Ghosts of the Ice Age

The last Ice Age came to an end roughly 15,000 years ago. But today we can still see ghostly remnants of extinct Ice Age ecosystems, preserved in living plants.

Osage Orange

It’s not easy being a plant. In particular, being immobile and fixed in one spot presents two obvious difficulties: since you can’t run away from enemies, you need to find some passive way to defend yourself, and since your young can’t move, you need to find a way to scatter your seeds so your offspring can disperse into new areas. Modern plants have solved these problems by using an arsenal of chemicals and thorns to make it harder to eat them, and by producing seeds inside fruits—which attract animals (especially birds and mammals) to eat them and carry them away, later depositing the undigested seed somewhere else (along with a helpful plop of fertilizer). In many cases, these strategies have become so specialized that particular plant species have become utterly dependent upon just a handful of animal species for their propagation—a process called “co-evolution”.

In Ice Age times, plants faced the same problems, and solved them in the same way. But as the “megafauna” of giant North American mammals went extinct at the end of the Pleistocene, many plants were left without their animal partners. Most of those plants also went extinct, unable to survive without the biological allies they had co-evolved with. But a few survived until today—and they give us a glimpse at ancient ecological relationships that are now long dead.

One example is the Osage Orange, Maclura pomifera. This is a small tree with sharp thorny branches which bear distinctive green knobbly softball-sized fruits. When first discovered, the Osage Orange had a range that was limited to parts of Texas, Oklahoma and Arkansas. Although the large inedible fruits were full of seeds and exuded an unpleasant milky sap, the tough springy wood was perfectly suited for making bows, and Native Americans traded this valuable material for hundreds of miles.

When white settlers arrived, they found another use for the thorny tree. Since Osage Orange readily reproduces by sending out lateral runners, it was often planted along the edges of cattle pastures to act as a sort of living barbed wire fence. Later it became prized as an ornamental garden tree, and was widely cultivated. Today, the Osage Orange has escaped captivity in many areas, and can be found growing in the wild throughout most of the United States and into Canada.

But botanists noticed some peculiarities about the tree. In its original wild range, the Osage Orange was found mostly in river valleys—but when it was widely cultivated, it was found to grow best on drier upland hills. Further, fossil finds indicated that it had, recently in geological terms, been more widespread and diverse in the past: 20,000 years ago, during the Ice Age, there were at least eight species of Maclura which could be found from Mexico all the way to Canada. It was a mystery why the formerly abundant tree had vanished, and why, in its remaining range, it seemed to be restricted to a habitat that it did not really prefer.

The answer came when biologists examined the manner in which Osage Orange seeds were dispersed. In the fall, the trees become filled with large knobby green fruits, each packed with seeds and covered by a layer of sugary flesh. When ripe, the fruits would all drop to the ground at once, producing a windfall of food for any herbivore which ate them.

It was an enormous investment of energy for the tree, and it would only have been made if the species got a big payoff in return—in the form of herbivores who would eat the fruits, swallow the seeds, and disperse them elsewhere in their droppings. And here was the mystery: ecologists found that the large fruits were virtually ignored by local herbivores. Occasionally a rat or rabbit or deer would nibble at them, but they only chewed away the rind and left the seeds behind. The only animal large enough to actually swallow the seeds and transport them was the horse—and horses had been unknown in North America for millennia until they were introduced by Spanish settlers in the 16^th century.

The only effective method for the Osage Orange seeds to be dispersed was water. During heavy rains, the large fruits rolled downhill, and were often carried by floodwaters downstream and washed up onto the river’s flood plains, where they sprouted.

This explained why the Osage Orange was limited to growing in river valleys (at least it was until humans had begun transporting the seeds everywhere and planting them). But since the tree had once had a much more extensive range in the past, it must have had some other more effective way of dispersing its seeds, one that it was, for some reason, not using now.

The answer came when ecologists noted that a closely related species of tree in Africa was dependent upon the forest elephant for its seed dispersal. The elephants could swallow the large fruits intact, digesting away the pulp and allowing the undamaged seeds to pass through their digestive tract and be passed in the droppings, days later and miles away from where they were eaten. The elephants also unwittingly carried the seeds to upland terrain where they grew better.

During the Ice Age, the Osage Orange had the same strategy; the large fruits were intended as food for Pleistocene megafauna. At that time, 20,000 years ago, North America was a different place, ecologically. Most of Canada was covered by glacial ice sheets over a mile thick. Most of the United States consisted of dry grassland with patches of forest. These were populated by a variety of herbivores including the American Camel, the Giant Bison, the Giant Beaver, the Giant Ground Sloth, the Colombian Mammoth, the Mastodon, and the Gompothere (an extinct North American elephant). These in turn were preyed on by carnivores larger than any found today, such as the American Lion, the Dire Wolf, the Sabertooth Cat, and the Short-Faced Bear. The large herbivores fed on a wide variety of plants—including Osage Orange.

Then, about 10,000 years ago, the North American Ice Age megafauna suddenly went extinct. Coincidentally, this is about the time that humans began spreading across the continent: it is also the time that the Ice Age ended and the glaciers melted. It is still a matter of scientific debate whether the megafauna were killed off by human hunters or died as a result of a warming climate.

But once the large herbivores disappeared, the Osage Orange lost its specialized reproductive partners, and was doomed. Most of the Pleistocene members of the Maclura genus died out, unable to effectively disperse their seeds anymore and unable to adapt. (To humans, 10,000 years may sound like a long time for the trees to evolve an effective response, but to the trees, which can live over 200 years, this was a mere 50 generations—far too short a time for evolution to act.) The sole survivor was reduced to a tiny refuge where it was able to eke out a bare existence by depending on gravity and water to carry its seeds downhill to a barely habitable zone. Today, the Osage Orange has recovered most of its former range—relying upon human gardeners to play the role that extinct Mammoths and Ground Sloths once did by carrying and planting the tree’s seeds.

A similar story can be found with one of North America’s favorite fruits—the Avocado. The Avacado has all the characteristic traits found in a fruit that is intended by the plant to be eaten by mammals rather than by birds—it is much larger than any bird would be able to swallow; it has a strong odor upon ripening but a dull green color (which allows nocturnal mammals who lack color vision to find it by scent); and the fruits drop just as they ripen, which prevents them from being eaten by seed-destroying monkeys before the ground-dwelling herbivores can find them.

But the Avocado is far too large to be effectively dispersed by any living mammal. Although the fruits are sometimes gnawed by tropical rats or squirrels, they tend to just eat the sugary pulp and leave the seed—which is ineffective for the tree. The only large wild animal that occasionally eats the fruit today is the Jaguar (which eats a surprising amount of vegetable matter). Obviously, the Avocado fruit was intended to tempt mammalian dispersers that were much larger—the Gompothere, the Giant Sloth, and the Toxodon (a cousin to the rhinoceros). When these disappeared at the end of the Ice Age, the Avocado began a slow and inevitable decline.

Once again, it was humans who saved it. About 5,000 years ago, Native Americans in Mexico discovered the sugary pulp of the Avocado and began cultivating it. Over the centuries, human selection led to thicker and sweeter pulps: meanwhile, the tree was saved from probable extinction by humans who planted the seeds and tended the trees. Today, despite the fact that all of its effective seed-dispersers are gone, the trees can be found all over the world, and some one million tons of Avocado fruits are grown each year. The species is now completely dependent upon humans for its reproduction and dispersal.

But the relationship between plants and their herbivore seed-dispersers is not entirely benevolent. Browsing mammals do not limit themselves to only eating the ripe fruits (which is what the tree wants them to do)—they also eat the leaves, buds, and young shoots (which is harmful to the plant). One of the methods used by modern plants to prevent their tender parts from being eaten by herbivores is to place prickly thorns on their branches to deter browsers. The modern champion is the Acacia tree in Africa, which has to defend itself against plant-eaters the size of giraffes and elephants. In Pleistocene times, the herbivores were even bigger—and tree weaponry was correspondingly more fearsome.

Remnants of this can still be seen today. The Osage Orange has thorns on its trunk and branches. But these have little effect on today’s browsers such as Deer: the thorns are spread too far apart, and the Deer can easily poke their snouts between them to pick off the leaves and twigs. Clearly, the Osage Orange’s thorns were not intended to defend against something the size of a Deer: they were intended to deter much larger herbivores. A Mastodon or Giant Ground Sloth who poked into the branches of an Osage Orange tree would, unless it were slow and careful, get a face full of sharp spines.

The best example of this defense strategy is found in the Honey Locust Tree. In autumn the Honey Locust produces long pods of beanlike seeds surrounded by a sweet pulp. These dangle below the branches invitingly, to entice large herbivores (like ancient Mastodons) to eat them. Today, with the Mastodons gone, the Honey Locust has become another species that is now dependent upon human gardeners and landscapers for their dispersal.

But at the same time, to deter the herbivores from chomping on the twigs or bark, the Honey Locust arms itself with some of the most fearsome thorns in the world—wicked-looking multi-branched clusters of needle-sharp spikes up to half a foot long, which extend as much as 15-20 feet up on the trunk and out at the ends of all the branches.

These may appear to be massive overkill when it comes to deterring today’s White-Tailed Deer, but these impressive weapons were originally intended for Mastodons and Mammoths—some of the largest mammals ever to live in North America. So, if you find yourself getting skewered by the thorns of young Honey Locusts as you walk through the woods, take comfort in the fact that the tree is still gamely trying to defend itself against an immense twig-browser that it doesn’t know no longer exists.