Research shedding light on South Dakota’s ants

Farm Forum

PIERRE, S.D. (AP) – They are tiny immigrants who arrived to help build the tallgrass prairie after the last glacier retreated from the ice-scarred surface of what is now eastern South Dakota.

That’s why entomologist Paul J. Johnson at South Dakota State University and his graduate student, Laura Winkler, believe South Dakota’s ants may be a good indicator insect to help evaluate success in reconstructing native grassland communities.

As the U.S. Fish and Wildlife Service and the Nature Conservancy attempt to restore some lands in eastern South Dakota that had previously been converted to uses such as cropland or pasture, Winkler is using ant diversity to measure how healthy those reconstructions are when compared to prairie remnants in the same region.

“We’re going from utter destruction of the natural biota and trying to reconstruct it,” Johnson told the Capital Journal.

Winkler said vegetation and soil chemistry are more often used to measure the success of restorations, and when invertebrates have been used, beetles, grasshoppers, butterflies and bees have been more typical. There’s been less study of ants in that role, in part because they’re not as easy to identify by species. That makes it harder for people to take a quick inventory of what’s there.

“It is my goal to simply offer ants as an alternative taxa to measure along with vegetation and other environmental factors,” Winkler said.

“They also cycle quite a bit of organic nutrients through the soil by bringing food particles into their colonies, which may include insect and plant parts, along with seeds,” Winkler said.

From the viewpoint of sheer abundance, too, ants are simply a good yardstick to measure life.

“In most habitats on earth, ants come out to be the dominant organisms in terms of biomass in most communities. But we don’t see them because they’re down in the earth,” Johnson said.

Johnson said Winkler’s study, looking at reconstructed grassland projects from one to four years old, has not yet produced firm conclusions. But the trend is in line with what classic theories about ecology suggest – the longer it has been since restoration of a grassland, the greater the diversity of ants and other organisms.

Just as human immigrants to what is now South Dakota settled down in ethnic communities, ants settled down by species in different parts of the state, finding different niches in the state’s ecosystems.

But the way they’re dispersed across the state may seem counter-intuitive to people who aren’t biologists. Fewer species are found in the richer soils of the east, even though rainfall and vegetation are more abundant there.

“West River has many more species than East River does, and that’s because of the diversity of habitats,” Johnson said. “The east half of South Dakota is much less biologically diverse than the west half.”

That’s simply a matter of the greater range in elevation and terrain, and perhaps the time allowed for colonization, he noted.

“For example, the Black Hills were not glaciated. That means things could have lived there continuously for millions of years,” Johnson said.

The eastern half of the state, meanwhile, was overrun by glaciers, so it’s flatter, with few greatly distinct elevational gradients, Johnson said.

An important pair of scholars named George C. Wheeler and Jeanette Wheeler, in a 1987 study of South Dakota ants, singled out several groups that they found particularly interesting, ranging from thatching ants to mound builders to harvesters to slave makers.

Johnson said the Wheelers’ checklist gets at some of the most fascinating things about the study of ants.

While some ants remain hunter-gatherers, other ant societies followed a similar path as humans, developing some of the same innovations but coming up with them long before humans. It was arguably the ants who first invented cities, for example, and the ants who first invented agriculture. And yes, it was probably the ants who first invented slavery.

“Ants have some of the most complicated social developments, more complicated than any other species besides ours. A part of that is making use of other species to carry out functions within their nests – slave-making,” Johnson said.

But there’s an important difference from human slavery, Winkler added, in that ants are not held captive against their will.

“They can and have been known to rebel,” Winkler said. “As has been mentioned, there are different types of slave-making ants and as many different tactics. Some steal the larvae and pupae and rear it as their own workforce. Others will take over a colony. However, through all of this the slave-making ants will and do work alongside their host colonies. It is only the queen who does little more than get pampered and create more brood. This is true of all ant colonies.”

Even here on the Great Plains, where there are fewer ant species than in, say, the tropics, there are ants that exhibit many innovations: Ants that keep slaves, ants that tend aphids and harvest and store seeds, and ants that live in communities comparable to cities.

Ant specialists Bert Hšlldobler and Edward O. Wilson believe ants first began practicing something very like human agriculture, including the culturing of fungus as a food source, some 50 million to 60 million years ago. In comparison, human agriculture is usually dated to about 10,000 years ago.

Though South Dakota doesn’t have the leafcutter ants that use the vegetation they cut to nurture fungus, it does have other species that tend aphids, similar to the way humans tend cows; and it has species that gather seeds.

But is that truly agriculture?

“That’s more a philosophical question of what qualifies as agriculture,” Johnson said. “In this case we have ants that have evolved with a nutrient source and they’re doing what they can to protect and propagate that nutrient source. Is that agriculture? What are we doing with other species that we eat? It’s the very same thing.”

In the same way, Johnson said, ants are among the social insects that invented much more populous communities than human cities, and much earlier than humans did.

“Each of those mounds is a portal into a subterranean community that extends over several acres and can include millions of ants,” Johnson said. “It’s one huge community, one city, really. Each mound can be thought of as village or town, connected by the highways that go between them.”