I have previously written a blog on the sloth-loving President of the United States. Last week France and I visited the Mojave Desert Ecoregion (NA1308), in the vicinity of Las Vegas, NV and Death Valley National Park, CA. Curiously, sloths were again on my mind, as we drove through patches of striking Joshua Trees.
The range of Joshua Trees (recently split into Yucca brevifolia to the west and Yucca jaegeriana to the east) is a reasonably good fit with the extent of the Mojave Desert, making them a Mojave indicator species. These fascinating trees are not trees at all, but succulents of the Asparagus family (Asparagaceae).
Many Mojave desert animals and birds depend on Joshua Trees for cover, nesting areas, or for food. Over very large landscapes they are the only “tree” available and so may be the only available nesting sites of Red-tailed Hawks. Woodpeckers drill their cavities into them, which in following seasons then go on to serve as nest holes for owls, titmice, wrens, and others. Scott’s Orioles are particularly associated with Joshua Trees; when we visited the Mojave they had unfortunately not yet arrived back from their Mexican wintering grounds.
The photos below are from near Las Vegas, NV. Note nest holes in one individual and one rare inflorescence (generally this year, conditions were not good for flowering).
Of the many fascinating things about Joshua Trees, each Yucca species has co-evolved with a single moth species; the Yucca species is uniquely pollinated by the moth and that moth is exclusive to that one Yucca. In the case of Joshua Trees, after the female has done her duties transferring pollen from one tree to another, she lays her eggs in the flower’s seed chambers. The larvae hatch, feed on some of the growing seeds, then drop to the ground, burrow in, and spend several years or decades underground, before emerging again to repeat as above.
In pre-Presidential times, the fruit of Joshua Trees were consumed by the giant Shasta Ground Sloth (Nothrotheriops shastensis). Mummified dung, which apparently can still be found in the desert, often contains Joshua Tree remnants. This has led to the oft-repeated assertion that the disappearance of sloths, about 8,000 years ago, may now be a constraint to the survival of Joshua Trees, due to a limitation on seed dispersal. Although sloths may have certainly had some dispersion role, it does not seem likely that Joshua Trees depended on them for seed dispersion; they are still widespread through the Mojave so are getting around somehow.
The real hero for that somehow is likely the White-tailed Antelope Ground-Squirrel (Ammospermophilus leucurus), a common chipmunk-like denizen of the desert. They avidly consume the seeds and cache them away in holes, some of which are forgotten in senior moments. The relationship between Joshua Trees and antelope ground-squirrels may be almost as tight and mutualistic as is the case with the Yucca moths, but less appreciated. Interestingly, in another example of a complex ecological interaction, often the only Joshua Tree seedlings that survive their first tender years, under intense browsing pressure from herbivores such as jackrabbits, are those that start to grow in the middle of spiny “nurse plants”.
Very worrisome for the Mojave ecoregion, and for its birds, are concerns that the long-term survival of Joshua Trees is compromised. Not because of the absence of sloths, but because of an evolving climate causing the Mojave to become drier and hotter, meaning optimal fruiting and seeding conditions becoming less frequent. Human-caused climate change is speeding up such changes. For Joshua Trees to survive, the delicate balance must be maintained with Yucca moths, antelope ground-squirrels, and their nurse plants, each with their own challenges adapting to changing conditions. A 2011 study by Cole, Ironside, and others on expected changes in distribution of the Joshua Tree has suggested that by the end of this century, they may be largely gone from Joshua Tree National Park, victims of climate change. Aah, Joshua Trees, you too?!
A few sources I particularly found interesting:
Cornett, James W. The Joshua Tree Second Edition, copyright 2018.
Clarke, Chris. The Joshua Tree: Myth, Mutualism, and Survival. Very nicely written essay at mojaveproject.org (consulted in 2019).
Thank you for teaching us about the Mohave Dessert and the Joshua plant. You missed the Scott’s Orioles but also missed the searing heat soon to arrive. I will feel one with the rest of the animal kingdom, and the squirrels, when I have my next senior moment.
Most interesting. Dad and I nearly got to the Joshua Tree Park. The heat got to us.
.
How can a little squirrel be called an antelope? I am glad to know that they, too, have senior moments.
Joshua trees are most unlike trees and I can’t see that they give very much cover. But asparagus family – a good one. I wondered why they were called trees – no else knows either. Beautiful they are not, but fascinating, yes.
Your Photos are absolutely first class Douglas.
Thank you Douglas, interesting biological connections, and great pictures. I was in the Las Vegas deserts in October 1990, pregnant with Ruskin (didn’t know it was him at the time) with Marg and Bill. We were camped in Death Valley, setting up and getting ready for a meal when this creature whizzed through our campsite! Memory may serve me wrong, but I’m sure it skidded to halt in a cloud of dust and as the dust settled, we grabbed our binoculars, and then moments after it checked us over with it’s beady eye…whiz…it was gone! We were soooo excited to see a real live road runner.
Great stories, thanks again Douglas.