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Anachronistic Fruits and the Ghosts Who Haunt Them

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Anachronistic Fruits and the Ghosts Who Haunt Them Anachronistic Fruits and the Ghosts Who Haunt Them Connie Barlow hirteen thousand years the Age of Great ago,Mammals came crashing to a close in the Western Hemi- sphere. Lost were the giants of the elephant clan: the mammoths, mastodons, and gomphotheres, which had maintained a presence in North America for twenty mil- lion years. The native horses, a tall camel, and all but one species of pronghorn-each group from a and mastodon. Shown here is the lineage thought to have originated Reumted: osage orange surely first time m thirteen thousand that the in North America-vanished from years fruit of osage orange, Maclura pomifera, has touched a molar of its missmg partner in the were the plains. Gone, too, evolutlon, Mammut amencanum. strange beasts that had evolved in South America during millions of years of con- fers, and many deciduous trees of our temperate tinental isolation: ground sloths as massive as forests. (And of these, some produce pollen that elephants, hippo-like toxodons, and lumbering, can be distinguished only at the genus or even spike-tailed glyptodonts, which looked uncan- family level.) Indeed, fossil pollen from strati- nily like the ankylosaurs that had shared the fied bog and lake sediments is the primary evi- Cretaceous landscape with T. rex. dence used to reconstruct the vegetational shifts These large herbivores (along with the biggest that accompanied the repeated coolings and bear, the biggest canid, and several big cats, all warmings of the Pleistocene epoch. of which depended upon the plant eaters) disap- The Pleistocene pollen record shows only one peared in a geological instant. Evidence is tree extinction (a species of spruce) in North mounting that newly arrived humans with for- America near the end of the epoch.z Is there rea- midable stone-tipped spears of Clovis design son to suspect that plants pollinated by insects, were to blame.’ This "extinction of the mas- birds, or bats-that is, plants with little or no sive" that marks the end of the Pleistocene pollen record-might have been more vulner- epoch ravaged the megafauna. What happened able to extinction? to the plants? The answer is yes, but vulnerability to extinc- We cannot be sure. Leaves and flowers and tion in this case has nothing to do with the mode seeds are not preserved as readily as bone. Plant of pollination. Many plants that are pollinated by lineages restricted to upland habitats may come animals rather than by wind produce fleshy fruits and go without a trace. Bogs and lakes, of whose seeds are dispersed by other, larger ani- course, receive a shower of pollen, often from mals. Plants dependent on megafauna for dis- vast distances. But detectable quantities of pol- persal would indeed have been vulnerable to len preserved in sediments are restricted to range reduction or even outright extinction when wind-pollinated plants-notably, grasses, coni- their partners in evolution and ecology vamshed. 15s Riddle of the Rotting Fruit plants are living in a time warp; they are adapted Ecologist Dan Janzen speculates that although for a lost world. Their missing animal partners fossil evidence is lacking, some fruit-bearing are "the ghosts of evolution." plants probably did follow the megafauna into Anachronisms and ghosts caught the early extinction. These plants would have begun to attention of Robert E. Cook, who is the director decline when their megafaunal seed dispersers of the Arnold Arboretum. Citing Janzen and vanished. Some might have gone extinct rela- Martin, Cook published an essay in 1982 in tively soon after losing their partners. Others, Natural History that described the avocado, especially the long-lived and those that regener- Persea americana, as an ecological anachromsm ate clonally from their roots as well as from that has been stunningly successful in attract- seed, still survive but may ultimately be on ing a replacement dispersal agent: us.~ Within track for extinction. Still others have been the past few hundred years, avocado has been regenerated as domesticated cultivars by taken from the New World tropics to orchards humans fond of their fruit or other botanical in Florida, California, northern Mexico, and far qualities. But by and large Janzen thinks that beyond this hemisphere. Wild elephants who those that are still here today suffered signifi- raid village fruit trees in Africa are now "plant- cant reductions in range when important mem- ing" American avocado on that continent.’ bers of their disperser guilds were extinguished. In their landmark paper, Dan Janzen and Paul The narrowly restricted ranges of some of Martin concentrated on Costa Rican plants. But today’s big-fruited plants suggest that Janzen in the final paragraph they extrapolated the may be right: plant extinctions may, in fact, anachronism concept to large-fruited plants of have stemmed from the animal extinc- tions. An indicator that something is amiss is evident in the case of trees whose fleshy fruits fall and rot beneath the canopy of the parent. It was this "riddle of the rotting fruit" that cap- tured Janzen’s attention twenty-five years ago while he was studying the ecology of Costa Rican plants. It made no sense for plants to waste energy by building pulp that attracted few if any dispersers. Worse, in the case of many fleshy fruits, when the pulp rots, the embedded seeds are killed as well. What was going on here? In 1982 Dan Janzen, with paleoecolo- the Domesticated gist Paul Martin, published a paper in Strategies of megafaunal dispersal syndrome. vaneties three native to the New World the journal Science titled "Neotropical of tropical fruits demonstrate a range of attractions and seed Anachronisms: The Fruits the pulp defenses Ripe papaya frmt, Canca papaya, is soft enough to mash rather than Ate."’ had car- Gomphotheres Janzen chew, so the tmy seeds require no physical protection A ried out field studies in Costa Rica to mammal that madvertently crushes a papaya seed is, however, learn whether introduced livestock deterred by a sharp, peppery flavor-and thus the toxms so (horses and cattle) served as surrogate signaled. Pmmate fruit thieves (hke us) can eat around and Pleistocene megafauna for the bereft discard the concentration of seeds, thus foiling the papaya’s’s mtent Avocado, Persea americana, produces a shppery and plants. The result: a list of some thirty dense seed, whose potent toxms taste bitter to mammals In of trees and vines of the Costa species contrast, the seeds of the camstel tree, Poutena campechmana, Rican forest whose fruits bear the dry have a mild flavor and are protected mstead by a tough coatmg. physical and ecological characteristics All but the biggest frugmores could be expected to eat around of "ecological anachronisms."’ These or spit the seeds of avocado and camstel 166 anachronisms in the forests and fields of North America. Indeed, scientists currently breeding honey locust trees to increase the fodder value of their pods, and those developing improved pawpaw cultivars and promoting the use of their fruit, have been doing so unaware that the fruits are anachronistic. Osage Orange-An Extreme Anachronism Recently, I spoke with an archeologist, Frank Schambach, who felt frustrated Honey locust and her new partner m evolution. Strung along because nowhere in the published lit- the top are the usual number of seeds m a honey locust pod, erature could he find the information such as the one restmg on the author’s arm. he needed to solve the ecological puzzle of osage orange-information crucial for validating his thesis that before set- tlers began rearranging the landscape, the wood of this tree, highly valued for making hunting bows, may have been traded far and wide in North America under the complete control of a single indigenous tribe.8 Schambach suspected that osage orange (also known as bois d’arc, "wood of the bow") occupied a very constricted range that could in fact be claimed by a single tribe. Osage orange would more appropri- ately have been named osage breadfruit. A close relative to Maclura, of America, The most anachromstic legume m North Amemca. The mpe pod and to its sister genus Cudrama, of of kentucky coffee tree is toughened byresms, and the seeds are eastern Asia, is the breadfruit genus, mvulnerable to msect attack The is sweet but green pulp Artocarpus. All are linked through the reputedly poisonous to humans Early colomsts m Kentucky mulberry family, a largely tropical roasted and ground the seeds to make a coffeehke brew. The dominated the and ground "cof fee" shown here is courtesy of Carl Mehling. family by figs striking for its members’ compound the eastern and central United States: kentucky fruits and the white latex some exude when coffee tree (Gymnocladus dioicusJ, honey cut. Osage orange was named after the Osage locust (Gleditsia triacanthos), pawpaw (Asimina Indians of Missouri, who first introduced white triloba), persimmon (Diospyros virginiana), and traders to this strange fruit-the color of a glow- osage orange (Maclura pomi fera).). green tennis ball and about the size and firm- To a plant lover, the notion that ghosts may ness of a softball. be haunting some of the most magnificent Inhabitants of the plains and prairie states native fruits of one’s homeland is a revelation. It know this fruit by another name: hedge apple. was quite a surprise, therefore, when I began the Until the invention of barbed wire in 1874, library research four years ago for my book The there was no more effective or economical way Ghosts of Evolution (2001) to discover that vir- to fence free-ranging livestock out of one’s veg- tually no effort had been made to test these five etable garden or cornfield than to plant a hedge temperate plants or to identify other possible of thorny osage orange stems, later interweav- 177 been reintroduced into North America in the sixteenth century.
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