Warning: Reading this article may cause a whiplash-inducing paradigm shift. You will no longer view wild areas the same way. Your concepts of "pristine wilderness" and "the balance of nature" will be forever compromised. You may even start to see ghosts.
Consider the fruit of the Osage-orange, named after the Osage Indians associated with its range. In the fall, Osage-orange trees hang heavy with bright green, bumpy spheres the size of softballs, full of seeds and an unpalatable milky latex. They soon fall to the ground, where they rot, unused, unless a child decides to test their ballistic properties.
Trees that make such fleshy fruits do so to entice animals to eat them, along with the seeds they contain. The seeds pass through the animal and are deposited, with natural fertilizer, away from the shade and roots of the parent tree where they are more likely to germinate. But no native animal eats Osage-orange fruits. So, what are they for? The same question could be asked of the large seed pods of the honeylocust and the Kentucky coffeetree.
To answer these questions and solve the "riddle of the rotting fruit," we first need to go to Costa Rica. That is where tropical ecologist Dan Janzen of the University of Pennsylvania noticed that the fruits of a mid-sized tree in the pea family called Cassia grandis were generally scorned by the native animals, but gobbled up by introduced horses and cattle. Janzen, who received the Crafoord Prize (ecology's version of the Nobel) for his work on the co-evolution of plants and animals, had the idea that the seeds of Cassia grandis, and about 40 other large-fruited Costa Rican trees, were adapted to be dispersed by large mammals that are now extinct. He teamed up with Paul Martin, a paleoecologist at the University of Arizona, to develop the concept of ecological anachronisms.
An anachronism is something that is chronologically out of place: a typewriter or floppy disc in a modern office. Leather helmets at the Super Bowl. Or, hopefully, the internal combustion engine in the near future. An ecological anachronism is an adaptation that is chronologically out of place, making its purpose more or less obsolete. A tree with big fruits to attract huge mammals as dispersers of its seeds is anachronistic in a world of relatively small mammals.
In the case of Cassia grandis, Janzen and Martin figured that the foot-long woody seed pods were eaten for their sweet pulp by giant ground-sloths and elephant-like gomphotheres. These multi-ton animals had such big gullets that they didn't need to chew a lot, so most of the seeds passed through the animals unharmed and ready to propagate more Cassia grandis trees. However, the gomphotheres and giant ground-sloths disappeared about 13,000 years ago, toward the end of the last Ice Age of the Pleistocene.
Gomphotheres and ground-sloths? The Ice Age? What, you may be wondering, do they have to do with Osage-oranges, honeylocusts, and coffee-trees today?
In terms of evolutionary time, the difference between 13,000 years ago and now is like the difference between Friday, December 31, 1999 and Saturday, January 1, 2000. We may assign those two days to different centuries or millennia, but they are still part of the same week. Likewise, all the animals and plants of 13,000 years ago belong just as much in the present. In fact, they still live in the present, with just one major exception: most of the big and fierce animals are now gone. This happened just a couple thousand years before we invented agriculture and planted the seeds of civilization. Woolly mammoths actually survived on some Arctic islands until after the Egyptian pyramids were built!
Today, if you searched all of North America north of Mexico, you would find only 17 species of land mammals that could be called megafauna, a term for animals that exceed 100 pounds. …