photo by Jayne Belnap
LIFE IN 'THE POOR MAN’S RAINFOREST'
Probing climate change’s impact on desert soil crusts
n the high desert of Utah, at a research site about twenty miles outside of Moab, Deborah Neher tries to step on the bushes. She doesn’t want to hurt the soil. Or, rather, what lives on top of the soil.
Here, an inch-high layer of lichens, mosses, tiny fungi, cyanobacteria, and microscopic creatures stretches in a bacon-colored carpet between scattered clumps of creosote bush. It’s lumpy, pinnacled, scabrous, pointillistically beautiful, “and darn fragile,” says Neher, chair of UVM’s department of plant and soil science.
photo by Cheryl Dorschner
It’s a biological soil crust. “That’s a living community,” she says. Although they’re little known, biological crusts cover a lot of the planet. They dominate many of the dry places that comprise about 35 percent of global land area from Africa to the polar regions. “We’ve been studying crust in the Colorado Plateau, the Chihuahuan desert into Mexico, the Sonoran desert in Arizona,” Neher says, “but I’ve seen them in Ohio, too.”
Neher and her colleagues want to know what climate change is doing to these crusts. Rising temperatures might not seem like a problem in a desert. But their research suggests that increasing summer heat in the Southwest kills important species of mosses, lichens, and bacteria in the soil crust.
And this, in turn, reverberates throughout desert ecosystems. Soil crusts form a living mulch. Filaments from the cyanobacteria and microfungi weave together across the surface, gluing soil particles in place, slowing erosion. Soil crusts sponge up what little rain falls. And soil crusts capture carbon, nitrogen, and other elements from the air, enriching the soil and providing nutrients that surrounding vascular plants need.
In short, “with climate change, we’ll have more sparse desert communities, with fewer native shrubs and grasses,” she says. And this threat—combined with direct damage to crusts from livestock, off-road vehicles, natural gas developments and, yes, foot traffic—“means we’re likely to see more dust and erosion like we saw during the Dust Bowl days in the 1930s,” she says.
To simulate future climate conditions, Neher and her partners from the Southwest Biological Research Center and Los Alamos National Laboratory have been making patches of the hot Utah desert even hotter. For the last six years, with funding from the U.S. Department of Energy, they’ve been blasting infrared lights over experimental plots, pushing up temperatures by nearly ten degrees Fahrenheit. And, since climate change may bring more frequent and heavier summer rainfalls to the Southwest, some plots have been sprayed with extra water.
Not only have the scientists measured damage to the crusts, like bleaching mosses, they’ve also observed surprising changes in the “amazing and strange little critters,” as Neher calls them, that live in and under the crust: mites, springtails, nematodes, and protozoa.
Though it might seem unlikely in a desert, “many of these critters are really aquatic organisms,” Neher says. Nematodes and protozoa live in microscopic films of water in the soil. Extra rain should be a nematode’s holiday—but not when it’s combined with high heat.
“One of the things that we’re learning is that, when it gets hot, many nematodes and protozoa can go into a kind of suspended animation called anhydrobiosis,” she says. “They’re living, but their metabolism goes way, way down. They shut down until there is a little bit of rain.” And that’s where their problems begin.
Though some nematodes can tolerate temperatures approaching 140 degrees, anhydrobiosis requires dry conditions. “If you keep them wet and increase the temperature, that’s the worst thing you can do for these guys,” Neher says, “They’ll die.”
While Neher’s research shows that populations of nematodes and protozoa in the Southwest suffer under climate change’s double punch of higher heat and more rain, their crustal companions—mites, springtails, and other microarthropods—seem to do fine, she says.
“We’re hypothesizing that’s because they live in the airspaces in the soil, not water,” she says; “climate change won’t create as many differences in their habitat.” In other words, looked at closely enough, the mites and nematodes are not cohabitating at all: miniscule differences of depth and moisture may mean a vast chasm to creatures smaller than a period.
“There are some people who argue, ‘You’ve got lots of bacterial feeders, so who cares if you lose a few species?’” Neher says. This view contends that the ecological roles of many small creatures are redundant: if one species declines, another picks up eating where the first left off and the whole system continues to work. “Instead, what we’re seeing is that these habitats are highly partitioned in time and space,” Neher says, suggesting that many individual species play unique roles in the system. One size mite does not fit all.
But these soil crust dwellers are poorly understood. “We call the soil food web the poor man’s rainforest. There are just an incredible number of species in soil, but we don’t know much about them,” Neher says. “Whether in the desert or our own backyards, we only know about 10 percent of the species. The other 90 percent is microscopic soil organisms that remain unknown to science.”
That is why Neher, her senior technician (and husband) Tom Weicht, and graduate students spend a great deal of time peering into microscopes. “One day in the field means three months in the lab,” Weicht says. They’re cataloging who lives in a soil crust and how they make a living.