D. Andrew White, 14, 08, 2008

Anachronistic Plants

According to most botany text books, edible fruits are edible so as to be eaten. The adaptive function of being edible is to entice animals to eat the fruits, digest the pulp, and later defecate out the seeds. In this manner, animals can be conscripted to aid plants in the dispersal of their seeds. However, as if to complicate things, certain plants seem to be entirely mismatched. They lack suitable animal partners.

Osage oranges (Maclura pomifera) has a fruit like giant green mulberries that few native American animals like to eat. The seeds of osage orange are often viable. However, most animals, such as mice, that eat seeds chew these seeds up. Few animals eat the osage's fruit and defecate out viable seeds. But as far as is known, only horses (Equus caballus) seem to eat osage 'oranges'. (Some observations seem to contradict this premise.) But, wild horses were absent in the Americas for thousands of years before 1492 (Barlow, 2000).

Honey locusts (Gleditsia tricanthos) have huge seeds (beans) in huge pods. While many American animals will eat the edible pulp of the pods, few will swallow the seeds. In addition, the wild honey locust has enormously long thorns on the trunk. Since very few American animals are bark strippers, this thorniness seems over-done. Strangely, there are legume trees similar to the honey locust in Africa. These tropical legumes also produce gigantic seeds. They also have long thorns to stop elephants from stripping their bark. African elephants eat the pods and defecate out viable beans. However, elephants have not been native to the Americas for thousands of years (Barlow, 2000).

Kentucky Coffeetree (Gymnocladus diocus) is an American legume with even larger beans than honey locust's. A coffeetree bean is too large (2-3 cm) for most native animals to swallow. The bean is too bitter for squirrels, worse in fact than the buckeye nut. What then is the function of the over-sized bean? In Africa such large beans are often dispersed by large animals such as elephants.

Could these plants be anachronisms? Is it possible that the main disperal agents have become extinct? Obviously if the plants were absolutely dependent upon certain animals for dispersal, and the animals became extinct, then the plants would follow their hosts into extinction. Rather, what has been suggested is that if the optimum dispersal agents have become extinct, the plants would then depend on secondary dispersal agents. Since even the most anachronistic plants do reproduce, the 'anachronism hypothesis' is not beyond critique.

Megafauna

Horses were present in the Americas over 12,000 years ago. Therefore, it is possible that horses used to disperse osage orange seeds. Likewise, mastodons, and other elephant species, existed in North America at least 12,000 years ago. Possibly they dispersed the seeds of honey locust (Barlow 2000).

In 1977 Don Janzen, an ecologist at the University of Pennsylvania, came up with a theory that could explain certain American fruits which have no efficacious natural distributors. Janzen suggested that perhaps some of these fruits were distrubuted by animals in a manner similar to their counter parts in the Old World. Originally, Janzen's megafauna hypothesis was designed to explain various neo-tropical fruits. Soon honey locust and osage orange were added to the list of suspected anachronisms. There are many candidates for megafauna that could have been seed dispersers. Before about 12,000 B.C. there were wild horses and elephants in the Americas. There were other megafauna also: giant ground sloths, Glyptodont 'armadillos', the hippo-like Toxodon, pig-like enteledonts, large peccaries, and giant Geochelone tortoises (Janzen & Martin, 1982).

After the extinction of the megafauna, some plants may have lost their best dispersal agents. They may still have lingered on in declining numbers. As if to support this hypothesis, osage orange was once confined to a very small area around the Red River tributary of the Mississippi. Perhaps osage orange did have dispersal problems. Even honey locust had a more confined range before modern times. Now honey locust is widely planted (Barlow 2000).

What made the American megafauna die off? Paul Martin, of the University of Arizona, has suggested that human beings over-hunted the American megafauna. This inadvertent over-hunting could have taken place just after the original discovery of the Americas. Large animals are easier to exterminate because they have fewer offspring per lifetime, and because they cannot hide very easily. The Americas were not the only places to experience bouts of extinction. Other continents also had extinction episodes. The dates of these extinctions are suggestive:

The dates correspond roughly to the arrival of modern humans into these new territories. The fact that there was a relatively mild extinction event in Africa, may indicate that African megafauna were better adapted to human beings, as they had co-existed with humans for a longer period of time (Flannery 2001).

Osage Orange

Osage orange, maclura, hedge-apple, bois d'arc, or 'tennis ball tree' (Maclura pomifera) is a member of the fig-mulberry family (Moraceae). Like other members of it family it has some rather strange traits. Osage orange was native to Texas and Oklahoma, approximately the home territory of the Osage people. Today it has now been widely planted far outside its natural range. Not everyone considers it to be a picturesque tree. The female tree can look somewhat like an orange tree with its orange-sized fruits. There are less messy species that are just as beautiful.

Osage orange has alternate ovate-lanceolate leaves, on zigzagging twigs. In the southern states its autumn folliage is a clear yellow. Although, specimens in Ontario, that I have seen, are not so colourful. A pair of stout thorns occur near each leaf-base. Osage orange is dioecious, as mulberries are. Male trees have racemes of staminate flowers. Female trees have pistillate flowers in little radial buttonballs. The buttonballs grow into greenish apple-sized compound fruits of 6 to 8 centimetres width. The fruit-ball is structured like a giant mulberry. The heavy fruits fall suddenly within a day, or so, in the autumn. The sap is full of latex, as is common in the figs and mulberries. Both the sap and fruit juice are somewhat distasteful, at least to humans. The bark is rough, reddish and convoluted. The Osage orange can grow to 18 metres tall. It tends to be a squat spreading apple-like tree.

Osage orange's large fruit is one of its most curious features. The seeds are often able to germinate even without pollination. Nevertheless, root suckering is the more common means of natural propagation. The large fruit is not eaten by many animals. The benefit of the strange fruit to the tree's dispersal is an enigma. In Asia there is a related genus, Cudrania, that some taxonomists consider a synonym for Maclura. Some species in this genus can hybridise with the Osage orange. The well known Cudrania tricuspidata produces somewhat edible fruits. In this regard is is unlike the Osage orange.

In the nineteenth century 'hedge-apple' was planted by farmers as a type of spiny hedgerow barrier. When real barbwire was invented this tradition was discontinued. Still, Osage orange persists semi-wild even in southern Ontario. The heavy compound fruits can dent cars, or hurt heads, when they fall. The 'oranges' take a long time to rot away. Osage orange is more of a curio than an ornamental.

References

Barlow, Connie. 2000. The Ghosts of Evolution - nonsensical fruit, missing partners, and other ecological anachronisms. Basic Books. New York. viii - 291.

Flannery, Tim. 2001. The Eternal Frontier - an ecological history of North America and its peoples. Atlantic Monthly Press. New York. 1- 404.

Janzen, Daniel H., and Martin, Paul S. 1982. Neotropical Anachronisms: the fruits Gomphotheres ate. Science. Vol. 215, 19-27.

Cokinos, Christopher. 2000. Hope Is The Thing With Feathers - a personal chronicle of vanished birds. Jeremy P. Tarcher / Putnam. New York.

In the long run of things, all continental flora are the result of natural invasions. North America's plants are basically a mixture of species of South American and Eurasian origin. Natural cross-diasporas have been gradually mixing species for millions of years. However, human activity, since 1492, has vastly accelerated this mixing process. Often these diasporas are very imbalanced, with natural control agents not following their hosts to the new worlds. Sometimes the native flora are unprepared for introduced diseases. Dutch elm disease, chestnut blight, sudden oak death, emerald ash borer, and Asian longhorned beetle, are only a few examples. More infamous are the current bio-introductions that are human diseases: HIV, SARS, West Nile virus, avian influenza virus, swine flu, and other globe-trotting micro-organisms.

Are Invasives always Bad?

It is always good to consider critiques of any bold claim, and ‘invasion biology’ is no exception. It has often been claimed that invasive organisms cause harm to native ecosystems. They displace native niche holders, or even actively exterminate native species. However, this belief has become something of a cliché. One horticultural writer, David Theodoropoulos, rose to the challenge and questioned the widely held belief that ‘aliens’ are always ‘bad’.

Using mostly examples from botany, Theodoropoulos questions the assumption of the badness of aliens. Certainly it is questionable whether loosestrife (Lythrum salicaria ) causes any real harm to New World wetlands. The saltcedar (Tamarix gallica) seems to fit right in with native plants in many places. Likewise, the prickly pears (Opuntia spp.) do not seem to have harmed Old World ecosystems. Even the notorious caulerpa seaweed (Caulerpa taxifolia ) does not seem to be as harmful as some have claimed. Starting as an aquarium escapee, the seaweed has spread widely through the Mediterranean Sea. It does not always result in decreased native flora and fauna. Indeed, there is some evidence that native fauna populations are higher in caulerpa-beds than in the neighbouring beds of seagrass.

Now one may take issue with the overriding counter-claim that alien are usually beneficial. One would be hard pressed to claim that invasive fungi such as chestnut blight, or elm wilt, are good things everywhere. Nor could one claim that feral dogs, cane toads, or zebra mussels, cause no harm whatsoever. But as is often the case, the truth is sometimes in the middle. Certainly alien species should not be introduced willy-nilly. This is especially true when the species is a pathogen. Still, for all that, an explicit critique of invasive alarmism has long been overdue.

References

Harrington, R.A., Kujawski, R, and Ryan, H.D. 2003. Invasive Plants and the Green Industry. Journal of Arboriculture. 29(1): 42-48.

Theodoropoulos, D. 2003. Invasion biology: Critique of a pseudoscience. Avvar Books, Blythe, California.