The Anthills of Island Park, Green River, Utah

Boedicea P. Fox

While we were setting up camp on Big Island, a large sandbar in Island Park, along the Green River in Utah, I noticed a series of strange dirt mounds. Speculation amongst my campmates and myself resulted in a couple of theories; the mounds were possibly anthills or wood rat storage areas (a wood rat nest was seen in a tree in the area). I decided to conduct an investigation of the mounds to ascertain their origin.

The mounds were strangely restricted in location to the central eastern side of the island, perhaps due to the sparser vegetation on that side. They were located in areas dominated by sagebrush and cheatgrass.

I counted seventeen of the mounds in an area of about half an acre. The surface of the mounds appeared fairly smooth, and free of entry obvious tunnels or holes. They measured between one and three feet across, and ranged from three to nine inches high. The circular mounds were bare of vegetation, as was the surrounding soil, for a radius of another foot or so. Curiously though, the bare dirt around the mounds had a particularly high density of cheatgrass seed husks. Blown there by the wind, or discarded by the inhabitants? A deconstruction of some of the mounds gave no other clues as to their origin, since no tunnels were apparent.

It seemed unlikely that the mounds were wood rat related, given the lack of obvious tunnels or buried food sources, and yet no ants were seen around the mounds. Ants were observed, however, on the eastern most shore of the island, scurrying up the beach. The ants were about 10 mm long, and a dark red color. They were carrying round, white seeds, about 1.5 mm in diameter. I was unable to follow them to their source. A little later, while taking water quality samples on the west side of the island, I stumbled upon two similar mounds that were inhabited by ants. The mounds were about halfway across the island, at the western edge of the range in which I had observed the first mounds. The ants were similar in size and color to those on the beach.

This discovery, while possibly solving the mystery of the mounds led to questions for further study. Why were there so many abandoned mounds on the island? Why were the abandoned ones on the far east of the island, and the active ones in the center? Was it to do with the hydrology of the island? Perhaps some areas are too moist, or too dry. Maybe the anthill locations are related to vegetation structure. Another possibility is the effects of human disturbance—the east side of the island is most heavily camped, which probably reduces vegetation, but may also create ground disturbance for the ants. It would be interesting to return and conduct a research project on ant behavior at Big Island—a good excuse for a rafting trip anyway!

Some Anthill Examples