Prickly Pear: A Cactus in Need of a Camel?
By R. Gary Raham:
The prickly pear cactus, Opuntia polyacantha, thrives on our dry western prairies and xeriscaped gardens. Like other soft-skinned animals, I usually choose to avoid this hearty and spine-endowed creature, stepping around the fleshy, pincushiny pads whenever possible, and watching where I sit during lunch breaks in the field. Prickliness seems to come naturally to many prairie plants like yucca, acacia, and mesquite. Spines deter herbivores. But what herbivores? Some scientists speculate that our defensively equipped dry land plants are still haunted by a few “ghosts of evolution.”
Certainly, many animals manage to harvest some of the cactus’ water-rich, seed-packed, red fruits and fleshy pads. Prickly pear seeds traverse the digestive tracts of coyotes, foxes, peccaries, and tortoises to germinate in their feces. Even cattle and horses munch on cactus pads when times are tough, but they are meals of desperation rather than choice. However, animals roaming western ranges today are mere remnants of an ice age fauna that included tough-lipped mega beasts like giant ground sloths, mammoths, and camels. These latter grazers represent the ghosts of prairies past—ghosts some scientists would like to resurrect.
Cactus History and Modus Operandi
The genus Opuntia apparently arose in South America but made its way into North America, and diversified into many forms. Botanists have a hard time classifying the genus into species because the plant tends to hybridize readily with its neighbors.
Plants and animals today interact as the result of their co-evolution over long stretches of time.
Opuntia species reproduce quite effectively just by falling apart and dropping roots when they land. Opuntia fulgida, the so-called jumping cholla cactus, gets its name from it ability to break easily and use microscopic, backward-pointing mini spines to latch onto unsuspecting mammal transport. Connie Barlow in her book, The Ghosts of Evolution, said that the plants are so friable and the spines so adhesive “that passerby swear that the branch itself jumped out to hitch a ride.”
Prickly pear cacti “beat the heat” in several ways. Spines are modified leaves that not only make a point, so to speak, with passing animals, but also conserve water loss from evaporation. Wax coats cactus pads, which are modified stems. The wax shields the bright green underlying photosynthetic layer from bright sunlight. Unlike most plants, cacti practice water conservation by opening their pores (stomata) at night rather than during the day to collect carbon dioxide.
Dan Jansen, a tropical ecologist known for his work describing the co-evolution of acacia trees and ants, has learned to see landscapes from a long-term perspective. During the 60s, 70s, and 80s he worked with Pleistocene ecologist, Paul Martin, to develop the idea that plants and animals today interact as the result of their co-evolution over long stretches of time. And sometimes, because creatures can’t change instantaneously, they behave in ways that only make sense when viewed in the context of past history. For example, many tall varieties of Opuntia set fruit at heights modern herbivores can’t reach. As Martin more colorfully said, “We live on a continent of ghosts, their prehistoric presence hinted at by sweet-tasting pods of mesquite, honey locust, and monkey ear.”
In 1986, Jansen described the nopaleras—a Mexican/Indian term for dense stands of prickly pear 1-4 meters tall, with associated yucca, acacia, and mesquite companions—as anachronistic landscapes. In other words, as landscapes that are missing some of their old parts: specifically, large browsers that would have partnered with dry land shrubs to disperse their seeds while pruning their growth enough to help maintain open grassland. North American ice age camels and elephants helped fill the job of “large browser.”
Ironically, ancestral camels migrated to Asia from North America and today munch contentedly on prickly pear carried to Africa by humans. Jansen is one ecologist who proposed that one way to keep cactus and mesquite from taking over rangeland would be to reintroduce their historical browsers. An experiment was unintentionally run in 1857 by U.S. Army officer, George Beale, who used camels as beasts of burden to open the Great Wagon Road through Indian Territory from Texas to California. He noted in his journal that “It is certainly very gratifying to find these animals eating, by their own preference, the coarse and bitter herbs hitherto of no value, which abound always in the most sterile and desolate parts.”
Personally, I vow to pay more attention to the unfriendly prickly pear. I know they can be cultivated for food. I love to photograph their colorful blooms and watch bees and beetles wrestle food rewards from among the abundant stamens—while unknowingly giving the plant’s sex life a boost when they carry pollen from plant to plant. But in addition, I plan to look for the evidence of ghosts—ancient partners, like camels, who somehow ate their way into cactus genes and linger in their behavior yet today.
Who knows? Maybe camels will return for a long-delayed reunion on the Western plains.
Gary Raham is a nature writer and illustrator. He is the author of: The Restless Earth: Fossils, The Dinosaurs’ Last Seashore, A Singular Prophecy, and other titles of science fact and science fiction. Download a free sample of Confessions of a Time Traveler at www.biostration.com/books/CTTindex.html