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.

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