Athayde Tonhasca Júnior has returned with an edifying photo-and-text contribution, centering on his speciality, pollination. And once again, these phenomena show the powers of natural selection to affect both morphology and behavior.
Athayde’s captions are indented, and you can enlarge his photos by clicking on them.
O, what a tangled web we weave when first we practise to deceive
It’s a bright early morning in the American desert of southern Arizona, and a bee prepares to land on the flowers on the top of a fishhook barrel cactus (Ferocactus wislizeni). The approaching bee belongs to a group of solitary, ground-nesting species that specialize on the flowers of several cactus species, so unsurprisingly they are labelled cactus bees. These visitors are most welcome to the fishhook barrel cactus, as it depends entirely on them for pollination.
‘Fishhook’ says it all: the fishhook barrel cactus is well-armed against plant munchers © Bernard Gagnon, Wikimedia Commons:
Alas, the impending pollinating event does not happen because just before alighting, the bee spots danger: southern fire ants (Solenopsis xyloni) are milling about. Ants in general have a mood ranging from annoyed to furious, and fire ants live up to the family reputation: they readily attack perceived intruders, humans included, biting and injecting them with venom. Wisely, the bee flies away.
‘Fire’ says it all: despite its small size, the southern fire ant is aggressive and inflicts painful stings © Jake Nitta, Wikimedia Commons:
Fire ants are ground dwellers, spending their days scouring the soil surface for seeds and dead insects. They are also keen on sweet foodstuffs such as honeydew (the sugar-rich liquid secreted by aphids and some scale insects) and nectar, which explains their presence on the fishhook barrel cactus: they were gathering extrafloral nectar exuded from specialised spines on the top of the plant.
Close to 1.8% of all vascular plants, or ~4,000 species distributed across 108 families (these numbers are ever increasing), bear extrafloral nectaries (EFN): nectar-secreting glands located anywhere in the plant outside the flowers. Darwin believed these glands had the function of discarding waste or toxins. One of his correspondents, the Italian botanist Federico Delpino, rejected the idea: plants wouldn’t squander nectar, an expensive resource, just to flush out unwanted substances. Based on information he gathered from various plant species, Delpino concluded that the main purpose of EFN was to attract ants.
Extrafloral nectaries can be found on stipules (A), leaf bases (B) petioles (C), or more than one structure (D) © Holopainen et al., 2020:
Luring ants with nectar hints of pollinating relationships, but this possibility can be discarded for the majority of EFN-bearing plants. Most ants do not have pollen-collecting structures and are too small for the task. But the main barrier to ants’ contribution to pollination is their chemical defences that prevent the proliferation of pathogens but also inhibit pollen germination and the growth of pollen tubes. As a result, there are only 30 or so known cases of myrmecophily (ant pollination) among the thousands of insect-pollinated species. Plants entice ants with their EFN for an entirely different reason: to recruit a cohort of mean-spirited bodyguards.
Numerous studies have shown that ants attracted to EFN protect plants by attacking plant-munchers or disrupting their egg laying. Some trees, mostly in the tropics, have developed mutualistic associations with ants; they shelter their visitors in domatia (hollow chambers) and feed them with food bodies (nutrient-rich structures) and extrafloral nectar. In return, ants keep the plant’s enemies away: some even snip off epiphytes (plants that grow on the surface of another plant). If you ever leaned against one of these myrmecophytes (‘ant plants’) for a break during a hike, you probably were quickly encouraged to move along by some irate ants.
Ants patrolling the myrmecophyte tree bullhorn acacia (Vachellia cornigera) © Ryan Somma, Wikimedia Commons:
However, just like advertising a party fuelled by free beer on social media, encouraging ant visitors can have disastrous outcomes. In some circumstances, ants extend their foraging bouts from EFN to flowers where they may deplete the nectar – and even pollen – intended for pollinators. Ants may damage reproductive structures, sometimes castrating flowers by nibbling on stamens and pistils. And when ants are about, other flower visitors tend to give them a wide berth. That’s what happens when a fishhook barrel cactus is occupied by EFN-seeking southern fire ants. As a consequence, bees visit flowers less frequently and for less time, resulting in fruits with smaller and lighter seeds when compared to plants with no fire ants (Ness, 2006). This is not an isolated case. The presence of the common red ant (Myrmica rubra) on flowers can cause the common eastern bumble bee (Bombus impatiens), one of the most abundant bumble bees in eastern North America, to collect and deposit less pollen. The scent of ants alone is sufficient to put bumble bees off (Cembrowski et al., 2014).
The common red ant, a Palaearctic native and invasive to North America © Gary Alpert, Wikimedia Commons:
Because ants often decrease the species diversity of flower visitors as well as the frequency and duration of visits, the Austrian botanist Anton Joseph Kerner suggested another purpose of extrafloral nectar: bribery. EFN would turn ants’ attention away from the flowers, thus reducing their conflict with pollinators, while retaining ants’ protection for other parts of the plant (Kerner, 1878). The Distraction Hypothesis, as Kerner’s proposal is known today, may sound farfetched, but has been demonstrated experimentally. In one case, researchers simply blocked EFN on the Mexican ant‐plant Turnera velutina and observed the results. Blocking reduced the number of ants patrolling EFN, increased the proportion of flowers occupied by ants and reduced plant reproductive success (Villamil et al., 2019).
(b): EFN on the underside of a T. velutina leaf; (c): clogged and control leaves © Villamil et al., 2019:
What we know so far: ants can be plant guards, as suggested by Federico Delpino, or plant exploiters, according to Anton Joseph Kerner. As ant protection is mostly associated with EFN-bearing plants, which make up a minority of flowering plants, it’s safe to assume that ants on plants are mostly bad news. But, as it is invariably the case in biology, the plot thickens.
For the EFN-bearing T. velutina, the more aggressive the ants, the more vigorously flower visitors are deterred; but, surprisingly, deterrence reduces self-pollination and increases cross-pollination (Villamil et al., 2022), probably because pollinators that take their chances hang on to flowers just long enough to collect or deposit pollen, thus increasing the rate of flower visitation. For the South American scrubs Banisteriopsis campestris and B. malifolia, the presence of ants around flowers reduces visitation by smaller bees. But larger bees, which are likely to be better pollinators, are not bothered by ants, a positive outcome for the host plants (Barônio & Del-Claro, 2017).
Ants keep small bees away from these B. campestris flowers, but large bees are not affected © João Medeiros, Wikimedia Commons:
Ants are found everywhere except Antarctica and a few remote islands, and are incredibly important for all sorts of ecological services, from decomposition to recycling of nutrients, control of plant-eating insects, improving soils and dispersing seeds. They achieve all of that thanks to their mindboggling numbers. The renowned myrmecologist E. O. Wilson estimated that 1015 to 1016 ants crawl on Earth’s surface at any given time (that’s quadrillions, figures usually discussed in astronomy). A later appraisal fine-tuned the number to 20 × 1015 individuals, which corresponds to ∼12 megatons of carbon. This is more than the combined biomass of all wild birds and mammals, and is equivalent to ∼20% of human biomass (Schultheiss et al., 2022). Another study following a different methodology suggested a population size of 5 × 1016, excluding arboreal ants (Rosenberg et al., 2023). So Wilson wasn’t far off, as a billion here or a billion there is not that important when we are talking quadrillions. For comparison, there are some 7.9 × 109 human beings on the planet.
Depending on the species involved, ants may have substantial positive or negative effects on the pollination game, even though they play a mediocre role as pollinators. When ants are about, pollination is no longer a quid pro quo between pollinating insects and flowers. The party crasher must be accounted for.
A silky ant (Formica fusca) collecting nectar from a germander speedwell (Veronica chamaedrys) flower © Gilles San Martin, Wikimedia Commons:
Always a very special treat!
Superb read – always something new to learn, e.g. the origin of that famous aphorism (are all aphorisms famous? I suppose not…).
Fascinating — thanks. Cleansing the brain after the news.
Great post. Fascinating.
Ants are a complex lot. Thank you for the excellent pictorial lesson on these fascinating creatures.
Thanks for the interesting post!
That gave me a new perspective on fire ants. Thanks!
Wonderful, as always. I liked this sentence:
“Ants in general have a mood ranging from annoyed to furious”
Sounds like me these days. Posts like this help, as #3 from Tom suggests.