Small Worlds in a Big Place

Red-spotted toad (Bufo punctatus) calling | Photo: Tim Graham

There are places in Arches National Park where you can stand on the edge of a cliff, gaze out over the Big Empty of the Colorado Plateau and swear with both hands on a bible that nothing, nothing lies between you and eternity. These wide-open spaces, framed by flying buttresses of red rock arches, are so knee-buckling beautiful that they attract visitors from around the world. One is even featured on the Utah state license plate. 

On today’s visit to the park, though, my eyes are glued not to the horizon but to the ground to keep from twisting an ankle as I pick my way up ledges or drop over slickrock balds. I’m hiking off the beaten track on this hot day in late July with ecologist Tim Graham, scrambling to keep pace with his deliberate, but steady, clip. Now and again, though, I interrupt the rhythm of our crunching boots to lag behind and admire the fluted grain of wind-scoured Navajo sandstone as it flows across canyon walls like long, loosening braids of human hair. 

Tim, by contrast, navigates this unmarked terrain with the sure-footedness of someone who could traverse it with his eyes closed. He has spent most of his adult life in and around Moab, Utah, including a stint as a biologist for the U.S. National Park Service in southeastern Utah. He has crisscrossed the Colorado Plateau, from Grand Staircase-Escalante National Monument to the San Rafael Swell, probably about as many times as he’s run up and down the ball field in town where he plays pick-up soccer several times a week. 

About 45 minutes into our trek, Tim slips off his pack and settles down next to a shallow depression the size of a Thanksgiving turkey platter. It’s the first stop on a tour that I have been looking forward to for years. “Time for some belly science,” he announces with a smile. I look at Tim, then down at the ground. Although the rock is not quite hot enough to fry an egg, it is, nonetheless, mid-day in mid-summer in the middle of the desert with air temperatures pushing triple digits and not a cloud or speck of shade in sight. But I’m not missing this show for the world. Living here in the dimples of slickrock—weathering pits known as desert potholes—are the tiny descendants of species that have called this wind-scoured place home since the Mesozoic era. To appreciate them, you have to adjust your sights. So I drop to my knees and then gingerly lower my stomach onto the toasty rock. 

Left:

Ephemeral Pools (Potholes) Photo: brewbooks, 2010 | Flickr cc; Right: Water boatman |

Photo: sam dredge, 2012 | Flickr cc .

Desert potholes have been on my list of Ten Things to See Before I Die, ever since I had read about them in a guide to canyon country five years ago. Southern Utah is ground zero for potholes. They are so common that the map is, well, riddled with them: Pothole Point in nearby Canyonlands National Park, Swiss Cheese Ridge around Moab and Waterpocket Fold at Capitol Reef National Park, for instance. 

My guide doesn’t get any more expert than Tim, who has studied these diminutive ecosystems since 1987. He even has the discovery of a new species to his credit—a yet-unnamed oribatid mite that lives only in pools on the Colorado Plateau. Just one other species within this new genus has been identified, and it lives clear across the country in the granite domes of Georgia. Another very similar mite, in the genus Aquanothrus, is found in ephemeral pools in South Africa. 

Potholes, like the shallow depression we’re looking at, typically begin to form when precipitation collects in low points along fractures in the sandstone. Studies by Jim Davis of the Utah Geological Survey and his colleagues reveal that the sandstones are composed almost entirely of grains of silica with only small amounts of calcium carbonate and iron binding them together. Over time, the standing rainwater in these self-contained pools can dissolve this weak cement and liberate the sand grains, as can the mechanical action of freezing and thawing. Strong winds carry off the loose sediments, in essence excavating the pits like a slow-motion backhoe. Natural forces acting on vastly different time and spatial scales produce potholes that range from hollows the size of a teacup to ephemeral ponds that are some 50 feet deep. 

Tim pokes around the thin, powdery layer of sand that has collected at the bottom of the pothole. It looks dry and lifeless. A black bathtub ring of live, but desiccated, microorganisms around its rim, however, indicates that it contained rainwater at some point during the year. This community, which includes several species of bacteria, forms an impermeable biofilm that keeps the water from seeping through the sandstone. And where there’s a sign of water in a pothole, there’s a good chance of finding other kinds of life too. Tim carefully isolates a few dark particles that look, to my untutored eye, like tiny flakes of cracked black pepper. They are oribatid mites. 

SEM photos, dorsal view and ventral view, of yet-to-be-named species of oribatid mite in Colorado Plateau pans. | 

Photo: John Gardner, BYU 

Tim pulls out his water bottle, pours a few tablespoons of liquid over them and then pauses to deliver a brief introduction to mite ecology. Most oribatids are terrestrial and live on plants or in duff on the forest floor where they consume fungi and organic detritus. The three pool-dwelling species of mites, on the other hand, have adapted to life in water. And unlike their vegetarian cousins on land, these mites are omnivores, supplementing a diet of algae and detritus with invertebrates such as nematodes and tiny hunter-orange animals known as rotifers. 

Tim pauses in his lesson and trains a magnifying lens on the mites. In this short time, they have been roused from their torpor and have begun kicking around on the edge of the dirt. Their movements are labored and unsteady because mites have pointy, sickle-shaped structures at the end of their feet. In terrestrial environments, they serve as grappling hooks for hanging on to plant material. Even though the appendages are ill-adapted for life in aquatic environments, the pool-dwelling mites have retained these features. The mites are especially ungainly when trying to maneuver through long strands of slick algae, a little like walking in high heels through a bathtub of spaghetti. “They’re not very graceful,” Tim points out. “They’re falling all over each other. They get tangled up.” 

Dry rotifers | Photo: Tim Graham 

Tim had observed this behavior early in his pothole studies and confronted a puzzling question. How did the mites survive, he wondered, when one of their staple prey—rotifers—slip in and out of the pores between sand grains with extraordinary ease? These rotifer movements, he says, “are very much like a ballet. How do these clumsy guys feed on these very coordinated, elegant rotifers?” 

The answer, it turns out, seems to lie in their differential responses to drought. Every member of a pothole community possesses some extraordinary adaptation for surviving the vagaries of desert rainfall, which is scarce and unpredictable. Even when it does rain, many pools evaporate quickly in the desert’s dry air. In this sense, growth and reproduction in desert potholes are a lot like life on the battlefield— long periods of down time during dry intervals punctuated by spurts of frantic energy during wet ones. 

Pothole animals ride out the wild surf of boom-and-bust uncertainty by using one of three survival strategies. The drought escapers include vertebrates such as red-spotted toads and winged insects such as backswimmers and mosquitoes. They possess fast-whirring biological clocks that allow young tadpoles and larvae to quickly mature into adulthood and leave the pool before it dries up. 

Left: Tadpole shrimp nauplius larva (about 7 hours old) | 

Photo: John Gardner, BYU; Tadpole shrimp nauplius larva beginning first molt (<10 hours old) | 

Photo: John Gardner, BYU 

The drought tolerators stay put and endure punishing conditions that would kill most other animals. Through a process known as cryptobiosis, tiny organisms such as tardigrades and rotifers, as well as the eggs of a trio of freshwater crustaceans—fairy shrimp, tadpole shrimp and clam shrimp—can lose up to 95 percent of their total body water. The eggs can survive 50 years or more on a lab shelf. So tough are they in this cryptobiotic state that astronauts have taken them out of the shelter of space capsules and exposed them to the vacuum of outer space and the full ionizing radiation of the Sun with no deleterious effects. Biomimicry enthusiasts know these organisms as the biological models for creating long-lived vaccines that can be stored without refrigeration. 

Other pothole dwellers utilize a third approach, what Tim calls the “Tupperware strategy.” Snails retreat into their shells and close their openings using a structure known as the operculum, which means “little lid.” The mighty mites produce sealants such as waxy cuticles and burrow into the mud to minimize water loss. 

Tadpole shrimp nauplius larva beginning later molt (about 15 hours old) | Photo: John Gardner, BYU 

Each strategy has its trade-offs. Because they are able to retain a large percentage of their body moisture, mites can spring to life as soon as moisture frees them from their dry matrix of soil. Without water, however, the mites can’t survive much longer than a year. The desiccated rotifers, on the other hand, can persist for longer periods in their cryptobiotic state. The downside, though, is that it can take anywhere from five to ten minutes to rev up their metabolic motors once rain falls. The mites are able to exploit this lag time, teetering across the sand on their tippy, high-heeled feet as they feast on the comatose rotifers. 

Triops longicaudatus tadpole shrimp. | Photo: Tim Graham 

After nearly an hour beside the dried pool, it is time to leave. We have several other potholes to visit before the day is over. By the time Tim has packed up his water bottle and magnifying glass, the sediment already has begun to dry and once again encase the mites in their sarcophagi of silica. I rise slowly to my knees and lean over to peer one last time into the pit. Ecologists call this a Mesozoic lifeboat niche. Hundreds of millions of years ago, the species they now shelter were more widespread. But aquatic predators, including fish and diving beetles, largely eliminated them from the more hospitable habitats of permanent water. So, pothole organisms hedged their bets and evolved over time to make the best of a tough situation. And tough it is. Time and again before the year is out, the tiny mites and their neighbors will endure wild swings in temperature, salinity, pH, and oxygen and carbon dioxide concentrations as their little pool refills and then dries out again. 

I read recently that the universal posture of awe and reverence across the world is to bow, kneel or prostrate oneself. The fact that I am ending my first visit to a pothole on my knees seems a fitting response to the wonder of the occasion.