The floral death trap

One of the most interesting features of evolution is coevolution -- where two unrelated species influence each other's evolutionary pathway.

It's usually framed as benevolent, providing a benefit for both species, as in the whistling thorn (Vachellia drepandolobium) of East Africa, which makes hollow spheres at the base of the spines.  These provide a home for various species of symbiotic ants, which help out the tree by attacking and killing insect pests that might eat the leaves.  (It's called "whistling thorn" because the wind blowing across the holes in the spheres makes a whistling noise.)

There's also the less pleasant sort, the best-known of which is the evolutionary arms race -- such as the cheetah and the impala.  The fastest cheetahs catch and eat the slowest impalas, thus pushing both species to (on average) get faster.  (The general consensus, though, is that the cheetah might have topped out in terms of potential speed -- the combination of their powerful muscles and flexible spine and limbs would make them prone to dislocations and stress fractures if they were driven much harder.)

A relationship that's usually portrayed as entirely positive, though, is the one between flowering plants and their pollinators.  One of the earliest vindications of Darwin's theory of natural selection came with his conjecture that the Star of Bethlehem orchid (Angraecum sesquipedale) of Madagascar, which has a ridiculously long hollow spur at the back of the flower (with the nectar glands all the way at the tip), must have a pollinator with mouthparts that fit it:

[Image licensed under the Creative Commons sunoochi from Sapporo, Hokkaido, Japan, Angraecum sesquipedale Thouars, Hist. Orchid. 66 (1822) (45523703575), CC BY 2.0]

Not long afterward, researchers discovered the Madagascar hawk moth (Xanthopan morganii praedictum), exactly as Darwin said they would:

[Image licensed under the Creative Commons Nesnad, Xanthopan morganii praedicta Sep 16 2021 03-58PM, CC BY 4.0]

In fact, the coevolution between flowering plants and insects is so varied and complex that some evolutionary biologists claim it's responsible for the explosion in biodiversity in both groups that started during the early Cretaceous Period.  This, they say, is why flowering plants outnumber all other plant species by a considerable margin, and insect species outnumber all the rest of Earth's species put together.  (My own opinion is that it's probably not that simple -- evolutionary drivers seldom are.  But I won't deny that coevolution played a significant role.)

What's kind of interesting, in a grim sort of way, is when the coevolution between plants and their pollinators takes a darker turn.  One example, that is so crazily complicated that I had students tell me I was making the whole thing up, is the bucket orchid (Coryanthes spp.) of South America.  These bizarre-looking flowers have some upright petals, but the lower ones are fused into a "bucket" that fills with sweet-scented nectar.  The pistil (the female part of the flower) is submerged at the bottom.  One genus of bees (Euglossa spp.) is attracted to the scent, but upon landing on the edge of the bucket, finds no place to hang onto and falls in.  It swims around trying to find a way out, but the sides of the bucket are coated with a slick wax that provides no traction.  There's only one place it can get out -- at the back of the bucket is a ladder (I shit you not) made of hairs the bee can hang onto.  But when it gets to the top of the ladder, it finds itself head-first in a long tube with no place to turn around, so it wriggles its way to the end -- in the process gluing the anthers (the male pollen-bearing structures) to its back.  At the end of the tube is a hole (whew), and the bee flies away.  But then -- not having learned its lesson, apparently -- it finds another flower, falls in again, and this time the nectar acts as a solvent.  The anthers it was carrying come loose and settle to the bottom of the bucket (remember, that's where the pistil is), and pollinates the plant.

[Image licensed under the Creative Commons Orchi, Coryanthes hunteriana Orchi 01, CC BY-SA 3.0]

But at least here, it has a happy ending -- the bee escapes eventually.  There are, however, plants that kill their own pollinators -- in fact, that's the reason this whole topic comes up, because another one was just discovered recently.

The first one I ever heard about that pulls this nasty trick is a species of African water lily (Nymphaea capensis).  It's deceptively beautiful:

[Image licensed under the Creative Commons Fan Wen, Nymphaea capensis (14) 1200, CC BY-SA 4.0]

The flowers open twice, on two successive days.  The first day, the pistils are inaccessible beneath a tight cone of stamens (the above photograph was taken on the first day).  Bees landing on the flower are doomed to disappointment, because the nectar is inside the cone, but in trying to get to it they coat themselves with pollen.  The flowers close at night, then when they open on the second day, the cone of stamens opens as well, exposing a tempting pool of nectar with (once again) the pistil at the bottom.  A bee, having just visited a first-day flower and come away with no food but lots of pollen, lands on the edge of the pool in the middle of a second-day flower, and falls in.  This time, though, there's no escape.  It drowns, and the pollen it's carrying settles to the bottom of the pool and fertilizes the flower -- thus ensuring the plant will cross-pollinate, not self-pollinate.

What brought this topic to mind was a paper I stumbled across yesterday in the journal Plants, People, Planet that describes the same sort of fatal deception, but using a different kind of lure.  A Japanese species of the genus Arisaema, which in the United States is best known for the spring wildflower we call Jack-in the-pulpit (Arisaema triphyllum), that is common here in the northeast.  They have bizarre flowers -- a long, inverted cone with a flap on top (it's actually a spathe, a modified leaf), and in the middle a narrow cylinder (containing the actual flowers themselves, which are tiny).  If they're not weird enough from appearance alone, wait till you hear what some of the Japanese species do.

Arisaema angustatum (left) and Arisaema peninsulae (right), two of the tricksters described in the paper [Photograph by Kenji Suetsugu] 

These flowers are pollinated by fungus gnats -- familiar as annoyances to anyone who owns a greenhouse -- pinhead-sized flies that are attracted to moist soil.  Well, the plants don't just lure them by pretending to be a source of food.  The researcher, Kenji Suetsugu of Kobe University, realized something else was going on when he found that inside the cone-shaped spathe there were dozens of dead fungus gnats...

... all of them male.

Suetsugu started analyzing what chemicals the flowers were producing, and found that like the water lily, the Arisaema flowers change during the time they're open.  At first, only the male flowers are open, and fungus gnats that find their way in (presumably lured by the musty smell the plant has) climb about and pick up pollen.  But then the male flowers close and the female ones open -- and the strategy changes.

When the male flowers only are open, there's a little escape hatch at the base of the flower, so the gnats can get out after picking up pollen.  But when the female flowers open, the escape hatch closes.  And that's when the plants start producing a new chemical: an analog to the sexual attractant pheromone female fungus gnats produce to attract the males.

So the poor male gnats, hoping to get laid, find their way into the flower, but there's no way out because (again like the water lily) the interior of the cone is slippery.  They climb around on the female flowers, pollinating them, but eventually die from a combination of exhaustion and frustrated horniness.

If you thought that the relationship between flowers and pollinators was all mutual happiness, think again.  "Nature is red in tooth and claw," as Alfred, Lord Tennyson put it.  He was thinking of predators and prey, but it isn't restricted to the animal kingdom.  There are plants that reward their pollinators with an unpleasant demise -- showing that once again, evolution finds a way to exploit just about every possible innovation you can think of, nice or not.

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The birds and the bees and the flowers and the trees

One of the most fascinating aspects of evolution, and one of the least appreciated (outside of the biology-nerd community, anyhow), is how insects and flowering plants have coevolved.

Coevolution occurs when two different species (or groups of species) reciprocally affect each other's evolutionary changes.  A commonly-cited example is the pair made up of cheetahs and impalas; the fastest cheetahs get more food than the slowest, and the slowest impalas get turned into food more than the fastest, so each species has a tendency to get faster and faster (at least until other considerations, like the limitations of physiology, kick in).  This specific type of coevolution is sometimes called an evolutionary arms race, and can occur not only with speed but with issues like toxicity (in the species being eaten) and toxin tolerance (in the species doing the eating).

The coevolutionary relationship between flowering plants and insects is a curious one.  Certainly, there are insects that eat (and damage, sometimes fatally) plants; witness the gypsy moths that this year have shredded trees in our part of New York state.  Fortunately for our apple and cherry trees and other susceptible species, most trees attacked by gypsy moths survive defoliation and are able to put out another set of leaves once the moths' caterpillars are gone, and because the moths are a "boom-and-bust" species, they seldom mount a serious infestation like this year's more than once a decade or so.

But there's a "nicer" side to coevolution between insects and flowering plants, and that has to do with pollination.  We all learned in elementary school how bees and butterflies pollinate flowers, but it's more complicated than that; insects and plants have in some senses opposite interests in pollination.  For insects, the more different species of flowers they can visit, the more potential nectar sources they can access; but that's actively bad for flowers, because if a bee visits (for example) a rose and then a clover blossom, any pollen transferred does the plant no good at all because the two species aren't cross-fertile.  That pollen is "wasted," from the plant's perspective.

A species peony from the Caucasus Mountains, Paeonia mlokosewitschii -- nicknamed "Molly-the Witch" because most people can't pronounce "mlokosewitschii" -- primarily pollinated by ants and wasps  [photo taken this spring in the author's garden]

Plants have adopted a variety of strategies for coping with this.  Some, such as wind-pollinated plants (oaks, maples, willows, grasses, and many others) produce huge amounts of pollen, because they don't have a carrier to bring it from one flower to the next, and much of the pollen never reaches its target.  (This is why wind-pollinated plants like ragweed are primary culprits in pollen allergies.)  The same thing is true of plants that are visited by many different kinds of pollinators, and for the same reasons.

But the other approach is specialization.  If a flower has a shape that fits the mouthparts of only one species of pollinator, the pollen picked up is almost certainly going to be transferred to a flower of the same species.  In stable ecosystems, like rainforests, there are flowers and pollinators that have coevolved together so long that both are completely dependent on the other -- the pollinator's mouthparts don't fit any other flower species, and the flower's shape isn't compatible with any other pollinator's mouthparts.

Anna's Hummingbird (Calypte anna) visiting Crocosmia flowers in San Francisco, California [Image licensed under the Creative Commons Brocken Inaglory, Humming flowers, CC BY-SA 3.0]

As my evolutionary biology professor put it, this strategy works great until it doesn't.  Specialists get hit hardest by ecological change -- all that has to happen is for one of the pair to decline sharply, and the other collapses as well.  Their specialization leaves them with few options if the situation shifts.

The topic comes up because of a paper this week in Biological Reviews that looks at plant species which try to do both at once -- attract various species of pollinators (increasing the likelihood that pollen gets widely distributed, and mitigating the damage if one species of pollinator disappears) while encouraging those pollinators to feed exclusively on the flowers of that species only (decreasing the likelihood that the pollen will be transferred to a flower of an unrelated species).

A trio of researchers -- Kazuharu Ohashi (of the University of Tsukuba), Andreas Jürgens (of Technishe Universität Darmstadt), and James Thompson (of the University of Toronto) found that this complicated "hedging your bets" strategy is more common than anyone realized.  Some of the solutions the plants happen on are positively inspired; the goat willow (Salix caprea) has evolved to be pollinated by two different pollinators, bees and moths -- and the flowers actually change scents, producing one set of esters (chemicals associated with floral fragrance) during the day, and a different one at night, to attract their diurnal and nocturnal visitors most efficiently.  Cardinal shrub (Weigela spp.) flowers change scent as they age -- young flowers have fragrances attracting bees and butterflies, older ones attracting species like drone flies.

"[Y]ou'd expect that flowers would mostly be visited by one particular group of pollinators," said study lead author Kazuharu Ohashi, in an interview with Science Daily.  "But flowers often host many different visitors at the same time and flowers appear to meet the needs of multiple visitors.  The question we wanted to answer is how this happens in nature... Most flowers are ecologically generalized and the assumption to date has been that this is a suboptimal solution.  But our findings suggest that interactions with multiple animals can actually be optimized by minimizing trade-offs in various ways, and such evolutionary processes may have enriched the diversity of flowers."

Evolution is more subtle than a lot of us realize, happening on solutions to ecological problems that nearly defy belief.  The bucket orchid of South America (Coryanthes spp.) has a flower with a complex "trap" that only appeals to one species of bee -- and is so convoluted that when I explained its function to my biology students, I had to assure them more than once that I wasn't making it all up to fool the gullible.  The strategies vary dramatically from species to species, but always fall back to that tried-and-true rule -- evolution is the "law of whatever works."

So there's something to think about when you're working in your garden.  The birds and the bees and the flowers and the trees are a lot more complicated and interconnected than they may seem.  Many of the sophisticated mechanisms they use to assure survival and reproduction are only coming to light now -- and papers like Ohashi et al. give us a new lens into how beautiful and intricate the natural world is.

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Most people define the word culture in human terms.  Language, music, laws, religion, and so on.

There is culture among other animals, however, perhaps less complex but just as fascinating.  Monkeys teach their young how to use tools.  Songbirds learn their songs from adults, they're not born knowing them -- and much like human language, if the song isn't learned during a critical window as they grow, then never become fluent.

Whales, parrots, crows, wolves... all have traditions handed down from previous generations and taught to the young.

All, therefore, have culture.

In Becoming Wild: How Animal Cultures Raise Families, Create Beauty, and Achieve Peace, ecologist and science writer Carl Safina will give you a lens into the cultures of non-human species that will leave you breathless -- and convinced that perhaps the divide between human and non-human isn't as deep and unbridgeable as it seems.  It's a beautiful, fascinating, and preconceived-notion-challenging book.  You'll never hear a coyote, see a crow fly past, or look at your pet dog the same way again.

[Note: if you purchase this book from the image/link below, part of the proceeds goes to support Skeptophilia!]

Meadow management

My wife and I live on a 3.5 acre bit of land in the hills of upstate New York.  We're lucky to have that kind of space, and the place itself is beautiful; it's crossed by a little stony-bedded creek, and has a swimmable pond and lots of big old trees for shade.

And lots of lawn.  At least it did when we moved in here eighteen years ago.  We've been gradually doing battle with the lawn for most of that eighteen years.  We replaced some with gardens -- which was a net loss of discretionary time, since weeding a garden takes a great deal more time than mowing an equal-sized piece of lawn.  My wife got the idea of replacing a lot of the grass in the back yard with clover, and she was helped out in that endeavor by our large galumphing dog Guinness, who essentially does a hockey stop whenever he's chasing his tennis ball, tearing up large strips of turf and thus earning himself the nickname "Skidmark."  But the clover's finally taken hold in a big way, and it not only looks great and needs way less mowing, it's a happy place for the honeybees.

That last bit was our incentive for turning a chunk of our front yard into a meadow.  A wonderful local nursery, The Plantsmen, specializes in native wildlife-friendly plants, so a couple of months ago we went down and came back with my Honda Element packed with such unusual finds as blue-stemmed goldenrod (Solidago caesia), nannyberry (Viburnum lentago), American scarlet elderberry (Sambucus racemosa), two different kinds of bergamot (Monarda spp.), fragrant sumac (Rhus aromatica), serviceberry (Amelanchier canadensis), and in a corner that has a permanent spring, the charming and water-loving buttonbush (Cephalanthus occidentalis).

The Meadow thus far.  Okay, it doesn't look like much yet, but just you wait until our plants have a couple years' growth behind them.

The reason this topic comes up is because of a study that came out this week in the journal Biological Conservation about the role of verges in preserving valuable pollinators and beneficial insects.  Verges, they tell us, are hotspots for flowers and pollinators, often containing a dramatic diversity of different species (not all of which are native, of course; but then, neither is the white clover we seeded in our back yard, and I still consider it to be on balance a beneficial plant).  

Of course, roads themselves are a necessary evil, replacing and fragmenting habitat, not to mention the never-ending problem of roadkill.  (Don't just think of mammals, here; think of the number of insects killed yearly by windshield collisions, and keep in mind that even the National Pesticide Information Center says that 97% of insect species are neutral with respect to humanity, or else actively beneficial).  So given that roads aren't going anywhere, the best thing to do is to figure out how to maximize whatever's positive about them, and minimize the negative ecological impact.

Verges seem to be the biggest positive feature, as long as they're managed properly.  The key, says the researchers (a group led by environmental scientist Benjamin Phillips of the University of Exeter), is to mow as infrequently as is practical for the spot.  Surprisingly, "don't mow at all" turns out to be a bad idea.  The authors write:

An observational study of 19 road verges in the UK found that mown verges (cut once between May and August, cuttings not removed) had on average 67% fewer flowers and 61% fewer pollinators across the summer than unmown verges, experimentally manipulated mowing frequency (cuts/year: 0, 1 (early autumn) or 2 (early summer and early autumn)) and removal of cuttings (left in the verge or removed) in a single road verge (with a species-rich plant community) in the Netherlands.  Increasing the number of cuts from 0 to 1 cut resulted in 3.5 times greater flower density and 2 times greater flower species richness, but no significant effect on pollinator density, though increasing from 1 to 2 cuts/year resulted in 3.5 times greater pollinator density.

This is good information for our own little meadow, because I wasn't sure if we should mow at all, but it sounds like it's a good idea; in our case, not only for the pollinators, but because in our area, completely stopping mowing is a good way to turn a meadow into a tangle of walnut saplings and nasty, fast-growing exotics like Tatarian honeysuckle and multiflora rose.

So it sounds like mowing once or twice -- maybe once in spring and once in fall -- might be the way to go.  That way the plants that die all the way back to the ground in the winter (like the goldenrod and bergamot) will get a fresh start each year, and we won't have competitors for the woody shrubs (such as the sumac, nannyberry, serviceberry, and elderberry).

I can't mow near where the buttonbush is, however, because the spring keeps that piece of the lawn soggy, and if I try to mow it our lawn mower sinks up to the axle and then I have to tow it out with my car.

Yes, that's the voice of experience, right there.

In any case, this paper was nicely timed from my perspective, as I was wondering what I'd do after the first frost when everything starts to die back.  Since we planted as we did to encourage the bees and butterflies, it's good to know that there's some solid scientific research to back up our choice of how to handle it.

With luck, in a couple of years we'll have something really beautiful to enjoy -- and a nice habitat for wildlife, as well.

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This week's Skeptophilia book of the week is for anyone fascinated with astronomy and the possibility of extraterrestrial life: The Sirens of Mars: Searching for Life on Another World, by Sarah Stewart Johnson.

Johnson is a planetary scientist at Georgetown University, and is also a hell of a writer.  In this book, she describes her personal path to becoming a respected scientist, and the broader search for life on Mars -- starting with simulations in the most hostile environments on Earth, such as the dry valleys of central Antarctica and the salt flats of Australia, and eventually leading to analysis of data from the Mars rovers, looking for any trace of living things past or present.

It's a beautifully-told story, and the whole endeavor is tremendously exciting.  If, like me, you look up at the night sky with awe, and wonder if there's anyone up there looking back your way, then Johnson's book should be on your reading list.

[Note: if you purchase this book using the image/link below, part of the proceeds goes to support Skeptophilia!]