How lowly termites save grasslands for lions, elephants, and people

Mound-building termites in Africa have the potential to buffer climate-sensitive grasslands there from the regional effects of global warming, at least for a while, according to a new study. In the process, the landscape above the colonies could well serve as the center of action for rebuilding vegetation following drought.

This relationship between colonies of the tiny bugs and their broader environment is likely to hold well beyond Africa to parts of Australia and South America, according to the researchers conducting the study.

That could be encouraging news for people who live in the world's arid or semi-arid savannas and grasslands. These make up less than 40 percent of the Earth's land area and support more than 38 percent of the world's population, the researchers note.

The study, published on line Thursday by the journal Science, underscores the role biological interactions can have in tempering the effects of climate change, notes Robert Pringle, an ecologist at Princeton University in New Jersey, and a member of the team that conducted the study.

It also highlights the importance to the health of an ecosystem of so-called cryptic creatures that do their work largely out of sight and so are largely out of mind.

In this case, these termites live in mounds that are hard to see, unless you know what to look for, and they do their work at night – modifying the soil in several ways that benefit plants with effects that can outlast the existence of the colony doing the work.

"If you pulled out the termites from the system, you'd get a dramatic decrease in its ability to support large populations of charismatic wildlife," Dr. Pringle says, referring to the animals that inhabit the continent's savannas and grasslands – as well as wall calendars, coffee-table books, and conservation-campaign handouts.

The discovery of this buffering role for termite mounds emerged from research into regular patterns of vegetation that can appear as grasslands dry out and teeter on the edge of a catastrophic shift to desert.

Scientists have noted self-organizing patterns of vegetation in landscapes for decades. Over the past 15 years, ecosystem modelers have shown that such lands dotted with islands of vegetation reach that state in stages, each with its distinct pattern of plant distribution. Those patterns are based on increased competition among plants for dwindling water supplies.

As seen from satellites, regularly space dots separated by bare soil appeared to represent the final stage before the system tips to desert.

The results, used in conjunction with satellite images of vegetation cover, suggested that the patterns could be used to steer conservation efforts toward the areas teetering on the brink of collapse.

In the meantime, Pringle and colleagues in the United States and Kenya had been studying a mound-covered area within the Mpala Research Centre in central Kenya with an eye toward gauging the effects the termite colonies individually and collectively could be having on biological activity above ground.

The mounds are hard to spot. They rise only about 1-1/2 feet above the surrounding surface, but typically measure about 65 feet across and are separated from neighboring mounds by anywhere from 65 to 400 feet.

The team found that the termites were in effect aerating the soil, allowing rainwater to reach deep into the mounds. They were changing the soil's texture. And they were loading it with nutrients. Nitrogen levels were 70 percent higher and phosphorus levels were 84 percent higher than in soils beyond the mounds. That turned the mounds into hot spots for plant growth. And the regular spacing between the mounds meant that collectively the mounds were enhancing plant growth and biodiversity over the full range of the mound field.

Enter Corina Tarnita, a Princeton University colleague with a background in modeling ecological systems, an eye for patterns, and the lead author on the paper in Science describing the study.

She noted a similarity between the landscape dotted with plant-rich termite mounds and the satellite images of landscapes dotted with the last vestiges of savanna – same patterns, but different mechanisms generating them.

Using a model that deals with plant competition for water in the "here comes the desert" scenario and adding representations of termite influence on soils and nutrients, she demonstrated that "plant islands" built by each of the two mechanisms could coexist in the same location. But they grow to different scales. Plant assemblages on mounds dominated the area but were interspersed with far smaller bunches of plants battling for water – bunches so small that, unlike the mounds, they can't be seen by satellite. This matched the mix that Pringle and Dr. Tarnita saw during a trip to the mounds.  

This diversity of process translated into a more robust ecosystem, with the mounds serving as the centers for slowing the effects of an oncoming drought or speeding recovery from a drought just ended.

What remains to be determined is how well the termite communities themselves respond to climate change.

Still, "even if individual colonies blink out in a given location, the concentrated nutrients, the altered soils, the tunnels and shafts -- those mounds will take years to decades to erode," Pringle says. "Productivity on the mounds themselves is going to persist, even in the short-term absence of termites maintaining the mound itself."

And termites can repopulate abandoned mounds.

Aside from demonstrating the buffering influence of the mounds, the study also highlights the importance of telling the difference between a landscape truly on the verge of going desert and a healthy one dotted with termite mounds. One doesn't want to send the conservation corps to the wrong location.

Satellite observations may help if the mounds give off a different spectral signature than comparably sized patched of plants in a region on the verge of becoming desert, Pringle says. That difference, if it exists, has yet to be demonstrated.

For now, the results suggest that hiking boots on the ground will still be important in distinguishing between the two.