival of community studies, encouraged largely by new tools borrowed from the theory of food webs that allow us to characterize and analyze the resulting patterns. For example, pollination networks show asymmetry – most specialist insects visit generalist plants, and most specialist plants are visited by generalist insects. This is a striking departure from the traditional implication of coevolved specialists!

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