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.