April brings us the wonderful wildflowers of another spring. As hard as it may be, if one looks up along a riverbank or narrow stream valley, you may find yourself standing beneath the dark burgundy flowers of the native pawpaw. Like many other such colored flowers (skunk cabbage, toad trillium, wild ginger), the pawpaw flower emits a musky odor, mimicking fetid carrion to entice insects to investigate, and subsequently, pollinate the flowers.
Later this season, the pawpaw will be the host of the zebra swallowtail butterfly, laying its eggs on the young leaves to hatch and feed. Look for greenish caterpillars with yellow, white and black stripes ringing the body. There are two broods per year in our region. Finally, come September, treat yourself to the wonderful banana-custard taste of the largest edible fruit native to the United States.
The large seeds of the pawpaw are designed to pass through the digestive system of herbivores in order for the microbes in the gut to ‘scarify’ the seed, enabling its successful germination upon final disposal. Deer do not generally eat pawpaw. In fact, no large herbivores exist that can swallow such a large seed. Only the black bear is large enough to consume the pawpaw seed, and it’s digestive activity does not penetrate the seed’s covering, thus rendering the seed unfit for germination.
So, how is the pawpaw seed dispersed? Scientists Dan Janzen and Paul Martin, in their 1982 landmark paper, investigated numerous large-fruited trees that had a tendency to drop their fruits on the ground, only to find much of it uneaten; left to rot. In eastern United States, such largely ignored large-fruited trees include the pawpaw, osage orange, honey locust, and the Kentucky coffee tree. This ‘riddle of the rotting fruit’ prompted their research to find the missing animal partners, known to them as the ‘ghosts of evolution’. Subsequently, Connie Barlow furthered their research with a book of her own (The Ghosts of Evolution – 2001).
Their research has found that, historically, the principle seed dispersers of these plants were the megafauna common to our region just 13,000 years ago. Such megafauna included such large herbivores such as woolly mammoth, mastodons, giant sloths, hippo-like toxodons, spike-tailed bony-shelled glyptodonts, camels, and horses. With the demise of the megafauna, the range of these trees has shrunk drastically, lasting in small refugium until human intervention, retaining their vestigial, or anachronistic, characteristics. In addition to the large fruit, the honey locust today still sports exceedingly large thorns, unfortunately inadequate to protect it from today’s smaller marauders.
So, how have they survived to today? One reason is that all of these species spread by root shoots. If even one seed finds a way to germinate, the roots will send up a colony of clonal stems (thus, the well-known ‘pawpaw patch’). Clonal communities (such as aspen groves) are known to exist for thousands of years. Certainly, humans have been largely responsible for the distribution of these trees; both by Native Americans and European settlers. Often, seeds carried by water have had the seed pods rot away, enabling germination in floodplains. And, yes, there are always a few foxes, raccoon and bear that will defecate a fertile seed.
Osage orange presents an interesting story. Without the mastodons and mammoths to disperse the seeds, the range of the osage orange had shrunk to a short stretch of the Red River watershed near the junction of Texas, Oklahoma, and Arkansas. This isolation enabled the local Spiroans of the Mississippian culture to control the trade of the valued wood (highly desirable for bow-making), explaining the extraordinary wealth accumulated by this people as found through archeological evidence. Until the invention of barbed wire in 1874, settlers of the plains and prairie states used this ‘hedge apple’ to create effective live fences around gardens, greatly expanding the distribution of the osage orange.
All of this discussion of anachronistic flora begs the question, what happened to the megafauna in our forests? This question is being hotly debated at this time, with four camps having differing positions:
Human overkill: The human invasion of North America around 14,000 years ago and the abrupt vanishing of megafauna from the fossil record around 13,000 years ago is likely not coincidental, but other factors could be involved as well. Evidence supporting the overkill hypothesis includes the persistence of island ground sloths on the Antilles Islands and the woolly mammoths on Wrangel Island long after their extinction on the mainland. Conversely, could what scholars acknowledge must have been a relatively small initial human population be responsible for emptying an entire continent of its megafauna virtually overnight?
Climate change: A significant and rapid cold snap that occurred between about 12,900 and 11,500 years ago (known as the Younger Dryas) may have stressed the megafauna and their habitats sufficiently to cause widespread die-offs. Contractors counter how could megafauna survive the many climatic shifts of the past two million years and then succumb to this last shift?
Disease: Some suggest a virulent ‘hyperdisease’ was brought by the human population, racing through a megafauna with no natural immunity, but there is no evidence to support this position.
Extraterrestrial comet: The most recent hypothesis in 2007 is based on a thin dark layer at the base of a ‘black mat’ of soil found at 50 Clovis sites across North America. Fieldworkers identify 15 genera of megafauna that are found just beneath the black mat, but not above. Stone tools of the Clovis culture also are found beneath, but not above the black mat layer. Researchers have found charcoal, soot, and microscopic diamonds at the base of the mat, indicative of a comet (rather than an asteroid). This event triggered widespread fires, releasing sun-blocking ash and dust, changed ocean circulation, and initiated the cold spell of the Younger Dryas. Opponents argue that 20 of the 35 megafauna cannot be shown to have existed up to this event, thus suggesting other causes were at play.