NATURE NOTES:
Underfoot in West Texas, A Mysterious World of Desert Ants and Their Uninvited “Guests”

By Andrew Stuart

Their social dynamics are intricate, and their legacy on the planet dates back some 150 million years. Ants can go unnoticed – underfoot as they are – but they’re a significant part of life on Earth. They comprise almost a quarter of the total biomass of animals on the planet. A study in Death Valley found that, in sheer weight, ants exceeded all the area’s mammals combined, and the numbers could be comparable in arid parts of West Texas.

Often surrounded by gravel and bare ground, harvester ant nests are a common sight in the plains and deserts of West Texas. But much of what transpires in these colonies – particularly between the ants and “guests” of other species – remains a mystery.

Bill Clark, director of the College of Idaho’s natural history museum, has researched ants in Big Bend National Park for 20 years. He’s documented a strange and complex world.

Big Bend is home to more than 100 ant species. Some are confined to the Chisos, others to the river corridor. Clark’s focus is desert ants – he’s recorded more than 50 species. 

Some of his most surprising finds have come with a common West Texas species: the rough harvester ant, Pogonomyrmex rugosus. The broad gravel mounds they create, and the clearings around those nests, make them highly visible. They also have a potent sting – similar in intensity to a honeybee. 

“Workers” – sterile females – do the “harvesting” of seeds. In nests, which can extend 6 feet down, the ants use different chambers for different purposes: to raise young, to cache food – or their own dead.

“If you were to take a shovel and slice one of those mounds in half, and look in chambers, some of them will be full of seeds,” Clark said. “Sometimes they may store ants that have died in one of those chambers.”

Once a year – usually after an early summer rain – the colony raises up winged, sexual individuals. If you’ve ever been swarmed by flying ants, it was likely a harvester ant “nuptial flight.” Males die after breeding; fertilized females may become queens of new colonies.

There is the bustling colony, with its tasks and roles – and then there are hangers-on. “Ant guests” – known as myrmecophiles, or inquilines – are a particular interest of Clark’s. Here we enter a truly mysterious world.

There are at least 16 groups of ant guests in Big Bend. There are ant crickets, mites and spiders, a variety of beetles. Silverfish are the most abundant – Clark found hundreds in some ant nests. In one rough harvester ant nest, Clark made the first record here of a beetle called Araeoschizus.

“The basic biology of those things is unknown,” he said. “No one has ever seen a larva of that beetle, or knows much about them. What exactly they’re doing in there is pretty much an educated guess.”

Most of these interlopers are thought to feed off the husks of seeds, which harvester ants cache in their chambers. 

“I mean – what a perfect environment: a safe place to live, a food supply sitting right there,” Clark said. “One of the question that comes up is, how do they survive in there without the ants killing them? Because the ants are pretty voracious on other critters.”

It’s speculative stuff – but some of the beetles have the same general shape as ants, in what appears to be a form of mimicry. Pheromones are a central means of ant communication. The guests may incorporate those chemicals into their bodies, as another way to fool their hosts and evade detection.

Bill Clark found an Araeoschizus beetle in a harvester ant colony in Big Bend National Park – one of only a handful of records for this “ant guest.” What the beetle is doing in ant colonies, and how it survives in such a hostile environment, are uncertain – but the fact that the beetle’s body has an ant-like appearance may partly answer the second question.

In terms of uninvited guests, the harvester ants aren’t even safe from their own kind. In two rugosus  nests, Clark found another harvester ant – Pogonomyrmex anergismus. These parasites can’t produce workers – they’re entirely dependent on their hosts to deliver food. They’d only been recorded at a few sites – near Lubbock, and in New Mexico.

“It’s an area where there’s even less known than some of the other ant guests,” Clark said. “It’s one of those things that it’s so little collected, that even simply collecting it, and putting the dot on the map, was really important.”

Native ants are Clark’s focus. But he also studied red imported fire ants. Originally from South America, these invasives appear – for now – to be confined to the Chisos Basin campground. But if they spread, they could decimate native ants.

And those natives are critical – as pollinators, as aerators of soil, as food for countless other creatures. Clark’s work hints at the complexity of their lives, unfolding at and below our feet.