Joshua trees and extinction

Yesterday’s mail included Keys Views, the membership newsletter of the Joshua Tree National Park Association. (The name is a play on Keys View, a popular viewpoint in the park.) The lead story, entitled “Oh Say Can You See… a Land Without the Joshua Tree?”, describes a study of the potential effects of climate change on the park’s namesake tree species.

From the article:

Predictions of the Joshua tree’s retreat from the Mojave were contained in a 2005 report whose lead author, Dr. Kenneth Cole, is a climate scientist for the federal government’s Colorado Plateau Research Station in Flagstaff, Arizona. Dr. Cole and his colleagues examined climatic conditions within the present range of the Joshua tree; they studied the Joshua tree’s climate tolerances and its potential to shift its range; and then they applied predictionsof future climate change to determine the likely effects of global warming on our beloved yucca grande. To quote from the report:

“The future potential range of [the] Joshua tree (Yucca brevifolia) is not only reduced and shifted northward by climate change, but the plant’s lack of dispersal mechanisms should reduce its actual extent by at least 90%”

Cole et al released some of their data last year in poster form at a meeting of the U.S. Climate Change Science Program in Arlington, Virginia. The maps below are from that poster. First, Cole and colleagues took what we know about current Joshua tree distribution:

20th Century Joshua tree distribution

…which is fairly accurate at this point: scientists have covered nearly every square mile of the plant’s habitat on foot, which wasn’t always the case. Then they took projections of future Mojave desert climate in a world with twice the atmospheric carbon dioxide we had in the 20th century, calculated the likelihood of Joshua tree survival due strictly to climatic factors, calculated the rate at which the trees disperse new seedlings to potentially more welcoming habitat, and mapped the species’ projected range in the mid-late 21st century.

Or they tried to. But the projections didn’t include any Joshua tree stands large enough to show up on the map. So they assumed that the trees would somehow disperse their seeds ten times more efficiently than they have been found to, and they got this map:

projected 21st Century Joshua tree distribution

Even in this extremely optimistic scenario, the Joshua tree will become extinct in Arizona and Utah. No Joshua trees will survive in Joshua Tree National Park, nor will these at my usual campsite on Cima Dome:


The problem is seed dispersal. Joshua trees don’t do it well. Oh, ladder-backed woodpeckers will hammer at fallen fruit to get the worms that live inside, thereby spilling seeds all over, and packrats will pick up seeds and fruit and carry them fifty feet from the tree or so, and there are a few other birds and rodents that play a role in moving Joshua tree seeds around. Every once in a while, a coyote might eat a fruit and shit it out ten miles away. But unless the tree is on a long, steep hill, its golf-ball-sized fruit tend to stay within a few meters of the parent tree. The seeds are large, and seem to have no adaptations allowing birds to carry them long distances by accident: no sticky burrs, no sweet pulp surrounding a gut-proof seed coat, nothing but a delicate little black flake. Most of the time when something eats a Joshua tree seed, it kills the seed.

I started wondering a few years back, as I learned more and more about Joshua trees, whether there might not be some missing puzzle piece. Joshua trees are tall, heavily defended plants with fleshy fruit growing at the top, sometimes thirty feet off the ground. There are quite a few plants in the Americas that, like the Joshua tree, seem ill-adapted to seed dispersal because their fruit isn’t designed for any living animal to disperse efficiently: the Osage orange, the pawpaw, the avocado. Many of these plants have been linked to the giant extinct animals of the Pleistocene, big critters who could denude a pawpaw or avocado tree of all its fruit, walk a ways and excrete the tree seeds, seasoned only slightly by digestive juices. Could Joshua trees be another such plant, dependent for seed dispersal on an animal species that will never come back?

Turns out at least some scientists share my suspicion.

Nothro y nosotros

Nothrotheriops shastense, the Shasta giant ground sloth — pictured here with a couple no-good desert layabouts in a painting by Carl Buell, who by the way started blogging again while CRN was temporarily out of service — lived throughout the range of the Joshua tree in the Pleistocene, up until about 12,000 years ago. That’s recent enough that you can still find its mummified dung in caves in the southwest. Said dung has often been found to contain the remains of Joshua tree fruit, with seeds that likely would have been viable when excreted.

The Joshua tree’s historic range, in other words, may already be a fossil of sorts, an artifact of a relationship with a species that no longer exists. The big problem with extinction is not so much that those species go extinct — though each is a huge loss — but in the extinction of the relationships that species had with every other species it encountered. No species exists in an ecological void, and each provides either sustenance for or competition with many other species. Each species lost is a node untied in that mesh of relationships.

There is nothing to keep Joshua trees from growing well outside their current range. A grove of them has been growing at the Tonopah Airport, one valley north of their present limit, since World War Two. There are mature Joshua trees growing in Ione and Fallon, Nevada, more than a hundred miles north of their current range. The trees would likely find the climate of the hills around the San Joaquin Valley salubrious, and if they dispersed as fast as oaks do they’d be there in fifty years — there are several distinct populations within an hour’s blue jay flight. But unless the scientists’ deliberate optimism pans out, an unlikely prospect, global warming may very well kill off the last of the wild Joshua trees. And that’s not even accounting for increased fire danger, and then there’s the fact that small mammals eat Joshua tree “bark” in dry years, a measure of desperation that may have killed thousands of trees in Joshua Tree National Park alone

I need to get that book written. Major life decisions that would free up writing time have been put off. They can’t wait much longer.

3 thoughts on “Joshua trees and extinction

  1. miscellanneous

    Yes, you need to get that book written. I have attached time to this message, just for you, to write.