In comments on this post, Spyder offered a plaintive observation:
In the year-end surveys on global climate change, one of the big 12 CA issues for the future was the potential loss (eventual extinction) of Joshua Trees. If i can dig up the research report (i believe it came out the middle of December), it compiled an overview of how global climate change will likely impact CA over this century. There were 12 worst case scenarios, and one of them regarded the demise of the species. Say it ain’t so!
It is in fact so, and CRN reported here on the possibility that climate change may render Joshua trees extinct, due to the trees’ relative inability to disperse their offspring into viable new populations, meaning that the trees can’t migrate north to escape the worst of the augmented desert heat. As I mentioned in that post, the trees likely evolved to depend on large mammals such as the giant ground sloth to move their seeds to new places, and when the megafauna went extinct, so did the tree’s main method of dispersing its population. That’s conjecture, if well-supported. It’s not conjecture, however, that the trees depend on yucca moths in order to set seed at all. Joshua trees thrive well to the north of their native range: there’s a nice old one growing as a street tree in Fallon, NV, 200 miles north of the tree’s northern native limits. But unless populations of Joshua trees and yucca moths migrate in tandem, each population will go extinct.
From that previous CRN post, describing a poster Dr. Kenneth Cole, of the Colorado Plateau Research Station in Flagstaff, and several other scientists presented at a 2005 climate science workshop:
First, Cole and colleagues took what we know about current 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:
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.
Soon after posting that a bit more than a year ago, I learned I’d misspoken. The data Cole and his associates were working with was, apparently, suspect. I took a closer look at the maps in the poster Cole et al. presented. It’s hard to gauge just where the green splotches are in the above maps, as only state and county lines are portrayed. So I took a portion of Cole et al‘s map and superimposed it on a National Geographic map of the same region:
At first glance, the map looks roughly accurate: the trees along I-40 between Needles and Kingman are there, as well as the Pearce Ferry Road and Joshua Forest Parkway stands, a swath from Mesquite, NV to Beaver Dam, AZ at the top margin of the map, and some near the southern corner of Nevada by Laughlin.
Those last are problematic, though. There aren’t many Joshua trees that close to Laughlin. There are big stands about thirty miles north and east of there, but not in the Newberry Range where that splotch sits. And look more closely at the little spot just east of Yucca (and the BIG spot just east of Yucca is wrong, too, but let’s leave it):
That small patch centering on Wikieup is perplexing. There is one Joshua tree in Wikieup, that I know of, and it was planted by the owners of the general store to make their parking lot look more Wild-Westy. Another patch visible on the bigger map just east of that, on Burro Creek? No Joshua trees there.
And this is the kind of criticism I’ve heard of Cole’s report: the data are not ground-truthed. Much of what Cole’s maps show of the western part of the tree’s range is more or less accurate, though he has the trees heading too far south in Joshua Tree National Park, and there are a number of other discrepancies.
What does this mean for Cole’s conclusions? Well, given that his study was based on data that was unjustifiably optimistic in its assumptions about how many Joshua trees exist now, and given that he had to fudge by a factor of ten the ability of those Joshua trees to disperse, it means that his study, as bleak as it may seem, is almost uselessly optimistic. Given current thinking about climate change, unless we can figure out a way to assist the migration of both Joshua trees and their moth partners, as Connie Barlow is trying to do with Florida Torreya, it seems almost certain that Joshua trees will be extinct in the wild by the end of this century.
Unless we stop burning carbon the way we’re doing. But that would require actual inconvenience, so I don’t hold out much hope.
I think stopping burning carbon the way we do would actually involve a lot of humans starving to death, at least in the short term. But probably if we quit using artificial fertilizers, more than half starve, I think.
I only mention this so maybe you can forgive them a little for wanting to keep burning carbon. The human species IS an animal population, after all, and like all other animal populations, it seldom chooses voluntary starvation, if ever.
Also the possibility EXISTS that the human race will think of a way out of this Greenhouse Problem. I mean, its POSSIBLE.
(puts on TOS uniform)
After all, Captain Kirk always thought of a way!
Why can’t *we* disperse the seeds? I know what you mean about the Yucca moths, but there is evidence (somewhere, don’t ask me to find it, because I can’t!) that insects are adapting to climate change, and changing their ranges. (problem here in San Antonio TX - we’ll get “new” tropical diseases eventually).
Just a thought.
Holy crap.
When am I going to stop going holy crap about this stuff?
But on a less unhappy note, thanks for the link to Connie Barlow’s work; that’s really interesting stuff.
Thanks for this, Chris.
And if this is the sort of analysis we can expect from your book on the Joshua trees, then I very much look forward to reading it.
sravana;
We *can* disperse the seeds, and for that matter we can take the seeds, grow them carefully so that far more of them make it past the very vulnerable seedling stage than would in nature, and then plant them out. And there’s an argument that we ought to be doing that, though people in favor of preserving native biomes as they were in the 19th century generally disagree.
Counting on the yucca moths’ migrating is much more problematic. If you check out my piece on yucca moths over at qarrtsiluni, you’ll see that the moths are basically capable of migrating only for a couple days out of a years-long lifespan, and they have to breed and lay eggs successfully in those couple days as well. If they get blown away to a place where there are no Joshua trees in flower for their egg-laying, they don’t leave any descendants to migrate farther.
It’s possible that we could successfully transplant populations of yucca moths along with the trees. The trees do not generally bloom until they are some decades old, and yucca moths have a rather obscure life cycle, spending years, sometimes decades, underground in a hibernation-like state called diapause. Transplanting Joshua tree forests that reproduce themselves, that aren’t just evolutionarily sterile arboreta, would not be quite as straightforward as planting milkweed and hoping the monarchs show up.
Biologist Olle Pellmyr, who I think knows as much about yucca moths as anyone alive, is an occasional reader of CRN. Perhaps he’ll weigh in here on some of the issues that we’d face if we were to assist the Joshua tree forests’ migration.
In the meantime, I’ll be addressing some of the controversy I allude to above in an upcoming post.
Thanks for your response, Chris.
That article was fascinating! I had no idea that the 2 species were so tied together.
Hopefully Olle will show up to enlighten us further.
I knew about yucca but not yucca moths! I learn so much from your work.
I’d be happy to take up where the giant sloths left off, dispersing seeds (or seedlings); but the yucca moth transplantation problem seems like the work of a lifetime, stretching over decades with no guarantee of success.
Wonder if the Long Now Foundation would be interested in rewilding as a form of long-term gardening. Their mission is to promote long-term thinking (and they have some desert land up in Nevada, near the Great Basin National Park. hmm).
um. OK, add to the annals of read-before-commenting: I just took a look at the pro and con arguments from the Florida Torreya site. The con has some good points.
I still want to be a giant sloth, though.
Chris and others,
We’ve been dabbling over the last 3+ years with JT across the range. What I can offer are some promises of data to come and the view from a natural historian on the moths.
Re accuracy of past range maps, it is important for several aspects, including ecological niches. Past distribution maps are from primary, secondary, and sometimes tertiary sources. And yes, one look will show problems that have implications for this field. Beyond mentioned Arizona issues, note JT in Coloradan section of JTNP on shown map. A record for one Granite Mountains is placed on the map in a totally different Granite Mtns. And there is more, much more. To cope with this issue, we decided to compile from primary observations a rather large presence/absence data set for JT. Lots of miles and GPS memory. So, I think this particular source of error can be much reduced in future work.
Re seeding in places that in the future will be within ecological niche, I admit to seeing several complex issues to handle. First, we will need data on pollinator mobility (i.e., flight range). No hard data on hand, and it is hard to collect; can in principle be done ecologically by labelling and recovering female moths. Sisyphean task in the best of years. What about flight range in moths? Wing shape is often a good indicator of flight ability in moths, and by this measure the pollinators of JT are likely not particularly strong fliers (that said, an eastern yucca moth with stronger-flight wings has expanded its range greatly in the last century). Can be estimated indirectly by looking at genetic structure of populations, and we should have some info on this very soon. Last observation is considerable morphological variation in flowers and moths that will have effects on compatibility of plant and moths; mismatch in floral shape and moth egg laying structures prevents successful mutualism. Another key factor would be to keep track of moth and plant phenology. The adult moth emerges from the ground to live a few days at best. It has to be synchronized with local flowering. If the emergence (which can happen after several years of diapause) is triggered purely by local environmental cues, timing may be appropriate, but there is often a genetic component to this, in which case local adaptation must be considered in choosing moths for possible introduction in new sites. Getting the answer to this question alone would take a few years.
Perhaps it sounds as if I’m overtly pessimistic. I am not, but having dabbled with the yucca-yucca moth association for longer than I care to tell, I have learned the empirical way what works easily and what takes more work with yucca and moths. JT is a glorious beast, but it is in my experience logistically the most difficult of all yuccas to work with (bar one Mexican species). We can do it, but it takes lots of resources to do it well - field hands, $$, and time; a species that may not flower for years carries a disadvantage against other models in the grants race so we have to show just why we should go to the trouble of working with the prickly, remote, tall, closed-flowered, iconic JT. I think the passion is out there, not only among us JT huggers, but also with key people in the pertinent federal agencies to maintain the JT ecosystems into the future. We hope to continue it in my lab in years to come, and if it bears more directly on how we can carry the mutualism forward into the future, that would be a life well worth living.
Olle Pellmyr: thanks very much for that deeply interesting response. Glorious, and Sisyphean, indeed!