A Tale of Hungry Bats and Lush Flowers

Gary Paul Nabhan and Stephen L. Buchmann in The Forgotten Pollinators detail the remarkable relationship between the bats and the agave. Nectar-feeders like the long-nose bats travel a looping 3,200-mile (5,150 km) migratory route every year that appears precisely timed to take them from wintering caves deep in Mexico to caves in which they raise their young as far north as Tucson, Arizona, and Big Bend, Texas. The migratory route is timed to take advantage of the flowering of 16 different plant species, including tree morning glories, century plants, and saguaros.

Sadly, the bats have been battered by the loss of roosting sites in both Mexico and the United States, as well as the effects of pesticides and other hazards. Caves that once harbored millions of bats are now empty, and some of the migratory bats have dwindled to threatened status. As a result, biologists have noted a decline in the plants the bats pollinate, including the agave and the saguaro.

The agave and its cousins have also long sustained human beings. Native Americans quickly discovered that the agave offers a treasure trove of nutrition if they are harvested just before the plants pour that hoarded energy into growing the 30-foot-tall (9.1 m) stalk. Many Native American cultures hacked out the rich heart of the plant or the young stalk, then roasted the great mass in rock-lined pits. From the sweet, pulpy heart they would make a nutritious, high-energy snack that would last for months when dried. They also made an alcoholic beverage. Today, the agave are still used to make mescal and tequila, but the popularity of these alcoholic beverages has resulted in a worrisome overharvesting of agave in Mexico, with an estimated 1 million plants a year chopped down to make bootleg liquor in Sonora, Mexico, alone.

The agave come in many varieties in the Chihuahuan Desert, including the lechuguilla, commonly referred to as the shin dagger. The leaves of this small plant roll at the tips into a finger-length needle filled with sap that contains a strong muscle contractor that makes the wicked wounds it inflicts especially painful. The lechuguilla, which puts out flowering stalks every 25 to 35 years, can also reproduce by sprouting from rootlets. As a result, a whole hillside of stalks may spring from a single plant. Another common plant is the sotol, a normally slow-growing plant that puts out a stalk that spurts upward by a foot a day after 12 or 15 years. It also yields a rich food source from a heart about the size of a cabbage and can also be used to make a sweet, alcoholic drink.

The high altitude, cold winters, and dry summers have impoverished the variety of cactus in the Chihuahuan, with the exception of the seemingly indestructible prickly pear, which remains widespread. The prickly pear makes energy from the sun with the chlorophyll in its green skin, which allows it to convert its leaves and stems into long spines and hairlike tormenters called glochids. The quick-growing prickly pear stores water in its thick pads and can shrivel in the dry season and swell up at the first rain. Moreover, the cactus has high concentrations of calcium oxalate crystals just under its skin. This prevents evaporation and also makes the pads toxic to most animals.

The prickly pear uses a specialized method of turning the sun’s rays into food common to many desert plants. Normally, tree leaves convert the sun’s energy into food as the light hits the leaf. However, that requires the plant to open up its pores, which means it loses a lot of moisture. By contrast, cacti and other desert plants use a version of photosynthesis called crassulacean acid metabolism (CAM), which reduces water use by 70 percent. Plants that use CAM open their pores, or stomata, to take in carbon dioxide from the air only at night. Come sunlight, they close their stomatas so they can complete the process of photosynthesis while still sealed up, preventing water loss. The tradeoff is that they cannot make energy from breaking up the carbon dioxide nearly as fast as plants that do take in light and carry out photosynthesis all at one time, which explains why CAM plants like cactus and agave grow much more slowly than plants that don’t have to fanatically hoard their moisture.

In nature, no matter how elaborate a plant’s defenses, some animal will evolve the ability to break through them. So the thorned, poisonous prickly pear is the mainstay of the collared peccary, a small, bristly relative of swine with wicked teeth, a sensitive, fleshy nose, and an endearing repertoire of grunts used to keep track of each other in thick brush. The peccaries consider prickly pear a delicacy, despite the darning needle spines, the glochids that can blind cattle, and the calcium oxalate crystals that deter almost all other diners. The peccaries manage this feat with tough tongues, thick hides, and heavy-duty kidneys.