I am still running low on photos, though I have a handful of contributions, and would greatly appreciate any readers sending in their good photos. Thanks!
Today we have a text-and-photo natural history and medical lesson from Athayde Tonhasca Júnior; his narrative is indented and you can enlarge the photos by clicking on them.
Suspicions and proofs
From a hundred rabbits you can’t make a horse, a hundred suspicions don’t make a proof (Fyodor Dostoyevsky)
Few people contributed more with entries to medical dictionaries than renowned German physician, anatomist and pathologist Friedrich Gustav Jacob Henle (1809-1885): Henle’s fissure, Henle’s layer, Henle’s ligament and Henle’s tubules are some of the several terms named after him. But there’s more from the good doctor: in a 1840 paper quirkily titled Von den Miasmen und Contagien (On Miasmas and Contagions), Henle championed the theory that microscopic organisms caused diseases, which was questioned by his peers. Later, Henle proposed the necessary steps to prove the theory, such as detecting the suspected agent in every case of the disease and establishing its absence in healthy people. These ideas were refined by Henle’s most famous student, Robert Koch (1843-1910), a future Nobel laureate, and are known today as the Henle-Koch postulates – although often, and unfairly, called just Koch postulates. These guidelines hold that the suspected microorganism must be present in every host affected by the disease but absent in healthy organisms; the microorganism must be isolated from a contaminated host and grown in a culture; the cultured microorganism should cause the disease when inoculated into a healthy organism; and the microorganism must be isolated again from the inoculated organism and identified as being identical to the original agent.
Dr Henle helped debunk the belief that diseases were caused by miasma, or bad air. Image in the public domain, Wikimedia Commons.
The original Henle-Koch postulates are no longer universally applicable because we learned quite a lot since they were formulated: for example, many pathogenic organisms are regularly found in healthy hosts, and the rules are not valid for viruses, which had not yet been discovered. Nonetheless, the postulates gave a rigorous scientific foundation to the emerging field of medical microbiology. By establishing a reliable causal relationship between microorganisms and infectious diseases, doctors could explore options for prevention and treatment.
Drs Friedrich Henle and Robert Koch were entitled to be proud of their work, but they probably would have been surprised to learn that their postulates are also relevant in the field of pollination ecology.
Pollination is the fundamental mechanism of plant reproduction. Since plants can’t go out on a date, they need an agent to transfer pollen for them. For nearly 90% of all wild flowering plants, this work is done by animals, mainly insects. And considering that more than 75% of the world’s most important crops benefit in some degree from animal pollination, identifying pollinating agents is enormously important for the economy and humanity’s well-being.
The apparently obvious way of recognizing pollinators is by checking out which creatures visit flowers. However, it has long been known that an insect or any other animal landing on a flower does not necessarily contribute to its pollination. Some visitors are nectar thieves: they take the flower’s nectar without touching stigmas or anthers. Others are nectar robbers: they get it through holes in the flower made by themselves or by previous visitors. Visitors in search of pollen may also contribute zilch to pollination if they eat pollen on the spot and take away few or no pollen grains. Sometimes they are too good at gathering it, carrying the entire loot back to their nests, leaving nothing for plant reproduction. In some cases, these pollination cheats can constitute the bulk of flower visitations. To complicate matters, visitors can be cheats and pollinators at the same time, or under different circ*mstances.
The sixteen-spot ladybird (Tytthaspis sedecimpunctata) is a keen flower visitor but an abysmal pollinator because it gobbles down pollen to its heart’s content © Gilles San Martin, Wikimedia Commons.
Pollination will happen only when pollen grains from the anthers (male parts of the plant) make their way to a stigma (female part). Evidence for this crucial event can be obtained by a sequence of conditions analogous to the Henle-Koch postulates. It must be demonstrated that pollen is transferred from anthers to the vector (the suspected pollinator), transported by the vector, and deposited on a receptive stigma by the vector (Cox & Knox, 1988).
The process of cross-pollination © Ali Niaz, Wikimedia Commons.
These steps seem straightforward, but they are in fact not easy to prove. King et al. (2013) had a go at it with observations from two Scottish sites and a deciduous forest in Costa Rica. They kept an eye on recently opened flowers from 13 species, waiting for the first visitor to alight on a bloom. Once that happened, the stigma from that flower was taken to a laboratory, where pollen grains were recovered, identified and counted. By carrying out repeated observations, the authors obtained estimates of single-visit deposition (SVD), which measures a visitor’s ability to take pollen from a given plant and deposit it in another plant where it can lead to fertilization. By estimating SVD values of the main flower visitors, the authors discovered that about 40% of them were not effective pollinators.
In southeast England, bramble (Rubus fruticosus agg.) flowers are frequently visited during daytime by flies, bumble bees (Bombus spp.) and the European honey bee (Apis mellifera). But an analyses of SVD revealed that most pollination is done at night by moths (Anderson et al., 2023). In a similar vein, Ballantyne et al. (2015) evaluated potential pollinators of heather (Erica tetralix, E. cinerea and Calluna vulgaris) and gorse (Ulex europaeus and U. minor) in Hyde Heath, England. By combining the frequency of flower visitation with SVD values, it was possible to establish who was pollinating what. Hoverflies were frequent visitors, but they deposited few pollen grains. The European honey bee, another regular visitor, was an efficient pollinator of common heather (C. vulgaris), but not as good as bumble bees for the other plants. In fact, SVD data for 76 plants from 30 families in Kenya, Israel and UK have confirmed the frequently reported observation that the European honey bee, despite often being the most abundant flower visitor, is a less effective pollinator than are solitary bees and bumble bees (Willmer et al., 2017).
Networks illustrating a combination of frequency of flower visitation and mean SVD for the main pollinators of heather and gorse. 1: Bombus terrestris/lucorum; 2: B. pascuorum; 3: B. lapidarius; 4: B. jonellus; 5: B. hortorum; 6: A. mellifera; 7: Halictidae; 8: other solitary bees © Ballantyne et al., 2015.
If you suspect that estimating SVD is hard work, you are right. There are alternatives, such as percentage of flowers that develop into fruit, fruit weight, fruit production, and so on. But these methods are equally laborious and not as precise as SVD. A great number of studies have used visitors’ features such as their abundance, hairiness and size, their pollen load, the number of stigmas touched, and the frequency and duration of flower visits. These are indicators of visitors’ potential, but not proof of effectiveness. For example, pollen attached to a visitor’s body may be lost on the way, or end up on incompatible or unreceptive stigmas. We need evidence of pollen deposition that may lead to fertilisation.
A marmalade fly (Episyrphus balteatus) on a grey-haired rockrose (Cistus creticus) flower. Pollination in action? Possibly, but the presence of pollen grains alone does not guarantee it © Aka, Wikimedia Commons.
Conservation organisations, academics, the press and social media have reiterated—often exaggeratedly—the imperiled state of pollination services. These concerns heighten the importance of safeguarding pollinators’ abundance and welfare, and the quality and extent of their habitats. Pollinators are in the spotlight, which opens opportunities for public involvement, new projects, funding – and to bandwagon jumping. A range of flower visitors have been claimed to be pollinators based solely on the fact that they are flower visitors. Even a tree frog was recently reported in the press as a new member of the pollinators club because one specimen was observed with pollen attached to its back. Could it be a pollinator? Possibly, but not likely: a frog or any animal may accidentally fertilise a flower, but that does not make them reliable, consistent pollinating vectors. Like any other scientific endeavour, progress in our understanding of pollination ecology and processes requires data resulting from hard work. Just listing creatures that fancy a pretty flower won’t do.
Flower-loving Pepé Le Pew is not likely to contribute to pollination © Prayitno, Wikimedia Commons.
