The Biogeographical History of the Prickly Pear Cactus (Opuntia sp.)

Prickly pear fruit tastes like watermelon but with an even higher ratio of seed to fruit.  They ripen about mid-fall and hang on  the cactus pads through winter when consequently, they often taste like overripe mushy watermelon.  They are high in Vitamin C and anti-oxidants and provide much food energy for humans and wildlife.  I pick them with tongs and use a paring knife to remove the skin and the attached, irritating spines, known as glochids, that don’t seem to stop any creature from eating them.  The prickly pear genus originated in South America, and the seeds, carried in the guts of small and large animals, were transported to North America when a landbridge formed between the 2 continents about 3 million years ago.  Humans have planted them in Africa and Australia where they’ve become an environmentally harmful invasive species.

Opuntia humifusa (eastern prickly pear) in flower.  This species ranges from Florida to eastern Canada.

Eastern prickly pear fruit.  It tastes like watermelon.  During the growing season this species grows upright, but during winter it grows flat on the ground.  The fruit of various species of opuntia can be red, purple, yellow, or even white.

Cactus patches like this are common in Texas.  Texas was a gateway for Opuntia into North America.  Javelina and wild hog spread Opuntia seeds in their dung, and the cactus grows in patches.

During the Pleistocene many Opuntia species evolved to become resistant to frost.  Prickly pear cactus thrives on poor sandy soils because they can retain water better than most other plants.  This would have made them especially well adapted to southeastern North America during arid stadials when sandstorms smothered many square miles of territory.

61 species of prickly pear live in North America today.  Texas hosts the most species–20, while 7 frost sensitive species are endemic or almost endemic to Florida.  An additional 7 species are found throughout the southeast.  2 species are cold tolerant enough to live in Canada.  A study of prickly pear DNA determined that southeastern and southwestern regions of North America provided refuges for prickly pear cactus during the Last Glacial Maximum.  When prickly pear cactus recolonized the midwest and Canada after the retreat of the glacier, closely related species came into conact with each other and hybridized.  This suggests that many more species of Opuntia may be the result of hybridization events that occurred when isolated populations reunited after thousands of years of separation due to climate-initiated environmental changes.  The commercially grown Opuntia ficus-indica is itself a manmade hybrid originally cultivated in Mexico.  Its exact parentage is unknown.  It has been bred to produce large spineless fruit called “tunas.”  I tried growing this cultivated variety from seed I extracted from store-bought fruit, but none germinated.  However, I do have a cactus in a pot that I grew from seed I got from a local wild species–Opuntia humifusa.  This species is quite common in my neighborhood because the soil here consists of Eocene-age sands.

Prickly pear cactus flowers attract all kinds of insect pollinators such as bees, wasps, butterflies, and flies.  Peccaries, deer, ground squirrels, rabbits, skunks, coyotes, foxes, turkeys, box turtles, and gopher tortoises all eat the fruit and spread the seeds in their dung.  Much of Texas’s landscape is covered with cactus patches, probably borne from javelina feces.  Pleistocene megafauna must have created cactus patches across much of the south, especially during dry climate stages when Opuntia was a dominant plant.

Collared peccary (Pecari tagaeu) eating a prickly pear cactus pad.  I thought I’d make it 3 blog entries in a row with something about peccaries by writing an essay about their favorite food.

The cactus pads are also edible, and herbivores sometimes forage on them, but to a lesser degree.  There is less food value in the pads, and the penalty of dealing with spines discourages animals from chewing them down to the root.  Opuntia bugs (Chelindrea sp.), though, plague cactus.  They suck the juice from the pads and can kill the cactus.  Last summer, I thought Opuntia bugs killed the prickly pear cactus I’m growing in a pot, but that tough old cactus did grow back from the roots this winter when the cold weather wiped out their nemesis.

Opuntia bugs (Chelindrea sp.) engaging in sexual intercourse.  There are about as many species of Opuntia bugs as there are of cactus.  They look like squash bugs–the scourge of gardeners.

Reference:

Majore, L.C.; W.S. Judd; P.S. Soltis, and D.E. Soltis

“Cytogeography of the humifusa clad of Opuntia (Cactacaea, Opuntiodoene, Opuntaeae) Correlations with Pleistocene Refugia and Morphological Traits in Polypoid Complex”

Comparative Cytogenetics 60 2012

Tags: Chelindrea sp., collared peccary, Opuntia bugs, Opuntia humifusa, prickly pear cactus