Picky eaters

Last week a gardener friend and I were talking about the fact that some plants are extreme specialists -- they only thrive in a very narrow range of conditions.

The classic example of this are orchids.  Virtually all orchid species only do well if you can somehow replicate the exact conditions of temperature, soil pH, soil mineral content, sunlight, and so on that they need.  Some also require the presence of symbiotic fungi (such as mycorrhizae) that infiltrate the orchid's roots and aid in nutrient and water uptake.  All of this is why if you ever are lucky enough to see an orchid growing in the wild, resist the temptation of digging it up and bringing it home for your garden.  The chances are nearly one hundred percent that all you'll succeed in doing is killing it in short order.  (Also, if you live in a place with laws against harming endangered species, you might be looking at serious fines if you get caught.)

It's an interesting question to consider why such extreme specialization evolves.  On first glance, it seems like it'd be better for all species to evolve toward becoming generalists -- able to handle a wide range of conditions.  The thing is that while generalists (like dandelions and crabgrass) do thrive just about everywhere, giving them a competitive edge in disturbed habitats (like cities) where not much else grows, they get beaten by the specialists in old, stable ecosystems.  The specialists have evolved to tolerate those specific conditions better than anything else.

It's why in old-growth rain forests, just about everything you see -- plant and animal -- is a specialist.  Along roadside ditches, they're all generalists.

Some recent research suggests that this drive toward specialization in stable habitats is very old.  A study of the distribution of animals in Ediacaran (very late Precambrian) sandstone in Australia found that some of the peculiar animals characteristic of these ecosystems showed a distinct preference for particular parts of the habitat -- a clear hallmark of specialization.

The researchers focused on a handful of species that have no living descendants, including Obamus coronatus (which looks like a French cruller) and the hubcap-like Tribrachidium heraldicum, one of the only known animals to have triradial symmetry.

Artist's reconstruction of Obamus coronatus [Image licensed under the Creative Commons Nobu Tamura (http://spinops.blogspot.com/), Obamus NT, CC BY-SA 4.0]

Both animals were grazers, feeding on the microbial mat on the seafloor, but their habitat choices differed.  Obamus turned out to have a distinct preference for places where the mat was thickest; Tribrachidium was much more evenly dispersed.  And since both animals were of very low mobility -- similar to modern barnacles -- this didn't just reflect the chance arrangement of where they were when the a layer of sediment, probably stirred up by a storm or landslide, buried them for eternity.

This was a habitat choice -- and the first known example of specialization in the natural world.

"We think about the very oldest animals and maybe you wouldn't expect them to be so picky," said Mary Droser of the University of California - Riverside, who co-authored the study. "But Obamus only occurs where there is a thick mat, and it's a pretty sophisticated way of making a living for something so very old...  There are a limited number of reproductive strategies, especially for animals like these.  There are more strategies today, and they're more elaborate now. But the same ones used today were still being used 550 million years ago."

"It's not like studying dinosaurs, which are related to birds that we can observe today," said Phillip C. Boan, also of UC - R, and lead author of the new study.  "With these animals, because they have no modern descendants, we're still working out basic questions about how they lived, such as how they reproduced and what they ate...  This is really the first example of a habitat-selective Ediacaran creature, the first example of a macroscopic animal doing this.  But how did they get where they wanted to go?  This is a question we don't yet know the answer to."

It's fascinating that we can get some insight into the behavior of a species that lived so long ago, during a time where there was no life at all on land.  Imagine it -- everything alive is in the sea, and the continents were vast, barren expanses of rock, sand, and dust.  The first land-dwelling plants and animals wouldn't exist for another fifty million years (and even then, they were clustered around bodies of water; the central parts of the continents would have been lifeless for a great deal longer).  

But despite how alien this landscape would have seemed, organisms were already evolving through natural selection to have many of the same traits we see today -- including the fact that some of them, like modern orchids, know exactly where they want to be.

****************************************

The moth and the flower

Ever heard of an evolutionary arms race?

It's a twist on coevolution, where the adaptations in one species affect the selection -- and therefore the evolution -- of another, unrelated species.  In arms races, as the name implies, it's specifically about biological weaponry, either for predation or for self-defense.  Finding lunch, or avoiding being lunch.  The most commonly-cited example of an arms race is speed and maneuverability in the cheetah and the impala.  The fastest cheetahs take down and eat the slowest impalas; the fastest impalas escape, and the slowest cheetahs starve.  (That's an oversimplification, but it'll do for now.)  The upshot is that over time, both the cheetah and the impala evolve the ability to run faster.  The most common outcome of arms races is that it continues until one of the species kind of maxes out on how far it can take the adaptation.  The cheetah might well be at that point; it's hard to imagine how they could be any faster without the strain on the joints, tendons, and muscles causing serious injury.

The odd thing about an arms race, though, is that sometimes it can backfire on one of the participants.  A relationship like this was the subject of a paper last week in the journal Alpine Entomology, which looked at a beautiful -- but highly poisonous -- plant, the alpine rose.

The alpine rose isn't a rose at all; it's a species of rhododendron (Rhododendron ferrugineum) that lives in a habitat not much else can tolerate.  It thrives only in rocky, acidic soils, just above the tree line in the Alps, Jura, Pyrenees, and Apennines.  Besides the obvious difficulties of living in a cold, windswept place, there's the issue that in such barren areas, any animals are going to find survival as tough as the plants do, so the local herbivores are going to eat pretty much anything green.  The alpine rose has responded by evolving a nasty cocktail of toxins, including the glycoside arbutin and the alkaloid arecoline, so even the hungriest of plant-eaters leave it alone.

[Image licensed under the Creative Commons MurielBendel, Rhododendron ferrugineum Valais4, CC BY-SA 4.0]

Well, most plant-eaters.  The current paper is about the discovery of a population of moths that have evolved to specialize on this plant.  It's the danger of an arms race; if one of the participants can exploit the adaptations of the other, it can actually end up better off.  Here, the moth has evolved tolerance to the alpine rose's toxins, and the result is it has a food source essentially to itself, with no competition.

The more the researchers looked into it, the more interesting the story got.  Not only had these moths evolved to specialize on eating alpine rose leaves, upon undergoing genetic analysis, they were shown to be conspecific with the species Lyonetia ledi, the Ledum leaf-miner moth.  This is a widespread species of moth that feeds mostly on the leaves of Labrador tea (Ledum palustre) and bog myrtle (Myrica gale).  This, however, left two puzzles: (1) most Ledum leaf-miners won't eat alpine rose; and (2) the nearest population of Ledum leaf-miners is over four hundred kilometers away.

What seems to have happened is that the moth, which feeds on plants that live in boggy areas at high altitude, was once more common -- during the last glacial period, when the favorable habitat was pretty much everywhere in Europe that wasn't actually covered in ice.  Then as things warmed up, the valleys became too warm for its host plants, and the range of both the plants and the moths moved upward in altitude, and broke up into patches.  One of them got isolated in the mountains of western Europe, where the alpine rose was way more common than either of the usual host plants for the species.  This created enormous selective pressure; as soon as there were moths that could at least tolerate the alpine rose's toxins, they were at such an advantage that they outcompeted their cousins and more or less took over.  Over time, this preference for alpine rose became a requirement.  Now, the moth feeds only on alpine rose -- something no other insect species can manage.

So that's today's cool science story; a poisonous flower and a relict population of moths stranded in the high mountains after the last glaciation.  Once again illustrating what Darwin meant by saying that evolution had created "many forms most beautiful and most wonderful" -- some of which are still out there waiting to be discovered.

***********************************

It's astonishing to see what the universe looks like on scales different from those we're used to.  The images of galaxies and quasars and (more recently) black holes are nothing short of awe-inspiring.  However, the microscopic realm is equally breathtaking -- which you'll find out as soon as you open the new book Micro Life: Miracles of the Microscopic World.

Assembled by a team at DK Publishers and the Smithsonian Institution, Micro Life is a compendium of photographs and artwork depicting the world of the very small, from single-celled organisms to individual fungus spores to nerve cells to the facets of a butterfly's eye.  Leafing through it generates a sense of wonder at the complexity of the microscopic, and its incredible beauty.  If you are a biology enthusiast -- or are looking for a gift for a friend who is -- this lovely book is a sure-fire winner.  You'll never look the same way at dust, pollen, algae, and a myriad of other things from the natural world that you thought you knew.

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