Indicator Minerals and the Gift of the Ants

While ants are considered pests by many, their biology is of interest to many entomologists. In the desert southwest the Harvester ants of the genus Pogonomyrmex are a common occurrence on the landscape.  In fact at the American Museum of Natural History Southwest Research Station a yearly course on southwestern ants is taught.  Harvester ants nests are easily found since they occupy large open areas which the ants clear of plant material and the entrance mounds may be over a foot tall looking like small volcanic cones.  The detritus that makes up the mounds is a combination of excavated material from chambers in the colony, plant refuse, and other organic material.  Harvester ants colonies essentially sample the soil while building chambers within their nests by removing small gravel and dirt which is conveniently placed just outside the entrance for inspection.  The volume of soil sampled is a function of nest size and in the images below a profile of an abandoned nest over 4 ft in depth and 2 feet across resulted in a sampling of 50 cubic feet of soil.  These naturally produced soil samples are used by geologists in explorations for commercially important minerals and in Australia, ant nest mounds are explored to locate gold deposits.  Recent explorations of harvester ant nest mounds at the Painted Pony Resort did not identify any specific mineral deposits but did uncover some interesting crystals.  While small in size and not very common in nest mounds the crystals are quite beautiful with some interesting crystal structures. 

In the process of looking for nest mounds there seemed to be variation in the frequency of harvester ants colonies across the landscape and this may be related to landscape quality.  Areas to the east with little ground cover seemed to have fewer ant colonies while on the western side of the San Simon riverbed, where grazing is excluded, there seemed to be more colonies.  This observation suggests that harvester ants may be useful a proxy indicator species for range land health and an experiment to test this idea is under development. 

The surface expression of a large mature ant colony, probably leaf cutter ants.  The mound of excavated material is over 1 ft in height.

Underground expression of an ant colony.  Four foot deep and approximately 2' wide this nest sampled approximately 50 cubic ft of soil.

Ant nest, closeup of underground chambers.  Note the connections between chambers.

clear crystals from nest mounds.

More clear crystals from harvester ant colony mounds.  The rounded specimen in the bottom left is chalcedony which is weakly fluorescent.