New Mexico Geological Society Annual Spring Meeting — Abstracts

Large anthills attributed to harvester ants were examined on several types of soil substrates across Sevilleta National Wildlife Refuge. Typically Pogonomyrmex rugosus build broad, low, shield-like mounds with entrances off-center whereas Novomessor cockerelli build more compact, steeper cone-shaped mounds with entrances in a central crater. Single-rock-type substrates include weathered granite, schist, limestone, and gypsum. Mixed-granule substrates are primarily on alluvial fans and ancestral Rio Grande gravel. No harvester anthills were found on thick petrocalcic soils or on pebbly sand terrace deposits of the Rio Puerco. Densities of anthills ranged from 74 hills per hectare on grus west of the Los Pinos Mountains to less than 1 per hectare on Popotosa badlands, Permian gypsum, and Baca Formation (conglomerate). Densities per hectare did not correlate to major vegetation types. Three types of grain-size distributions are exhibited on linear-log-differential plots of coarse sand and granules (typically between 1 and 4.5 mm with modes ~2.6 mm, perhaps related to ant mandible size) gathered at the surface by the ants. One type is a log-hyperbolic distribution with relatively steep decreases in coarse and fine amounts. A second type of distribution is lognormal with broader parabolic shapes. The third type appears to be lognormal with more coarse grains (possibly due to sample locations on anthills). Due to the lack of ant activity at the surface during the summer heat, we could not determine whether specific granule distributions can be attributed to a single species. On most substrates, the ants gathered locally available granules and did not appear to choose particular minerals or rocks over others. It is possible that on some local substrates, ants chose quartz grains more than other grains and avoided ironstone concretions and sandstone. The ants did not choose rounded grains over angular grains. Limited tests concerning seed preferences showed that P. rugosus chose blue grama seeds and ignored juniper and creosote seeds. N. cockerelli ignored all three seeds and must prefer other local seeds.