Harvester Ant Nests Research Articles

Variability in soil chemistry and arbuscular mycorrhizal fungi in harvester ant nests: the influence of topography, grazing and region

Harvester ants are important disturbance agents across western North America, but the effects of ant disturbances on soils may vary considerably with topography and land use. We examined how soil properties and arbuscular mycorrhizal (AM) fungi in harvester ant nests varied across spatial scales according to topography, grazing regime and region. Soils from undisturbed areas were compared with nest disturbances created by two species of harvester ants, Pogonomyrmex occidentalis on shortgrass steppe in Colorado and P. rugosus on Chihuahuan desert grassland in New Mexico, in 1996 and 1997. Nests of both ant species were enriched with NO3 –-N, total P and roots colonized by AM fungi. Soil moisture was higher in P. rugosus' nests and lower in P. occidentalis' nests compared to surrounding areas. Soil pH was consistently lower in ant nests. Broad-scale factors such as grazing, topography and site affected most soil properties in and away from ant nests. Site exerted a strong influence on soil organic matter, pH and moisture. Within sites, topography had a significant affect on pH. Mycorrhizal colonization was influenced by site and topography in 1996 only, a substantially wetter year at both sites. Lastly, nutrient levels were largely determined by the fine-scale effects of ant disturbances. Principal components analysis revealed that, after removing site-level effects, harvester ants have similar functional roles in creating soil heterogeneity in these two different semiarid ecosystems.

A survey of arbuscular mycorrhizal fungal root inoculum associated with harvester ant nests (Pogonomyrmex occidentalis) across the western United States

Soil organisms are known to affect the population dynamics of mycorrhizal fungi in many ecosystems. The western harvester ant, Pogonomyrmex occidentalis, is widespread in its distribution and creates patch disturbances that affect both plant and soil microbial communities. Harvester ant-plant-fungus interactions can result in the formation of dense root mats in nest soils and abundant viable mycorrhizal root inoculum. We surveyed mycorrhizal root inoculum in nests of P. occidentalis across its distributional range to address whether root mats and fungal enrichment were a common consequence of ant activity. We concluded that mycorrhizal root mats were unique to particular plant communities (sagebrush steppe) and were not a common feature of P. occidentalis' nests across its distributional range.

Prairie Dog Effects on Harvester Ant Species Diversity and Density

The purpose of this study was to determine if black-tailed prairie dogs (Cynomys ludovicianus Ord) influence harvester ant nest density and species composition within the shortgrass prairie biome of southwestern Kansas. Two treatments were established: areas colonized by prairie dogs and areas not colonized by prairie dogs. We recorded 183 harvester ant nests of 3 species. Harvester ant nest density did not differ significantly between prairie dog colonies (3.08 nests ha -1 ) and non-colonized shortgrass prairie sites (4.54 nests ha -1 ), but species composition did. Pogonomyrmex rugosus Emery was the most frequent species on prairie dog colonies where it accounted for 87% of ant nests present, as opposed to 33% on sites where prairie dogs were absent. Pogonomyrmex barbatus Smith was the most abundant species on non-colonized areas, making up 49% of the ant nests sampled. Pogonomyrmex occidentalis Cresson comprised 11% of ant nests sampled, and was nearly absent from prairie dog colonies (20 nests on non-colonized sites vs. 1 nest on prairie dog colonies). The average number of harvester ant species found per site was consistently greater on sites where prairie dogs were absent.

Prairie dog effects on harvester ant species diversity and density

The purpose of this study was to determine if black-tailed prairie dogs (Cynomys ludovicianus Ord) influence harvester ant nest density and species composition within the shortgrass prairie biome of southwestern Kansas. Two treatments were established: areas colonized by prairie dogs and areas not colonized by prairie dogs. We recorded 183 harvester ant nests of 3 species. Harvester ant nest density did not differ significantly between prairie dog colonies (3.08 nests ha-1) and non-colonized shortgrass prairie sites (4.54 nests ha-1), but species composition did. Pogonomyrmex rugosus Emery was the most frequent species on prairie dog colonies where it accounted for 87% of ant nests present, as opposed to 33% on sites where prairie dogs were absent. Pogonomyrmex barbatus Smith was the most abundant species on non-colonized areas, making up 49% of the ant nests sampled. Pogonomyrmex occidentalis Cresson comprised 11% of ant nests sampled, and was nearly absent from prairie dog colonies (20 nests on non-colonized sites vs. 1 nest on prairie dog colonies). The average number of harvester ant species found per site was consistently greater on sites where prairie dogs were absent.

Harvester ant nests, soil biota and soil chemistry.

Many ant species accumulate organic debris in the vicinity of their nests. These organic materials should provide a rich resource base for the soil biota. We examined the effect of harvester ant nests (Pogonomyrmex barbatus) on the soil community and soil chemistry. Ant nest soils supported 30-fold higher densities of microarthropods and 5-fold higher densities of protozoa than surrounding, control soils. The relative abundances of the major groups of protozoa differed as well: amoebae and ciliates were relatively overrepresented, and flagellates underrepresented, in ant nest versus control soils. Densities of bacteria and fungi were similar in the two soil types. Concentrations of nitrate, ammonium, phosphorus, and potassium were significantly higher in ant nest soils, while concentrations of magnesium, calcium, and water were similar in nest and control soils. Ant nest soils were marginally more acidic than controls. The results demonstrate that P. barbatus nests constitute a significant source of spatial heterogeneity in soil biota and soil chemistry in arid grasslands.

Founding, Foraging, and Fighting: Colony Size and the Spatial Distribution of Harvester Ant Nests

This study examines how the spatial distribution of nests is related to the behavioral interactions of conspecific neighbors in a population of the seed—eating ant, Pogonomyrmex barbatus. Colonies live for 15—20 yr, reaching reproductive age and a stable size at $\approx$5 yr. Spatial distributions were measured for 6 yr (1988—1993) in a population of $\approx$250 colonies of known age. The probability that a 1—yr—old colony occurs in a given location is related to the distance to, and ages of, its five nearest conspecific neighbors. One—year—old colonies are most likely to occur near small, 2— and 3—yr—old colonies. Neighboring colonies encounter each other when foragers of both colonies search the same area on the same day. The probability of an encounter between two colonies decreases with the distance between their nests. For colonies of all ages, encounters are most likely with their larger neighbors, $\geq 5$ yr old. Encounters are more likely if there was an encounter the previous day, and this effect can overwhelm the effect of distance. Two—yr—old colonies are more likely than colonies of other ages to lose ground in repeated encounters with a particular neighbor. Though encounters are frequent, their costs are low: few of the foragers on a trail that meets a neighbor's actually encounter an ant of the neighboring colony, and interaction with a neighboring colony does not increase the typical duration of a foraging trip. If foragers of two colonies do meet, most fights are brief, with few resulting in injury or death. These results suggest that the cost of conspecific neighbors searching the same ground for seeds may be greater than the cost of behavioral interaction itself. Exploitative competition may have more important effects than interference competition on founding colony survival and thus on the spatial distribution of nests.

Variability in soils and vegetation associated with harvester ant (Pogonomyrmex rugosus) nests on a Chihuahuan Desert watershed

The effects of harvester ant (Pogonomyrmex rugosus) nests on the density and cover of spring annual plants and on soil characteristics were measured at three locations characterized by different soils and dominant vegetation on a desert watershed. There were few differences in vegetation and soils associated with harvester ant nests at locations at the base of the watershed where brief periods of flooding and sediment deposition occur at periodic intervals. At mid-slope locations, there were significant increases in total nitrogen, inorganic phosphorus, and cover (biomass) of four species of spring annuals at the edges of nest disks when compared with reference sites. The spring annuals that exhibited increased cover were species that increase biomass as a function of available nitrogen. At a clay-loam, Scleropogon-Hilaria, grassland site, there were significant reductions in the concentrations of Ca2+ and Mg2+, significant increases in nitrate and total nitrogen, but a significant increase in cover in only one species of annual plant. The data demonstrate that the effects of ants on soil properties and vegetation vary with site location and soil type.

Chaff piles of harvester ant (Messor spp.) nests in a desert ecosystem

The plant species composition of the chaff piles of three species of harvester ant (Messor spp.) and the contribution of the chaff to the organic pool were studied from August 1985 to July 1987. There were distinct differences in the plant species composition of the chaff of the three species. We attribute this to the different diets of the three species, which reflect the relative sizes of their individuals and their foraging strategies. The amount of chaff accumulated varies greatly between the three species (Messor rugossus: 127–196 g · ha−1 · y−1;Messor ebeninus: 2823–4437 g · ha−1 · y−1;Messor arenarius: 2165–2535 g · ha−1 · y−1), although the number of nests per hectare is virtually the same. We found that the amount of chaff is related to the rate of activity and the size of the individuals of each of the three ant species. The total chaff accumulated during the study period was 19.2 kg · ha−1, which is an important contribution to the organic matter in the soil in the Negev desert ecosystem.