Harvester Ants as a Rangeland Indicator Species

Ants, especially harvester ants utilize seeds as a food source and might be a useful indicator species for the grassland restoration efforts at the Painted Pony Resort.  A number of reports suggest that both soil composition and grazing intensity can alter harvester ant colony frequency on the landscape (1, 2, 3). Variation in ant colony density across the estate was noted while preparing a previous post on indicator minerals in ant colony mounds so a small comparative study was designed to test the hypothesis that harvester ant colonies varied with the grazing intensity on the estate.  A 3 km transect across the estate from west to east, starting on grazed New Mexico State land was chosen and at 14 stations a 30 m diameter circle (area = 730 meters²) was examined for harvester ant colony entrances.  A colony was defined on the basis of a circular pile of small (1-2 mm) stones, and an entrance hole with or without harvester ants on the surface. Although other ant species were observed along the transect these were not included in the colony totals.
Constantly grazed land had a average of 1 colony/730 m² while ungrazed land averaged 4 colonies/730 m² and there was a significant difference in the frequency of harvester ant colonies between the 2 grazing regimes, P = 0.003, single tailed t-test.  Since soil type also varies across the estate the number of colonies by soil type was also tested. Forest-Pinleno association is found on the west side of the estate while Eba very gravelly loam predominates the eastern portion of the estate. There was no significant difference in harvester ant colony density between these 2 soil types, P = 0.12, single tailed t-test. This suggests that soil composition, and secondarily plant species diversity, are not a major contributor to harvester ant colony frequencies on the estate in the semi-arid southwest, but rather the frequency of grazing (constantly grazed versus ungrazed land) is the more important determinant of harvester ant colony frequency in and around the estate.

Satellite view of the 756 acre Painted Pony Resort showing 3 km transect and locations of test sites.  The red line indicates the property boundary but no fencing and accessible to cattle, while the white line indicates fencing around the estate where cattle are excluded.  The 3 stations furthest west lie on New Mexico State land which is grazed year round.

Frequency of harvester ant nest colonies.

Soil map of the Painted Pony Resort.

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.

Harvester Ant Nest Building, Pogonomyrmex.

Ants are everywhere, and Arizona has 324 species. The most commonly observed species around here are the Pogos. Recognizable by their distinctive pebble nest entrances and large vegetation free space, these ants are ubiquitous.



Time lapse video courtesy of captenmerika