PARENT SESSION
Poster Session # 19: Insect and Invertebrate Ecology.
Thursday, August 7 Presentation from 5:00 PM to 6:30 PM. SITCC Exhibit Hall B.
Documenting long-term persistence of the western harvester ant (Pogonomyrmex occidentalis).
Bramble, Dennis1, Turner, Raymond2, 1 University of Utah, Salt Lake City, UT2 U.S.G.S. - Desert Research Laboratory, Tucson, AZ
ABSTRACT- The nests of the western harvester ant, Pogonomermyx occidentalis, are conspicuous features on the landscape of the western United States. These structures, consisting of a pyramidal mound and a surrounding cleared zone, appear to be remarkably long-lived as well. Using repeat photographic and dendrochronologic techniques, we have obtained evidence for minimum site persistence of P. occidentalis ranging from many decades to over a century at two environmentally contrasting study sites in Utah. At a lower elevation (1920m), desert-scrub locality in west central Utah (Millard Co.), all anthills discernable in an historic 1901 photograph were still occupied by ants in 2001. Periodic repeat photography and direct monitoring indicate that most of these ant sites have been continuously occupied since at least 1969. Repeat aerial photography and big sage (Artemesia tridentata) dendrochonology were used to document ant persistence at a cooler, wetter, and higher elevation (2210m) pinyon-juniper site in south central Utah (Garfield Co.). At this locality, dozens of individual sites inhabited by harvester ants in 1960 still possess active colonies at this time. Growth ring series from sage shrubs bordering the cleared zones (together with other historic data) strongly suggest that many of these ant-dominated sites predate Anglo settlement and livestock grazing in the area. If so, such sites have been occupied by harvester ants more or less continuously for at least 120 years. Although targeted recolonization of abandoned sites is likely a factor in the long-term persistence of P. occidentalis, we so far have little direct evidence of this. Our observations indicate that P. occidentalis is probably the most enduring, site-specific animal in the American West. The extraordinary persistence of this ant is all the more impressive given the dramatic, anthropogenically induced changes in vegetation that have occurred at both study sites.
Key words: Pogonomyrmex occidentalis, repeat photography, harvester ants, site persistence