The Saguaros and Joshua Trees of the American South-West
The deserts of the United States and Mexico are familiar to filmgoers. The American West is surprisingly rich in a huge variety of ecological niches. Botanical icons, such as the Saguaro Cactus and the Joshua Tree, and mountain tracks attract wildlife. There are herds of Pronghorn and Bighorn sheep, big predators such as Puma, and scores of migrating birds. Thousand of kilometres of impenetrable thickets of cactus are widespread to the south. The Atacama in Chile is the driest place on earth, but even here the population of highly specialised birds and plants still exists. The deserts of the Americas are perfect cactus habitat, with a wide variety of species that often grow very densely. Impenetrable thickets of cactus are widespread and provide important shelter for many small mammals and invertebrates, as well as breeding habitats for birds.
For much of the year, these deserts were portrayed in many movies as an inhospitable environment seemingly empty, but in fact, it can claim one of the greatest diversity of vegetation of any desert in the world, as well wildlife. It has, for example almost 69 species of reptiles, including the legendary Gila Monster. The Sonoran Desert ecoregion covers 223,000 square kilometres, that include a large part of southern California and Arizona as well as the western part of the Mexican state of Sonora and much of upper Baja California. Part of this desert includes Death Valley where high summer temperatures can exceed 55C.
Mojave is the place where the celebrated Joshua Tree grows. These impressive trees with their shaggy limbs strike a spectacular pose across the landscape. A type of yucca, Joshua Tree is the largest member of their family and mature specimens can reach heights of 15 metres or more and live to be at least 300 years old. They were given their English name by Mormon pioneers, who saw in the tree’s upward-reaching branches the supplicating arms of the prophet Joshua, guiding them upwards. When conditions are right, the trees flower in spring; seemingly part of the equation is a sharp frost, which scientists believe damages the growing end of branches and by this stimulates flowering. The creamy-white flowers are produced in panicles and their pollination is provided by the Yucca Moth, which spreads pollen while visiting different trees and laying its eggs. Joshua Trees can also reproduce from their roots, a useful asset following devastating fires, for example. One of the best places to see Joshua Trees, which are endemic to the United States, is the Joshua Tree National Monument in the Little San Bernadino Mountains, which separate the Sonoran Desert from the Mojave and are home to wildlife typical of each ecosystem.
The largest reptile in the Mojave is the Desert Tortoise, a much-declined species but one that still survives here in reasonable numbers. It feeds on a variety of vegetation, from flowers to grasses and cacti. Desert Tortoises can survive up to a year without drinking, deriving moisture from the vegetation they eat. Their strong front claws enable them to dig hibernation chambers and in summer, shallow burrows where they escape the intense heat.
The Mojave is also home to the largest desert spider, the Desert Tarantula. It reaches a total span of 10cm, and the tarantula is a fearsome sight, but it is not easy to come across one as they are shy and retiring creatures, usually only emerging from their burrows at dusk in search of prey such as grasshoppers, beetles, and small rodents. Their hunting methods are different from other spiders as they chase down their prey as opposed to the ‘sit-and-grab’ method. They like to burrow into soil crevices around the Joshua Trees – their favourite habitat. Although tarantulas will bite humans only when greatly provoked, their bite can be painful and a too-close encounter is best avoided.
Interestingly, the tarantula is itself prey to a remarkable group of insects, the Tarantula Hawk Wasp. The female wasps seek out tarantulas as host for their larvae in what is an extraordinary, if not rather macabre, process. They locate the spiders by smell, entering occupied burrows to lure out the resident and then sting it. The tarantula is quickly paralysed by the wasp’s venom, soon becoming comatose; the wasp then drags it to a hole, lays a single egg on its body and then covers the spider with soil, leaving only a small entrance; it may even reuse the spider’s own burrow for this purpose. The egg later hatches and the wasp larva then feeds on the living body on an incapacitated spider, before finally emerging through the entrance as an adult wasp.
The warm, rocky slopes of Saguaro are home to the Saguaro cactus, which thrives here, and are one of the most biodiverse ecosystems, surrounded by a wide range of plants and wildlife. The beautiful Saguaro flowers open only for a few hours. By first light, after their nocturnal blooming, bees and other pollinators are delighted with the rich pollen supplies. By early afternoon the blooms have already started to wither.
The Sonoran Desert is renowned for its cacti and other succulents, but one species stand out as a familiar symbol of the desert ecosystem: the Saguaro or Arizona Giant Cactus. The Saguaro is a slow-growing but potentially large plant. After a few decades, it can attain 10 or even 15 metres in height. The oldest Saguaro may be 200 years old, with a girth of 3 metres plus, and carrying thirty or forty ‘arms’. Those limbs-branches contribute to the tree’s sculptural and anthropomorphic character. Saguaro has a single trunk which, together with the limbs, forms a central component in this plant’s impressive water-storing capacity. The structure of the plant is essentially a robust cylinder of wooden rods surrounded by absorbent pulpy tissue and encased in an outer skin made of ribs and grooves, giving it a fluted appearance. As the skin expands, it allows the Saguaro to absorb water and then retain it within the pulpy tissue. The bigger the plant, the more water it can store, and therefore it is more able to withstand periods of severe drought, and can visibly increase its size after rainfall. When they are ‘full’ the skin appears less fluted, and once fully charged with water, a Saguaro can survive for several years before needing to refill.
Late spring is Saguaro flowering season. Each of the creamy-white flowers will bloom for only a few hours, opening and closing in response to heat. The flowers comprise a garland of petals attached to a tube ten centimetres long, in the bottom of which is rich nectar. Each flower will start to bloom after dusk and will usually be fully open by midnight. This attracts a range of creatures which by reaching into the flower to get to the nectar will then pollinate it. The first to visit at night are bats and flying insects before diurnal pollinators take over at sunrise. By noon the flowers begin to close and die off, subsequently forming a red fruit which is a popular food for many types of desert wildlife.
The Cactus Wren lives mainly on insects, but takes advantage of the Saguaro seasonal fruit. It also nests in Saguaro, building its nest in existing holes or a dense clump of spines.
A common sight around Saguaro cacti is the Gila Woodpecker which uses the same nest holes for several years. Newly excavated nest holes are not used straight away as the woodpecker must wait for the sap to harden before moving in.
Perhaps, the most iconic American desert bird is the Greater Roadrunner which is a member of the cuckoo family and can run at speeds up to 32 km/hr. He isn’t a fussy eater and dines on anything to hand, including insects, small birds, mammals, and reptiles. Loony Tunes saw the comic potential in the Roadrunner’s antics and the bird was featured in many comics to the delight of children and adults everywhere.
Several desert bird species enjoy a strong ecological association with the Saguaro, including Gila Woodpecker, Gilded Flicker, and White-winged Dove. The first two species excavate nest sites inside the cactus, drilling through the outer skin and layer of pulp into the internal cylindrical structure, where they then peck out a cavity. This can leave Saguaros literally peppered with holes, and sometimes with big gaping wounds; yet they rarely cause serious problems for the plant, which is able to seal off any exposed water-storing tissue. Meanwhile, the largely migratory White-winged Dove is one of the Saguaro’s most important pollinators, feeding extensively on the plant’s flower pollen, nectar, fruit and seeds. So close is the relationship that the dove times its arrival in the Sonoran Desert with the Saguaro’s flowering season.
Despite being protected by law, smaller Saguaros are vulnerable to theft and large ones to vandalism, although sometimes they do strike back. There is at least one documented incident of a large Saguaro being shot at repeatedly by a man with a rifle, only for the damaged plant to then collapse on top of its assailant, killing him. Development and projects such as road-building and suburban sprawl have also taken their toll on this magnificent plant. One of the best places to see Saguaro in their full sculpted glory is the Saguaro National Park near Tucson, Arizona. The existence of such a remarkable relationship underlines the complex connections of the natural world and the urgent need to protect the wider landscapes and wildlife of the Mojave and Sonoran Deserts. They are highly vulnerable to growing pressure, including a rapidly expanding human population that needs the land and water resources, the encroachment of agriculture, as well as certain aspects of tourism, particularly uncontrolled off-road driving which can be disastrous for desert plants and wildlife.
Like many other desert reptiles, the Gila Monster only emerges to hunt in the cool of the night. Slow-moving and unaggressive, it is one of the world’s two species of venomous lizard, subduing its prey with venom delivered from grooves in its teeth.
The flowers of the desert spread after winter rains. Spring blooming annuals have to germinate, bloom and set seeds within a short season; their seeds then wait in the ground for the rain to return.