Missing Sloths, Modern Pollution, and the Fate of the Joshua Tree

Sunsets on the Joshua Trees. Photo by Ben Horton.
The sun sets on Joshua Tree National Park. (Photo by Ben Horton)

Photographer Ben Horton began his work with National Geographic as the recipient of the first Young Explorers Grant, and has gone on to receive an NG/Waitt Grant as well. When describing his work he says, “Getting people to fall in love with our world is the first step to getting them to protect it. Photography is how I can help that happen.”

Across the southwest, an unprecedented natural event is taking place. The endangered Joshua Tree has come into full bloom across its entire range. Typically, only a small percentage of the trees flower at a time, and of those only a few of their branches hold flowers. Right now though, nearly every branch of every tree is in full bloom. For the endangered tree, and for a the desert life that it supports, this means a year of plenty, and possibly even more seedlings taking root. That the flowering and subsequent fruiting of the trees is enough to save the species is unlikely, however it does help. The best place to see the bloom right now is at the higher elevations where the trees fruit slower, like Keys view road in Joshua Tree National Park. Below, discover more about this remarkable landscape and the creatures who shaped and are shaped by it.

By Ben Horton

I’m standing next to Ranger Josef Zarki looking over Joshua Tree National Park, we’re waiting with firefighters to get trucked out to a blaze that has consumed about 300 acres of a remote part of the desert. Fires have been eating away at an already endangered population of Joshua trees. Josef sweeps his hand through the air as if to take in the landscape and says “When it comes to habitat, this is it.” He is exaggerating, but only by a little bit. Outside of Joshua Tree National Park and it’s sister park The Mojave National Park there isn’t much in the way of a healthy population of these trees. Even in the park there are far fewer trees than one would imagine. The park’s is made up of about 750,000 acres of desert, but when you leave the 100,000 acres that makes up the most visited portion of the park, the trees quickly begin to disappear. It’s true, from the roadway there seem to be plenty, but you’re driving right through the last stands of trees. Fire, a lack of seed distribution, and a noticeable warming trend that has been attributed to global warming have created a perfect storm that has placed these trees on the endangered species list.

Nearby branches are nearly indistinguishable from distant trees in the otherworld landscape of Joshua Tree National Park. (Photo by Ben Horton)
Nearby branches are nearly indistinguishable from distant trees in the otherworld landscape of Joshua Tree National Park. (Photo by Ben Horton)


Scientific observation has confirmed two distinct varieties of Joshua Tree (var. brevifolia and var. jaegeriana McKelvey). The significance of recently splitting the already dwindling population into two species has major implications. How we save two tree species instead of one, with potentially different threats and solutions is compounded by the fact that now we have two trees with vastly reduced populations.

In very few places the two types of Joshua Trees overlap, and here is where one of the defining points is made for the segregation of the species. Where the trees overlap, their numbers are quickly declining. The Joshua Tree is in the yucca family, and each yucca is pollinated by it’s own species of yucca moth that has co-evolved with it’s plant. They come to the trees to lay their eggs inside of the flower in what is to become the fruit of the tree. Where the eastern and western trees overlap, the moths sometimes get confused as to which tree they should be attempting to lay their eggs in, and the pollination is a failure. The moths are closely related, but differ in size by approximately 30% and so find themselves either to large or too small to lay their eggs in a successful location, also failing to pollinate the plant.

Dust and smoke hover around a scorched tree. (Photo by Ben Horton)
Dust and smoke hover around a scorched tree. (Photo by Ben Horton)


Thankfully the pollinating moths are not endangered, many of the trees are still able to come to fruit. Fist sized fruit grow from the flowers with the moth larvae growing inside. Rows of black seeds inside the fruit wait to be consumed by the primary vehicle of their dispersal and deposited in nitrogen rich dung that will fuel the seeds as they sprout. They wait for an extinct species.

Up to about 11,000 years ago, bear-sized shasta ground sloth roamed the deserts of the southwest. A substantial amount of their dung has been found, fossilized, packed full of intact, undigested Joshua Tree seeds, as reported inĀ Harrington 1933:193. These fossilized heaps of dung have led researchers to believe the sloth was the main vehicle for the distribution of seeds. A few animals still transport the seed, rodents and birds carry them quite far actually, but not in the volume or distance that a shasta ground sloth would. Most of the time, when the seeds are passed through the insides of a rodent or bird, the seed dies from either being chewed up, or damaged by the stomach acids, and what is excreted is unable to sprout. When rodents cache the fruit underground, the trees may sprout, but without the seeds traveling great distances as they had previously, they are growing in the same environment as their parent trees. The problem is that this environment is changing and it is no longer the best place for a Joshua Tree.

A Yucca moth deposits her eggs in the tender interior parts of a blossom. (Photo by Ben Horton)
A Yucca moth deposits her eggs in the tender interior parts of a blossom. (Photo by Ben Horton)


Few phrases elicit as heated a debate as “global warming.” Although there is still a raging debate regarding the warming of the planet, in the Mojave and Colorado desert, scientists have found that nights are consistently and measurably warmer than they were a few years past, winter is milder. Although most people are not complaining, a strong cold snap is vital to the joshua tree. It still is not understood why, but observation has confirmed that without these cold snaps, fewer Joshua Trees come to flower. This year was an especially warm winter, and the bloom was over a month late. When the bloom did come, it was not every tree that flowered. These winters are coming more and more often in Joshua Tree, and fewer trees are seeding. The trees need something like the giant ground sloth that could carry the seed to a colder environment, but there isn’t anything filling that ecological niche anymore. They are stuck, unable to pick up their roots and move to a better environment, and because of that they are helpless against an even older threat that has only recently become far more destructive than it ever has before.

Dr. Edie Allen PHD and Professor of Plant Ecology at University of California, Riverside, has found a link between the nitrogen oxides from car pollution and wildfires. Wildfires didn’t always pose such a threat, Joshua Trees grow far enough apart that in the past, fires rarely leapt from one tree to another, and the desert between the trees had little plant life that could carry the fire from tree to tree. The nitrogen oxides from car pollution end up settling in the soil and acting as a fertilizer. In the desert soil that is more like gravel, the native plants have evolved to survive with much less nitrogen. The soil, now rich in nitrogen can support much more of the invasive annual grasses that wouldn’t have grown before, and when those annual grasses die they leave a bridge between trees for the fires to cross. When at one time a fire would take out a few isolated trees, fires now take out whole swaths of desert that is unadapted to such things. In the evergreen forests where fires have been more common, the fires can actually promote life. Some pine cones won’t even open and release their seed until heated by fire, the trees are tougher and can survive the flames. Even if a Joshua Tree survives the flames, it’s likely that with the bark weakened, bugs will enter the soft interior and the tree will die anyway.

An aerial view shows the ancient distant spacing of joshua trees being filled in by new plants taking advantage of the additional nitrogen reaching the soil through pollution and wildfires. (Photo by Ben Horton)
An aerial view shows the ancient distant spacing of Joshua Trees being filled in by new plants taking advantage of the additional nitrogen reaching the soil through pollution and wildfires. (Photo by Ben Horton)


It takes decades for a tree to be more than a few feet of green spines growing out of the gravelly desert, for a tree to grow to maturity takes hundreds of years. With the threats compounded as they are, and the increasing commonality of forest fires, it’s unlikely that the tree will successfully create many more generations of offspring. With the tree gone, the birds that rely on it’s elevated branches, the moths that have co-evolved so perfectly, and the biodiversity that is vital to the already challenged desert landscape will all be hit with a monumental obstacle.

The variety of species across the globe is in decline. We have seen the devastation caused when forests are cut down, but what hasn’t been paid enough attention is what happens when one species is removed. With the Joshua Tree it is apparent that the lives of so many species are interconnected in such a way that without one, the others will disappear. Something has to be done in order to preserve this ecosystem, or at least slow it’s demise until we have a better idea about how we can bring the tree back to a healthy population.

So what is being done? In my mind I imagine people carefully picking the fruit of the trees and replanting them in parts of the desert that still haven’t been affected by the warming, or perhaps in a place where there are not so many invasive grasses to fuel the fires. Finding those places is harder than it sounds though, years of studying the micro-climates around the park would be required in order to predict where the best place to plant a Joshua Tree might be, and we still could be wrong. Perhaps this valley or that ridge has the necessary factors at work to preserve the tree, but for how long?

Night falls and drapes shadows over invasive plants competing with the Joshua Trees, as it once did with the bear-sized sloths who were the trees’ partner in survival. (Photo by Ben Horton)


Better it is to let the tree’s remaining natural distribution channels work in their own way, but to provide protected landscapes so that whichever direction the trees take off in, it will be one that they will be able to continue in. Placing the Joshua Tree under the protection of the endangered species act would be a monumental leap forward. Doing so would keep people from cutting down trees to build roadways, parking lots and golf courses.

The public is largely unaware of the issues that are facing the Joshua Tree and the animals that rely on it, but it comes down to public interest to get these conservation measures put into place. The choices we are faced with are difficult, and the decision makers can be influenced by the support of a public who are fighting for the preservation of this ecosystem. Education, outreach, and communication on these matters is the first line of defense, until we figure out what we really can do to preserve this iconic landmark of the southwestern deserts.

NEXT: Q&A With Ben Horton


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Meet the Author
Andrew Howley is a longtime contributor to the National Geographic blog, with a particular focus on archaeology and paleoanthropology generally, and ancient rock art in particular. In 2018 he became Communications Director at Adventure Scientists, founded by Nat Geo Explorer Gregg Treinish. Over 11 years at the National Geographic Society, Andrew worked in various ways to share the stories of NG explorers and grantees online. He also produced the Home Page of nationalgeographic.com for several years, and helped manage the Society's Facebook page during its breakout year of 2010. He studied Anthropology with a focus on Archaeology from the College of William & Mary in Virginia. He has covered expeditions with NG Explorers-in-Residence Mike Fay, Enric Sala, and Lee Berger. His personal interests include painting, running, and reading about history. You can follow him on Twitter @anderhowl and on Instagram @andrewjhowley.