Submitted to: Environmental Entomology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/2/1999
Publication Date: 4/1/2000
Citation: NASH, M.S., WHITFORD, W.G., VAN ZEE, J., HAVSTAD, K.M. AND (HYMENOPTERA: FORMICIDAE) RESPONSES TO ENVIRONMENTAL STRESSORS IN THE NORTHERN CHIHUAHUAN DESERT. ENVIRONMENTAL ENTOMOLOGY. 2000. V. 29(2). P. 200-206. Interpretive Summary: The arid rangelands of the southwestern United States have experienced extensive and dramatic changes in species composition and physiognomy of the vegetation since the establishment of the livestock industry in the late 19th century. The desert basins of the northern Chihuahuan Desert were originally dominated by C4 perennial grasses, particularly black-gramma (Bouteloua eriopoda (Torrey) Torrey (Brown 1994). The northern Chihuahuan Desert grasslands are now a mosaic of shrublands and grasslands with patches of shrubs. These changes in vegetation are thought to be results of overgrazing by domestic livestock combined with naturally occurring drought. Currently, there are few shrub-free patches of grassland in the region, and there are no large areas in the intermountain basins that are not grazed or have not been grazed at some time in the past. Although the historical changes in vegetation composition and physiognomy are well documented, data of faunal community structure in these grasslands are nonexistent. Published faunal studies primarily involve woody shrub, such as mesquite (Prosopis glandulosa) or creosotebush (Larrea tridentata (DC.) Cov. The presence of these shrubs influences the fauna, especially the insect communities. Although livestock production is the primary land use in the region, there are few studies designed specifically to determine how grassland fauna is affected by livestock grazing. This study examined the effects of intense grazing by livestock and removal of a dominant shrub (mesquite, Prosopis glandulosa) on the numerical responses and spatial distribution of ants in Chihuahuan Desert grasslands. There were fewer colonies of a dominant ant species on plots where shrubs were removed and the nests were randomly distributed. On plots with shrubs, the nests of this species were aggregated at the base of shrub. A co-dominant seed harvesting ant species increased in abundance but did not change the spatial distribution of the nests. This study provides two metrics for ant assemblages that should be used in monitoring and assessment (numerical responses and spatial distribution responses) that considers ants as indicators. This information provides guidance to land managers who use ants as indicators for assessing and monitoring ecosystem health.
Technical Abstract: We studied responses of ant communities to shrub removal and intense pulse seasonal grazing by domestic livestock for four consecutive years. Weighted relative abundance and percent of traps in which an ant species occurred were analyzed using randomized, complete block design split in time analysis of variance to test for significant differences between means of ant groups. The ant community in the Chihuahuan Desert grassland is dominated by small, liquid-feeding ants, Conomyrma insana (Buckley), and large seed harvesting ants, Pogonomyrmex desertorum (Wheeler). The weighted relative abundance of C. insana was significantly reduced on the plots without shrubs. The relative abundance of P. desertorum was significantly lower on grazed plots without shrubs than on the ungrazed plots without shrubs. There were no detectable effects of shrub removal or intense, pulse grazing on the less abundant ant species. These results suggest the recent encroachment of shrubs into Chihuahuan Desert grasslands has increased the relative abundance of the dominant ant species in these communities. Intensive grazing by livestock has had an adverse effect on the most abundant seed-harvester, P. desertorum.