Page 32 - Black Range Naturalist, Vol. 2, No. 2

In pinon-juniper and ponderosa pine woodlands the estimated abundance of Rough Harvester Ant colonies varies from approximately 4 colonies per acre to as many as 10 colonies per acre. In these environments, Rough Harvester Ants reduce the plant cover around the nest disc by approximately 0.5 %
by cutting and
removing plants that
affect the foraging
activities of the
colony.
Rough Harvester
Ants are known as
central place
foragers where the
nest is the central
place. Foragers
collect seeds and
insects from
locations nearest the
nest disc and then
move to patches of
seeds at varying
distances from the
nest disc. The result
of this foraging
behavior is to deplete
organic materials
from areas around the nest and enrich the corona of soil adjacent to the nest disc. Experiments on cleared arenas with seeds colored with food dyes documented that the workers cleared the concentrations of seed nearest the nest while establishing trails to the more distant seed concentration. Foraging trails are marked by “scouts”, individual foragers that locate rich sources of seeds and establish a chemical trail back to the colony by dragging their rears along the soil surface. The chemical trail is produced by a gland near the stinger. If there are no rich sources of seeds, Rough Harvester Ants search for seeds as individuals rather than as a group that follows a chemical trail. When there is an absence of spring flora, harvester ant colonies remain closed. During one year when harvester ant colonies were not active, several Texas Horned Lizards, one of the predators that depend upon harvester ants as prey, were found dead near a colony. We thought that this may have occurred because ants are the primary source of water for these lizards. The dead lizards were emaciated and probably desiccated.
When winter rains are sparse, Rough Harvester Ants may not forage even when environmental temperatures are suitable for foraging. These ants are capable of moving at body temperatures between 45oF and 125oF. The numbers of Rough Harvester Ant workers that occupy established colonies in winter varies between 1000 and 6000, but these numbers do not necessarily reflect the numbers in the colony during a time of food abundance when the queen is producing a large number of eggs. Rough Harvester Ants frequently forage at night during the heat of summer. In mid- summer, Rough Harvester Ants can only be active between
dawn and the time that soil temperatures exceed 130oF which is usually around 10AM. Foraging at night allows Rough Harvester Ants to avoid competition with two other species of seed harvesting ants (Desert Seed Harvesters and California Seed Harvesters) which are limited to foraging in
32
A Rough Harvester Ant with a seed. Photograph by Walt Whitford.
daylight hours. These species do not produce large colonies and have more ephemeral nests than Rough Harvesters.
Foraging by harvester ants is determined by temp- erature but is in- dependent of air
relative
humidity. However, when the soil around a harvester ant nest was wet
by sprinkling water, the ants exhibited intense activity around the nest. One favorite trick to get Rough Harvesters to boil out of their nest is for a human to blow into the nest opening. The breath of a human must be close chemically to the “danger” pheromone because within seconds, ants emerge from the colony and will attack anything in the vicinity. After blowing into a harvester ant nest hole, scramble out of the way because the ants coming out of the nest are very agitated and will bite and sting if a human remains near the nest. Rough Harvester Ants are reported to have venom that is the most potent mammalian toxin based on the amount of venom produced. When harvester ants sting, they anchor themselves with their mandibles. The mandibles fasten the ant to the victim which allows the ant to insert the stinger with force. While fastened to the victim, the ant injects the stinger and venom multiple times. The sting wound is swollen hard and very painful. Multiple stings can result in anaphylaxis which may be fatal.
Rough Harvester Ants have been described as keystone species in the ecosystems in which they occur. These ants serve important ecosystem functions by stashing seed husks around the nest disc, bringing sub-soil to the surface when excavating nest chambers, and aerating the soil, and are the primary prey of Texas Horned Lizards. Animals that generate patchiness in arid and semi-arid ecosystems are critical for the normal functioning of the landscape.