I stopped to take a photo of colorful bentonite clay soils along Little Wild Horse Road south of Goblin Valley and on the eastern edge of the San Rafael Swell in Utah.
A large prickly pear cactus at my feet was beginning to flower, and while admiring the flowers I was astounded to see that the cactus had no spines.
I had enjoyed the pads or thalli of spineless prickly pear cacti in Sicily, where they are grown in large fields and served as a wholesome and tasty desert. The domesticated varieties are descended from the Indian fig, Opuntia ficus-indica, or from crosses between it and the Mexican prickly pear, Opuntia tuna; both species are native to Mexico.
Wondering if the cactus before me was one of the domesticated varieties, I gently touched one of the pads and was immediately able to reject the hypothesis that it was a domesticated, spineless variety. Although it lacked spines, it had glochidia, the hair-like, nearly invisible bristles projecting from tufts on the pads. This odd cactus was a native, but it was new to me.
I found two other of the spineless cacti, also much larger than the prickly pear cacti that I was familiar with. When I returned to Boulder, my colleague Tom Lemieux identified it as the beavertail cactus, Opuntia basilaris, which is native to the Mojave Desert, the Anza-Borrego Desert and parts of northern Mexico and the Colorado Plateau.
My first impression was that beavertail cacti were noteworthy for their lack of spines, large size and numerous flower buds. For example, the cactus in the photo is more than 4 feet across and has more than 200 flowers and flower buds.
I began to wonder if the noteworthy traits "spineless," "large" and "many flowers" were related. The growth of spines is not a trivial expenditure of energy, and energy saved by not growing spines could be invested in growth and reproduction. Furthermore, a study has shown that spines shade cacti and can limit both photosynthesis and growth.
The most abundant prickly pear cactus at the San Rafael Swell is the common prickly pear, Opuntia erinacea, and it has a dense layer of long spines. But those cacti were all very much smaller than the beavertails and typically had two to a dozen flower buds. So perhaps energy saved from plant defense can be shunted into growth and reproduction. However, this advantage of spinelessness immediately raises a question.
If cacti gain so much by not producing spines, why are spineless species rare, and why is the desert bristling with spines? My guess is spineless cactus species can only survive in places that have no mammalian herbivores. I mentioned that I had not seen beavertail cacti before, but now three very large specimens were in view. Was there something about this local environment that would exclude mammalian herbivores?
This site was at the base of the eastern rise of the San Rafael Swell, a structure large enough to produce a rain shadow. Looking around, I could see that this particular site was rocky and plants were few and far between— adjacent plants were often separated by two to three meters of bare soil. Areas just a few miles to the north had much less bare soil, many more plants, higher plant diversity and numerous prickly pears but no beavertails.
This local site had very low plant cover, but in addition it was at the margin of bentonite clay soils forming the Moroni Slopes. Bentonite clays result from the weathering of volcanic ash in the presence of water, and they have high levels of sodium, aluminum and calcium that make them inhospitable to most species of plants. I was looking over a landscape very nearly devoid of plants.
I suspect that herbivores such as pronghorn and desert bighorn learn to avoid areas where food is chronically insufficient, and smaller herbivores such as jackrabbits and ground squirrels are unable to colonize and occupy sites where plants are so sparse and meager. Perhaps I had wandered into a site where cacti did not need spines, and those without spines were at an advantage over those with spines.
Jeff Mitton, email@example.com, is a professor in the Department of Ecology and Evolutionary Biology at the University of Colorado.