Date of this Version
Southwestern Naturalist (September 1993) 38(3): 1,229-1,232.
Ecological communities respond to environ- mental changes as the individuals of the component species die and are replaced. Thus, pat- terns of population turnover form an important aspect of community processes. Much less is known about species of long-lived individuals than of short-lived ones (Likens, 1989).
Instantaneous observations of age structure can be used to infer long-term dynamics but not all species can be aged retrospectively. Inferring life- history dynamics from current populations requires assumptions that are hard to verify. Following marked colonies of long-lived species is slow but provides direct, non-inferential data on population dynamics, although those are specific to the period observed. This note reports 15 years of observations aimed at determining survivorship of individual harvester ant colonies. Harvester ants are important arid grassland herbivores whose dynamics are crucial parameters for patterns of change in the rest of the community (Brown et al., 1979; Coffin and Lauenroth, 1990).
Fifty-six mounds of Pogonomyrmex occidentalis Cresson (Hymenoptera: Formicidae), the western harvester ant, were permanently marked with aluminum tags in August 1977. The ant colonies were checked each August from 1977 to 1991; colony deaths were noted and new colonies were marked. Death was determined based on the following information: absence of foragers at times in which neighboring colonies were foraging and poor condition of the mound. It was verified by deterioration of the colony site in subsequent years. Unoccupied sites remain marked for a study of succession. The site, about one hectare in extent, just south of the University of Nebraska's Cedar Point Biological Station, Keith County, Nebraska, was within a pasture which received moderate, half-summer grazing during the period studied. The vegetation is typical of shortgrass prairie, dominated by Buchloe dactyloides, Bouteloua hirsuta and B. gracilis, interspersed with Stipa comata, Aristida purpurea and A. oligantha, and forbs such as Artemesia and Psoralea and woody perennials, including Yucca glauca and Juniperus virginiana (Kaul et al., 1983). The site lies between eroded canyons, which give the study area an irregular shape. Rock outcrops produce some areas with insufficient soil depth to support a harvester colony.