ANTS AS INDICATORS OF EXPOSURE TO ENVIRONMENTAL STRESSORS IN NORTH AMERICAN DESERT GRASSLANDS WALTER G. WHITFORD 1 , JUSTIN VAN ZEE 2 , MALIHA S. NASH 1 , WALTER E. SMITH 2 and JEFFERY E. HERRICK 2 1 U.S. Environmental Protection Agency, National Exposure Research Laboratory, Characterization Research Division, P.O.Box 93478, Las Vegas, NV 89196, U.S.A.; 2 USDA-ARS, Jornada Experimental Range, Dept. 3JER, New Mexico State University, Las Cruces, NM 88003, U.S.A. (Received 22 January 1997; accepted in revised form 2 July 1997) Abstract. The relative abundance of ant species was measured by pit-fall trapping at 44 sites in southern New Mexico and southeastern Arizona, U.S.A.. Sites were selected for study based on doc- umentation of a history of disturbance or protection from disturbance, exposure to varying intensities of livestock grazing, dominance by an exotic species of plant and vegetation change resulting from disturbance or restoration efforts. Ant community composition, relative abundances of species, and species richness were the same on disturbed and undisturbed sites. None of the metrics based on hypothesized responses of ants to disturbance clearly distinguished between disturbed and undis- turbed sites. Ant communities on sites where restoration efforts have resulted in distinct differences in vegetative cover and composition were similar to the ant communities on degraded unrehabilitated sites on the same soil type. Ant communities in riparian cottonwood gallery forests in Arizona and New Mexico were similar but differed from the assemblages in exotic salt cedar and native ash riparian woodlands. Ant species exhibited remarkable resistance to human-induced disturbances in these rangeland areas. In grasslands dominated by the South African grass, Eragrostis lehmanniana Nees, large seed harvesting ants, Pogonomyrmex spp., were greatly reduced in abundance compared to native grasslands. Other ant metrics were not different in E. lehmanniana grasslands and native grasslands. We conclude that ants cannot be used as indicators of exposure to stress, ecosystem health or of rehabilitation success on rangeland ecosystems. Ants are also not useful indicators of faunal biodiversity in rangeland ecosystems. Key words: environmental stressors, exposure indicators 1. Introduction Ants are among the most ubiquitous invertebrates of terrestrial ecosystems. They occupy habitats from the driest deserts to sub-alpine regions. Diversities of ants are high even in human-modified habitats (Holldobler and Wilson, 1990; Nuhn and Wright, 1979; Talbot, 1953; Torres, 1984; Van Pelt, 1963; Whitcomb et al., 1972). Assemblages of ant species include species with different life history character- istics and trophic relationships. Ants fill most kinds of consumer trophic levels as predators, omnivores, detritivores (including species that culture fungi), grani- vores, and sugar feeders. The complex interrelationships between ant species and their physical and biological environment should make ants sensitive indicators of environmental stress. These complex interrelationships may also make ants good indicators of the characteristics of environmental stressors. Indicators of ecosystem stress should be easily linked to essential ecosystem processes. Ants play many clearly identifiable, important roles in ecosystems. Environmental Monitoring and Assessment 54: 143–171, 1999. c 1999 Kluwer Academic Publishers. Printed in the Netherlands.