Ice Age Ghosts and Their Groceries

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Ice Age Ghosts and Their Groceries
By Heath Shive
 
Herbivores and plants have a tentative quid pro quo.  Herbivores get food.  Ideally, plants get seed dispersal.  However, many herbivores - like rodents - eat seeds rather than disperse them! 

No seeds, no future.  That’s why some plants adapt defenses like toxic seeds.  What we consider “spices” (e.g., black pepper) are really just seeds defending themselves. 

Therefore, symbiotic plant-animal relationships are very specific.  And some of those relationships could be over 10,000 years old!

To the plants...and their Ice Age ghosts!

The Case of the Useless Fruit
 
Years ago, in Costa Rica, ecologist Dan Janzen noticed that many of the larger tropical fruits were going uneaten, just rotting away. 

Together Janzen and Paul S. Martin wrote a paper entitled “Neotropical Anachronisms” where they hypothesized that many fruit trees might still be making food for Ice Age megafauna (giant sloths, mastodons, mammoths, glyptodonts, etc.). 

These great beasts were symbiotes.  Now they’re extinct.  But the trees still remember them and make Ice Age groceries to this day.

The Ghosts of Evolution
 
Connie Barlow’s book The Ghosts of Evolution explores this idea in depth.  Her anachronistic examples include honey locusts, Osage orange, calabash, paw paws, persimmon trees and others.  All of these plants produce large fruit harvests that go largely nowhere.  They’ve had to develop alternate means to survive without Ice Age megafauna, and their numbers were dwindling as a result. 
 
For example, the Osage orange produces a softball-sized fruit that looks like a green brain.  It’s made of a latex-like material, and it tastes worse than it looks.  No animal is known to eat this fruit.  By the time European colonists arrived in America, the population was restricted to a small tri-state range around Arkansas, Oklahoma and Texas.
 
Humans now propagate some of these “anachronisms”.  Osage orange trees make excellent natural fences for farms. Honey locust trees thrive in cities and arid climates to provide shade. 
 
Noted ecologist Henry Howe critiqued the idea of anachronisms in his 1988 book Ecological Relationships of Plants and Animals.  Though Howe thought the idea intriguing, he criticized the hypothesis on two points.  First, if the megafauna were the seed dispersers, then how could the plants survive “for 500 years, much less ten millennia” after the Ice Age megafaunal extinction?  Second, how could one even begin to prove this hypothesis?
 
But it should be remembered that even if megafauna were the primary dispersers, it doesn’t mean they were the only dispersers.  For example, without the honeybee, flowering plants wouldn’t go extinct, but many flowering plants certainly would dwindle. 

Besides, some plants find alternate means to survive.  For example, some plants will send out root suckers to form nearby clone trees.  Some of the oldest plants in the world have survived this way.       

Conclusion
 
This hypothesis is by no means perfect, but it seems better than any other alternative explanation for all the “nonsensical fruit.”  Kind of a lonely story, really.  Smacks of unrequited love. 

But my advice for these trees, or any lonely heart, is this:  They are plenty of other symbiotes in this big crazy world!  Good luck!

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Sources:  
Barlow, Connie.  The Ghosts of Evolution:  Nonsensical Fruit, Missing Partners, and Other Ecological Anachronisms.  New York:  Basic Books, 2000.   
 
Howe, Henry F., and Lynn C. Westley, 1988, Ecological Relationships of Plants and Animals (New York:  Oxford University Press)
 
Janzen, Daniel H., and Paul S. Martin, 1982, “Neotropical Anachronisms:  The Fruits the Gomphotheres Ate,” Science 215:  19-27.  

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