Rodents Seek Moisture from Sensitive Plants During a Severe Drought Year

Primary habitat (white soils) for Tiehm’s Buckwheat (Eriogonum tiehmii) in western Nevada

Jim Boone, Ph.D., Ecology (with decades of experience harassing rodents)

The summer of 2020 has been extreme in the Mojave Desert — definitely one for the record books. As of October 10, Las Vegas is still adding to the record number of days since the last rainfall (nearly 6 months with not a drop) and nearly matched the record for the number of days (97 vs. 100) with temperatures reaching at least 100 degrees. I knew the summer had been hot and dry, but living in my air-conditioned bubble, I was privileged to not experience it firsthand other than when I went out in my backyard to feed the birds and freshen their water.

After suffering a summer of hiding indoors avoiding the extreme Mojave Desert heat, and of course social distancing to avoid the global pandemic virus, I headed into the desert for a few days to photograph some plants and work on a few other small projects. Driving north out of town, I was stunned to see the desert so dry — the plants clearly were suffering. The creosote bushes were brown, and our usually bright-green Mojave yucca had turned sickly yellow — miles and miles of sickly yellow as I drove up the highway.

Looking for the northern-most Mojave Yucca, I turned off onto a dusty dirt road and pulled a few hundred yards off the highway. First things first after more than an hour in the truck, I hopped out to relieve myself on the desert sands. To my surprise, a zebra-tailed lizard, one of the most skittish of our desert lizards, ran up to me and lustfully tried to catch drops before they soaked into the parched soil. I knew that something was wrong. Not only were the plants suffering, but apparently other creatures were too.

Zebra-tailed lizard near urine soaked sandy soil

I hiked onto the low hillside photographing various species of plants as I went. These plants didn’t look quite so bad as the ones I had seen farther south, but everything was still very dry and dusty. As I photographed a Joshua Tree, I noticed that rodents (probably Desert Woodrats) had chewed off the ends of the stiff leaves (most people might call them spines) as they searched for a last source of moisture. I had seen this behavior before in other places, so although notable, it did not strike me as odd or out of place.

Joshua Tree leaves chewed by rodents

Before long, however, I stopped to photograph a Silver Cholla and noticed that rodents had been chewing on the plant. I don’t know how they got past the spines, which are quite dense in this species, but somehow they figured out how to clear away the spines and get to the moist tissue underneath. Again, I suspected Desert Woodrats were responsible because they are among the few mammal species that have learned the metabolic trick of detoxifying the acids that cacti use to defend their precious fluids.

Rodents got through the spines and chewed into the moist tissue of this Silver Cholla

Some distance farther north, as I sped along the highway at 70 mph, I started noticing white-trunked Joshua Trees along the highway. After seeing several, I stopped to investigate. To my dismay, rodents had stripped large patches of bark from many of the trunks and branches, leaving visible the white fibrous tissue below. Bark had been stripped from all the way around at least one Joshua Tree, and although the leaves were still green, the tree is dead.

Bark stripped from all the way around the base of this Joshua Tree, up the trunk, and on some of the branches. Bark chips litter the ground, but surprisingly, there were no scats that might identify the culpret.

A number of other Joshua Trees were similarly chewed, but not all the way around the trunks. Perhaps these will survive.

Bark stripped from the trunks of Joshua Trees.

I had never seen this behavior before, but the week before in Gold Butte (200 miles southeast), I had seen where Desert Woodrats climbed into the upper branches of the Joshua Trees, stripped off the leaves and bark, and chewed away the moist vascular tissue underneath. In this case and most others, the woodrats left the leaves and bark chips on the ground. This is somewhat unusual because when I’ve seen this before, the cut leaves are often added to the woodrat nest defense system. If the cut leaves are not removed, I wonder if perhaps White-tailed Antelope Squirrels might be doing this. Also, I learned that this same branch-chewing behavior was seen at Red Rock Canyon, west of Las Vegas, so perhaps this behavior is widespread in the desert.

Leaves and bark chewed from the upper branches of a Joshua Tree in Gold Butte National Monument.

I drove farther north and camped by an old corral and stone house in the far reaches of the western Nevada desert. Water is piped to the corral from a nearby spring, and the water attracted birds and bats in the evening. Curiously, not far from the watering trough, I found another cholla that had been chewed by rodents. It seems that they could have gone the 40 yards to the water, so this is curious.

The next morning, I met with a small group of environmental writers and biologists to investigate reports of environmental terrorism at a proposed mine site. Rare minerals and rare plants seem to go hand-in-hand in the desert. Often these soils contain chemicals that generally are toxic to plants. In some cases however, one or a few plants learn metabolic tricks for avoiding the lethal consequences. When this happens, the plants have the habitat all to themselves without competition from other species for space, nutrients, and moisture. 

An eroded shale deposit with naturally occurring chemicals that generally are toxic to plants

Tiehm’s Buckwheat (Eriogonum tiehmii), a low-growing, mat-like plant, grows on 21 acres of this white mineral soil and nowhere else. Note: pronounced Team, as in a sports team.

While botanists love these rare soils for the special plant species they support, the mining industry loves these rare soils for the minerals that can be mined for profit. Often rare species are listed as endangered or sensitive in some other way and protected because they live in such small habitats, and thereby generate opposition to mining on those mineral soils. Facing the loss of huge profits, sometimes measured in the billions of dollars, the mining industry often does all it can to push aside these plants (literally or legally) or try to relocate them. So when reports emerged stating that a large percentage of a population of rare plants had been dug up and destroyed, the environmental community went on war footing. Letters were written, newspaper articles were printed, and lawsuits initiated on the assumption that the company that wanted to mine these lands was responsible for this environmental outrage. 

Full of righteous indignation, our small group stepped onto the edge of the mineral soils and immediately saw that plants had been dug up. If the reported 17,000 plants (40% of the population) had been dug up and cleared away, surely we could find at least one case clearly showing the outlines of a shovel or a pile of shoveled dirt, proof that humans had dug up the plants. With proof, we could make things difficult for the mining company.

As we first walked onto the mineral soils, we saw what it looked like a classic botanical crime scene: divots of dug-up soil and no Tiehm’s Buckwheat (pronounced “team”) left on the surface, although other species were growing nearby.

At first glance, a botanical crime scene: plants appear to have been dug up and removed

Getting low to photograph the divots, however, something seemed wrong. There were no shovel marks. Intact plants remained in place. Plant parts remained on the soil surface. Large plants remained intact while adjacent small plants had been dug up. If this was environmental terrorism, the perpetrators did not do a very good job — and certainly did not clear all of the plants from any area.

At second glance, the “botanical crime scene” has several intact plants and lots of plant parts scattered about

Some plants dug up while adjacent plants remain intact

Despite all that we had heard and our various preconceived assumptions, it became apparent very quickly to the biologists and the environmental writers that this was not the work of environmental terrorists. The plants had not been dug up using shovels, they had been dug up using little paws.

Tiehm’s Buckwheat dug out by rodents: this is not the work of humans with shovels trying to remove the plant; rather, rodents dug under and around the plants

Another Tiehm’s Buckwheat partially dug out by rodents

In many cases, the red bark that surrounds the central root had been chewed off and the inner moist layers chewed away, leaving only the woody core of the root remaining. 

Dug out, all that remains is the bark-striped pith of a Tiehm’s Buckwheat root

Tiehm’s Buckwheat dug out and chewed off by rodents gaining access to the root stalk. The irregular cut is not consistent with shovel activity.

Tiehm’s Buckwheat root. The red bark was stripped off and the moist vascular tissue was removed by rodents, leaving only the white pith of the root.

One case in particular showed especially clearly that the damage to Tiehm’s Buckwheats was caused by rodents rather than humans. Here, a Tiehm’s Buckwheat is still growing on a steep slope at the bottom of the hillside. Careful inspection reveals that rodents dug around the main part of the plant and also dug down through older plant materials to reveal a piece of the stem. As the creature dug around and through the plant, dislodged materials slid down the slope and accumulated on the hillside. With the stem exposed, the rodents chewed through the bark and into the moist tissue below, but left the rest of the stem intact. As seen in the series of photos below, this is completely inconsistent with actions of a human, but entirely consistent with rodent behavior.

Tiehm’s Buckwheat growing on a steep hillside

Careful inspection reveals that dirt was dug out, and plant parts were scraped away from the plant, and that these debris slid down the slope to accumulate below the plant

Dirt and plant material was scraped aside to reveal part of a stem, which was chewed to access moist tissue below the root bark

Close-up of chewed Tiehm’s Buckwheat stem

Dislodged stones and plant material debris accumulated on the slope below the buckwheat

In one interesting case, a strip of soils up the hillside was covered with Tiehm’s Buckwheat. Near the base of the hillside (close to the “normal” desert vegetation), virtually all of the buckwheat plants were damaged. A bit higher on the hillside, where it would be more dangerous for rodents because they lacked cover to hide from predators, only some of the plants were damaged. Finally, towards the top of the hillside, where rodents would be fully exposed to predators, none of the plants were damaged. This example shows, at a scale larger than a single plant, that the pattern of plant damage matches the behavior of rodents.

Hillside with differing degrees of plant damage relative to the base of the hill where rodents could hide among desert shrubs

So, if not humans, who might be causing this damage? We noticed plenty of rodent burrows in the desert area below the mineral soils, including kangaroo rats, pocket gophers, antelope squirrels, and woodrats. We also saw scat from pronghorn and bighorn sheep on the mineral soils. Pronghorn and bighorn are known to scrape the ground with their hooves, but the digging here was inconsistent with that type of scraping. Although known to only eat dry seeds in the wild, kangaroo rats in captivity love carrots, but it seems unlikely that they did the digging around the Tiehm’s Buckwheat. The others, pocket gophers, antelope squirrels, and woodrats, should all be considered candidates for the digging. We saw one antelope squirrel at the site, but I don’t recall seeing any burrows at the main site, although we saw several burrows at other sites. Similarly, I don’t recall seeing any pocket gopher burrows at the main site, although we saw several at the other sites, but gophers do feed on roots. That leaves woodrats as most likely for the damage. Woodrats are good diggers, and they harvest plant material and bring it back to their nests. We found one woodrat nest complex adjacent to the main site and heard about another. At the nest we saw, I did not see any Tiehm’s Buckwheat fragments, but I did see fragments of Nevada Ephedra that had been chewed in a way that looks like trying to get moisture from the stems. I’ve seen plenty of cases where it looked like woodrats had eaten ephedra stems from the end, but I’ve never seen chewing from the side.

Entrance to a Woodrat nest (crack in a cliff face) with collected stones and plant debris

Close-up of plant debris: Nevada Ephedra stems chewed as if to consume only the green tissue

Game cameras have been set up at the main habitat and one of the adjacent habitat sites, probably by mine company biologists. Both cameras seem to be focused intently on plants already somewhat disturbed by rodents. I look forward to seeing the images and determining whether they solve the mystery of “Who done it?”

Game camera gazing intently towards an area well dug up by rodents

In a few cases at the main site, we found that the plants seemed to still be alive even though the dirt had been dug out from around the roots. Some of us kicked the loose dirt back around the plants hoping to save them. I’m not sure if this humanitarian gesture might be a crime against the Endangered Species Act, but I am sure it will feed conspiracy theories about the mining company trying to cover up alleged crimes. 

Despite the “natural” nature of the plant damage, Tiehm’s Buckwheat remains a critically endangered species, even more so now than before this drought disaster, and it clearly deserves consideration under the Endangered Species Act. It might be possible to mine the site for underground minerals without damaging the critical habitat for this species, but I urge state and federal agencies to move quickly to protect this rare plant and explore other options for this proposed mine site.

As much as we had expected the mining company to be at fault, this was an environmental disaster, not environmental terrorism. The drought has pushed rodents to the brink of their own disaster, and in trying to survive this unprecedented drought, they went for the last sources of moisture they could find. It is very disappointing that a rare plant was that last source of moisture — I would rather see them work on the cactus.

This, of course, brings up a good question — why are rodents focusing on this species when other woody species are available? It might be interesting to study this and other questions, but we need to protect the plant and the habitat before it is too late if we are to answer the questions.

One encouraging observation that we noticed is that Tiehm’s Buckwheat can, if left alone, re-establish on damaged mine sites. In at least one prospect (a bulldozer scrape on a hillside), several Tiehm’s Buckwheats were growing in the bottom of the cleared area. The surrounding hillside was covered with buckwheats, so there was a nearby seed source, and at least a couple of seeds survived and grew in the ditch.

Several Tiehm’s Buckwheats growing in a bulldozer scrape

Three Tiehm’s Buckwheat plants growing in a bulldozer scrape (note rodent digging here too)

With photographs and notebooks in hand, we departed in our separate directions. I headed south to spend a few days hiking in the hills of western Nevada. As it turns out, these hills are at the edge of the Joshua Tree range.

Joshua Trees are a species of iconic plants in the Mojave Desert. During the persistent droughts of the last decade, many Joshua Trees in the southern part of its range are dying. In part, this has been due to bark damage by woodrats. At the same time, Joshua Trees have been expanding their range northward and into cooler, higher elevation habitats. Hiking in these hills at over 6,000 feet elevation, there are lots of juvenile Joshua Trees, many of which have not yet flowered for the first time. But even here, woodrats have been at work gnawing away on Joshua Tree leaves. I saw no serious bark damage here, so perhaps this higher elevation provides enough moisture to encourage the woodrats to only chew on the leaves. As I hike about examining the damage on so many Joshua Trees, I wonder about the long-term persistence of this species even in these higher elevation, cooler habitats. 

Juvenile Joshua Tree with most of the leaves chewed off by rodents

Sitting on a hilltop as the sun sets over California, I see the red disk of the sun dip into thick clouds of smoke above the Sierra Nevada Mountains. Picking up my righteous indignation and putting it away, I lament the letters that I must now write apologizing for writing publicly that the mining company had committed environmental terrorism.

As I sit in camp reading and writing this essay by headlamp, I notice a deer mouse on the tailgate of my truck where I had left out a bag of homemade banana muffins. Fearing that the mouse would soon indulge in my muffins, I was surprised to see that in fact it was only interested in water that I had spilled earlier on the tailgate. Something is definitely wrong in the desert this year.

The mouse only wanted water, not the fresh banana muffins

On the last day of my expedition, I saw nothing new or shocking, only more chewed–on Joshua Trees and Beavertail cactus. The striking difference for today however, was that the wind has shifted to blow more from the west. This filled my local valleys with smoke, and I could taste the fire with each deep breath as I hiked the steep mountain hillsides.

Drought, rodents trying to survive, and mega fires blowing smoke across the landscape. Indeed, something is wrong in the desert this year, but at least we know that environmental terrorists have not been at work, at least not here and not now. Perhaps, however, whether the plants were damaged as a result of climate-change driven drought or environmental terrorism, the site is still a crime scene.