Formicidae - Trachymyrmex arizonensis
Family Formicidae Group Guild Status
Trachymyrmex arizonensis Trogloxene Fungivore Uncommon
Size: 4.40 mm
Members of the ant tribe Attini are fungus-gardening species, and are obligate fungivores (Vo et al. 2009). These ant-fungus relationships are mutualistic, with the ants providing protection and a suitable environment for fungal growth, and receiving the benefit of harvesting the fungal mycelia as food (Rabeling et al. 2007; Vo et al. 2009). The Attini are primarily tropical, and T. arizonensis is one of only 9 species of attines recorded from Arizona (Rabeling et al. 2007). This tribe also includes the true leaf-cutting ants (Atta and Acromyrmex), which grow their fungi almost exclusively on fresh plant materials, primarily leaves. Species of Trachymyrmex use primarily seeds and flowers (in season), but are more plastic in their selections than the true leaf-cutting species, and will use a variety of vegetative plant parts (Leal and Oliveira 2000; Rabeling et al. 2007). Trachymyrmex construct their sheltered gardens in the ground or beneath surface objects such as rocks.
All observations of this species have been in the Jackrabbit Shaft. The first two records, in April of 2010 and June of 2011, were of a single individual. In September of 2011 RBP observed a group of about 18 individuals near the top of the shaft, where they were observed moving chaff from their fungus garden to a refuse midden at the side, near the top of the shaft where there is a break in the wall between the drilled bedrock and the concrete cap of the shaft. The chaff had overflowed this repository and had cascaded down into the shaft. The yellowish-grey color of the chaff is diagnostic for this species (Rabeling et al. 2007). It appears the ants were using the shaft as an ant “landfill”. RBP observed the ants excavating a nest in a crack in the bedrock wall about 2.5 meters below the top of the shaft in May of 2012. There were about two dozen of the ants actively working the hole and removing pieces of soil, and a spoils pile had accumulated on part of the metal framework for the shaft ladder system. By August of 2012 the colony had expanded its presence in the cave and there were numerous individuals of the species operating out of the nest in the wall of the Jackrabbit Shaft. With its long-term presence in this part of the cave T. arizonensis is apparently at home in a karst environment. Because of their nesting presence in the cave we consider the species a trogloxene.
Army ants (Neivamyrmex spp.), many of which are primarily predators of other ant species, have been recorded preying on T. arizonensis in southern Arizona (Rabeling et al. 2007). Our records of Neivamyrmex in the cave are not in proximity to the Jackrabbit Shaft. If N. graciellae does prey on T. arizonensis, it likely occurs outside of the cave proper, in the soil profile. N. leonardi is probably too small a species to successfully predate Trachymyrmex spp.
T. arizonensis is essentially an epigean species, living in the shallow soil substrate or beneath objects on the ground, and foraging primarily on the surface. Their presence in the cave is opportunistic in that they have incorporated this small portion of the cave as a portion of their territory, where they are currently using it as a dumping ground for their chaff. Because they are using the cave for a portion of their activities we consider this species a trogloxene. There is some possibility that in habitats that contain numerous soil-filled fractures in the bedrock this species may be using these areas to build nests.
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