D. Andrew White, 30, 01, 2005
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:
Eurasia - mammoth etc.: 12 400 B.C.
Americas - mastodon etc. : 11 200 B.C.
Caribbean - ground sloth : 4 250 B.C.
Siberia, Wrangel Island - mammoth : 1700 B.C.
Madagascar - 'elephant birds' : 500 A.D.
New Zealand - giant moa birds : 1200 A.D.
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).
Critique
Some extinct dispersal agents may be from more recent times. The Carolina parakeet (Conuropsis carolinensis) did eat Osage oranges, as well as cockleburs and other strange fruits. The Carolina parakeet became extinct in 1918 (Cokinos 2000). Perhaps the parakeet dispersed the Osage orange seeds. Other people report that animals such as fox-squirrels do in fact drag the fruits about. Perhaps this would be dispersal enough.
Not every one agrees that such 'anachronistic' plants are actually lacking dispersal agents.
Perhaps they are not anachronisms.
Consider the observations of Elizabeth Moon of Texas:
Sir,
"On the website ontarioprofessionals.com, I note that you state that
members of the modern North American fauna do not eat the fruit of
Maclura pomifera, and that this fruit is readily eaten by horses.
"Whatever is true in Ontario, this statement is not true of the tree in
the southern part of its range, where fox squirrels and other mammals
readily consume the fruit and spread the seeds. We have a small
woodland in central Texas with many Osage orange trees in it, and we
have often seen squirrels at work on the broken fruit; we have also seen
evidence (from scat and tracks) that other animals (opossum, possibly
raccoon, probably grey fox) eat it. There have been no horses on that
acreage acres for over 20 years, but there are many seedlings, saplings,
and mature trees both in the creek woods and on the fencerows.
"A little more field observation would dispel the mistaken belief that
nothing eats the fruit, or that horses are eager to sample it. (I won't
say horses will never eat the fruit, but of the half dozen or so horses
that I've had in a paddock with the trees in easy reach, none has done
so. One is very fond of the leaves and bark, however, and tears
branches off the tree...but he merely sniffs the fruit and leaves it
alone.) Whoever first promulgated the notion that the native fauna
don't eat the fruit just didn't get out there in the field and look at
what happened. The fruit is not eaten on the tree, or immediately after
it drops (except in bad years when other food supplies fail), but
within a few weeks all the fallen fruit is ripped open and consumed." ----
Sincerely,
Elizabeth Moon
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 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.
Vegetation Types & Forest Regions
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Vegetation Zones
Even before C.H. Merriam, in 1898, and H. Mayr , in 1909, there have been attempts to define and map 'Vegetation Types'. Vegetation type is a description of the dominant plant life forms in a particular region. 'Vegetation' is not defined by specific species, but by general life forms. The life form may be tundra grass, montane trees, conifers, deciduous broadleaf trees, desert succulents, or chaparral scrub et cetera. The maps made at the turn of the last century are very similar to the vegetation maps made today.
Forest Regions
Forest Regions are tree dominated vegetation types tailored named to specific geographic regions. For example, the Temperate Forest vegetation zone of eastern North America is often called the Carolinian Forest Region, after the Carolinas. Coniferous forests, in general, are called Coniferous Forest vegetation zones. The Boreal Forest Region is basically the cool-temperate coniferous forest of the northern hemisphere. All forest regions grade into their neighbours.
Climate
In the 1960s the ecologist L.R. Holdridge discovered that maps of a few climatic factors almost exactly corresponded to older maps of 'Vegetation Types'. Growing season, humidity and annual precipitation were the main factors. Biotemperature, the mean temperature over and above freezing, is a simple measure of growing season. (Temperatures below 0oC are not averaged, plants are dormant.) Mean annual precipitation is another very important factor. Humidity depends on both temperature and available water. When mapped together the climatic factors share outlines with most of the vegetation types.
Soil Types
Soil types, when mapped, correspond very closely to both vegetation type and climate. Certain features of soil are related to the parent mineral on which they are based. However, climatic factors seems to have more influence overall. Acidic soils (pedalfers) tend to occur in moist forested areas. Basic soils (pedocals) tend to occur in dry areas. Tropical and sub-tropical soils (lateritic) tend to be leached and low in surface humus. Prairie pedocals tend to accumulate lime or salt (solod) under a fairly rich humus layer. Precipitation and temperature seem to be the two main factors determining both vegetation and soil type.
References
Eyre, S.R. 1968. Vegetation and Soils - a world picture. Second Edition. Aldine Publishing Company. Chicago.
Holdridge, L.R. 1967. Life Zone Ecology. Tropical Science Center. San Jose, Costa Rica.
Lawrence, George H.M. 1951. Taxonomy of Vascular Plants. MacMillan Publishing Co. Inc. New York.
Merrill, Elmer D. 1981. Plant Life of the Pacific World. Charles E. Tuttle Co. Tokyo.
Note: Taxonomy based on various Internet sources.
Invasive Species
'Invasive species' are plants of foreign origin that become naturalised in a new location. In the case of Ontario, the vast majority of these species come from Asia. Asia often gets a bad rep because of this. But actually, the same accusation could be made the other way around, Asia has many invasive species of American origin. The cause for the dominance of 'invasives' from Asia and Europe is that Eurasia has a range of climates similar to North America's. Eurasia is over twice as large as North America, hence it has over twice the number of species to share.
Invasive species often lack crucial natural controls, agents such as specialised insects, that keep their populations in check at home. Sometimes invasive species can push out native species. Japanese barberry (Berberis thunbergii) can smother-out other understory plants, as it has in parts of New England. The kudzu vine (Pueraria montana) is one of the worst of the smothering invasives. Luckily kudzu cannot grow in Ontario's cold clime. Naturalised Norway maple (Acer platanoides) is also suspected of reducing local biodiversity. Naturalised ailanthus (Ailanthus altissima) also seems to reduce understory diversity and to crowd out native seedlings. Sometimes there is no real solid evidence that the invasive does any real harm to the ecosystem to which it is introduced. European buckthorn (Rhamnus cathartica) does not seem to cause much damage. Buckthorn basically grows in the same kind of temporary habitat (fallow fields) as do the native hawthorns.
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.