Here's a little art assignment: Grab a piece of paper and draw a cactus. I'll wait.
Now, be honest: Is it tall and cylindrical with two arms sticking out the side with bent elbows, pointing upward? Congratulations, you've just drawn a saguaro cactus (Carnegiea gigantea)! It's the only cactus that looks like that, and in spite of its iconic silhouette, most people have never seen one in real life, as it only grows in a small area of the American Southwest and northern Mexico.
Saguaros are the tallest North American cacti, native to and common throughout the Sonoran Desert in the Southwestern corner of the United States and Northwestern Mexico. Saguaros are important to the Sonoran ecosystem, as they provide food and shelter for various desert animals. And because they're often the tallest thing standing in the desert landscape, they can fill the niche that is usually occupied by trees: They're the desert go-to for nesting and perching birds and protection for other animals. Once a saguaro has died and fallen, its decomposing body provides important organic matter to the desert ecosystem.
Saguaro cacti have also been an important food source for the indigenous peoples of the Sonoran Desert.
"The Tohono O'odham (Papago), Kimel O'odham (Pima), Hia c-ed O'odham (Sand Papago) and Seri have used the saguaro as a food plant and used saguaro ribs as construction material," says Kat Rumbley, marketing and media manager at the Arizona-Sonora Desert Museum in Tuscon, Arizona, in an email interview. "Many members of the Tohono O'odham Nation still harvest saguaro fruit to this day, as the harvest marks their new year and provides them with saguaro syrup for the year to come. In the cultural traditions of the Tohono O'odham, the saguaros are considered people, the ancestors of today's Tohono O'odham."
The Life of a Saguaro
While some other cacti spread by vegetative reproduction — essentially cloning — saguaros reproduce by seed. Therefore, they require a pollinator to spread pollen around.
"Flowers of the saguaro are pollinated by the lesser long-nosed bat and the Mexican long-tongued bat at night, and by bees and birds, such as the white-winged dove, during the day," says Rumbley. "Saguaro fruits develop right before summer monsoon rainfall and serve as a critical food and moisture source for animals after the famine period of dry, early summer."
When the heavy monsoon rains come in mid-summer and early autumn, the fruits are knocked from the tops of the plant to the base and can be carried by floods to be established in new spots. Their seed is also spread by animals that eat their fruits — especially ants.
"The conditions for successful saguaro seed germination are relatively specific, so you will notice large cohorts of cacti that are all the same age due to establishment in years with these specific conditions," says Rumbley.
Saguaros have a pretty broad geographic range for a plant that's so specifically designed for one ecosystem, but cold is their kryptonite. They avoid both the cold-air drainage basins in the lowest areas of the Sonoran Desert, as well as any elevations high enough to have much frost. Therefore, saguaros in the landscape keep to the warm-air belts in the foothills at the base of desert mountains within their range.
A Cactus in Danger
As iconic as the saguaro is, it faces threats such as wildfires, increased summer temperatures and inconsistent rainfall due to climate change, as well as loss of habitat due to livestock, invasive species and people building a bunch of stuff.
"Development and increased urbanization within its habitat removes mature plants, eliminates the favorable areas for saguaros to establish and influences climate change by producing urban heat islands that become inhospitable to saguaros," says Rumbley.
Not only that, changes in rainfall patterns due to the changing climate affect recruitment of young saguaros, as the babies need dependable watering and they suffer under constant record-breaking high temperatures. And the introduction of grazing to desert landscapes has reduced nurse plant cover and is thought to be negatively affecting saguaro recruitment.
"Human-introduced invasives, such as buffelgrass, create fodder for wildfires that would not traditionally be able to spread in the sparse desert landscape. Saguaros are ill-adapted to fire, as are many desert endemic species, and are not able to recover from significant burn events," says Rumbley.