Nature Notes – From the Backyard to the Biosphere. The Unlikely Allies: Cacti in the Human Sphere

Nature Notes – From the Backyard to the Biosphere. The Unlikely Allies: Cacti in the Human Sphere

Sometimes it isn’t the prettiest plant that most inspires admiration, but rather the most tenacious. It is the survivor that, despite all odds, manages to thrive in even the toughest of conditions. True to this description, the spiny, lumpy, bumpy, often-portly cacti may not typically be considered the most attractive of plants, but are deserving of our admiration for being some of the hardiest floral specimens out there.

These remarkable plants must contend with a host of conditions intolerable to most other flora, including extended periods of drought, intense sunlight, drying winds, dramatic temperature fluctuations, and poor soil. And while the cacti have had to evolve a multitude of specialized features to deal with the unfavourable growing conditions typical of their home environments, they have still managed to be of great value to human cultures for millennia.

Native almost exclusively to the Americas, cacti (members of the Cactaceae family) are found in such diverse locations as British Columbia, Arizona, Patagonia and southwestern Ontario; wherever they occur, they have proved invaluable as sources of food, building materials, medicines, and a host of other resources. Evolved from woody jungle plants, cacti exist today in a great diversity of morphological forms: from the forty-foot Saguaro (Carnegiea gigantea) of the Sonoran desert to southwestern Ontario’s low and spreading Eastern Prickly Pear Cactus (Opuntia humifusa) to the tropical Pereskia genus of leafy trees, shrubs and vines, cacti and the uses to which they have been put by humans are as varied as the environments in which these plants are found.

Of the 2,000 or so cactus species classified, the most ethnobotanically relevant may be those members of the Opuntia genus. Opuntia species are found throughout the Americas; three even exist in Canada. Known broadly as the prickly pear cactus or cholla, Opuntia cacti have provided people across a whole hemisphere with essential resources for centuries.

Like most cacti, Opuntia species have well-developed mechanisms for absorbing and retaining water when it becomes available, and for minimizing its loss once stored. Thus in the arid regions in which it is found, its most basic service to humans (and other thirsty fauna) has been its provision of that most precious commodity of all — water. Its fresh cladodes, or ‘pads,’ can (once carefully ‘de-spined’ using, for example, bunches of grass) be pounded to release a sweet juice, providing the thirsty traveler or desert dweller with a much-needed reprieve. The fact that the cladodes’ moisture content can be as high as 90% makes them significant sources of water in an otherwise dry environment.

Survival in the arid ecosystems in which most cacti are found requires adaptations aimed at dealing with two factors fundamental to plant life — heat and moisture. The succulent stem has become the be-all and end-all in the cactus world. Made of tissues specially modified for water storage, this organ serves the cactus through dry periods, efficiently swelling with water when this resource becomes available, and storing it for periods when external moisture is in short supply. Roots positioned close to the soil surface assist this process by ensuring that even minimal rainfall is captured and sent up to the water-storage tissues for later use.

The green cactus stem has also taken over from the leaves as the primary location for photosynthesis. Leaves on most cacti have been modified over evolutionary time, now existing as spines which serve the plant by deterring herbivory, deflecting incoming solar radiation, and (in combination with the downy hairs found on the stem) trapping a layer of air close to the epidermal surface to regulate temperature and decrease moisture loss via air movement. Unfortunately for the water-conservationist cacti, the most basic of all metabolic processes — photosynthesis — includes the process of transpiration: the release of water vapour from the plant’s tissues into the atmosphere. Given cacti’s need to minimize moisture loss wherever possible, the plants’ stomata (those tiny epidermal pores which allow for gas exchange during photosynthesis) have developed the trick of opening at night rather than during the day as most plants do. The cooler nighttime air has much less of a drying effect than the hot daytime air.

But cacti are much more than mere watering holes. Their fruits, often conspicuous and brightly coloured, have been prized for both their flavour and their nutritional value by people throughout the Americas for thousands of years. The fruits of Opuntia species are said to vary in flavour from that of strawberries to watermelons to figs. First peoples of Mexico and the southwestern United States made full use of the fruit of various cactus species by eating the rind, pulp, and seeds. Seeds were often dried in the sun and ground for later use as a nutritious gruel. Given cacti’s often-significant spines, gathering the fruit presents some real challenges; in the past, long forked sticks were often used to allow the gatherer to collect the fruit without danger of being impaled.

The use of cacti for medicinal purposes appears less significant than the importance of these plants to the human diet. However, several cactus species are, or have been, considered useful in treating a variety of physical disorders. For example, the Hopi chewed the cholla’s root as a treatment for diarrhea, the Blackfoot used Opuntia fuzz to treat warts, and some 19th Century doctors treated wounds with baked Opuntia cladodes. Today, cacti such as Selenicereus grandiflorus (night-blooming cereus) are being processed for use in homeopathic remedies to treat ailments ranging from urinary tract infections to heart disease. Tribes such as the Navajo have always had great respect for the Prickly Pear Cactus, with culturally embedded taboos and prohibitions regarding the plant’s use, as well as special ceremonies related to its harvest.

Opuntia has been of great significance to the history of peoples of the Americas. These plants have been domesticated and grown in family-controlled plots since 3000 BC. Opuntia‘s importance in Mexico and the southern US remains significant. Today, over 250,000 hectares (ha) of plantations can be found in Mexico alone. Over $50 million in revenue are gained each year as a result of Opuntia cultivation; tens of millions of dollars in Opuntia products are exported to Europe and Asia annually. Aside from human consumption of the cladodes as a fresh vegetable, and the pulp and seeds of the cacti as an important fruit, the scale of cultivation today, and the plants’ economic importance, is enhanced by the fact that Opuntia is an excellent fodder for livestock. In Brazil, over 300,000 ha of Opuntia are cultivated for fodder alone.

In addition to their nutritional importance to both humans and livestock, cacti have provided raw materials for a variety of technological applications. The woody skeleton of Opuntia spinosior (cane cholla) has proved very useful as the basis of house and fence construction, as well as furniture making, cuttings from Pereskia species root so easily that the plant has been used to create impenetrable living fences, and the mucilaginous properties of boiled cactus pulp has led to its use as an adhesive (e.g. in ensuring that whitewash adheres to walls). Nopalea cochenillifera (nopal cactus) was highly valued for centuries because the insect for which it is the host (the cochineal insect, Dactylopius coccus) could be processed to produce a much sought-after red dye.

However, while cacti have proved invaluable in so many applications throughout the Americas, their presence has been less welcomed in other parts of the world. Like many introduced organisms, various Opuntia species have become serious pests in far-flung locations such as Australia and parts of Africa. Roaming livestock and wildlife consume the cactus fruits and spread the seeds across broad areas. The result in many locations has been cactus infestations across millions of hectares of land. Australia has been fortunate in that its experimental introduction of the cactus moth Cactoblastis cactorum (a natural Opuntia predator in South America whose larvae dine on the cactus) has managed to bring the Opuntia infestation down to manageable levels, and all this without leading to an infestation of the moth species.

The situation regarding cacti closer to home is quite different. The native Opuntia of the southwestern US and Mexico are now being threatened by the northward movement of Cactoblastis cactorum, and there is concern that populations of these plants, of such great cultural and economic importance, could be decimated by the insect’s arrival. Insects aren’t the only threat to cacti’s survival. Southern Ontario’s eastern prickly pear cactus is listed as a species at risk due to invasion of its habitat by woody plants, and as a result of direct human disturbance in its limited range (e.g., trampling and collection from the wild).

Thus the story of the cactus is a familiar one for North American botanists, bringing together the significance of plants in human culture and history, the potential invasive nature of species introduced to exotic locations, and the threats that natural and anthropogenic changes can pose to wild populations.