Effects of Precipitation and Livestock Grazing on Foliage Foraging Ants in a Chihuahuan Desert Grassland

Yosef Steinberger,Walt G Whitford

doi:10.4236/oje.2021.111005

Abstract

We examined the relationship between seasonal livestock grazing (late summer and late winter) and the abundance of two ant species, Dorymyrmex insana and Forelius pruniosus, on three types of plants (mesquite shrubs, snakeweed sub-shrubs, and mixed grasses) dominated by black grama (Bouteloua eriopoda). Stocking rates were adjusted to remove 75% of the available forage. Since Chihuahuan Desert grasslands are not in transition to shrublands, the grasses and some herbaceous plants are the only available forage. We hypothesized that neither rainfall nor cattle grazing would affect the abundance of these ants on mesquite (Prosipis glandulosa) or snakeweed (Gutierrezia sarothrae). Linear regressions of monsoon rainfall on mesquite (Prosopis glandulosa) produced an r2 nearly equal to that with the annual precipitation. Monsoon rainfall on the evergreen sub-shrub, Gutierrezia sarothrae, resulted in June-July rainfall accounting for 47% - 83% of the variation in densities of D. insana on snakeweed. The number of D. insana was more than double the number of F. pruinosus on grasses, mesquite, and snakeweed. There were significant reductions in the abundance of F. pruinosus on the grass in the grazed plots; each year the plots were grazed. There were no significant effects of grazing on the abundance of either of the ant species sampled from G. sarothrae canopies. There were significantly fewer D. insana on mesquite in summer grazed plots than on P. glandulosa in winter grazed and ungrazed plots in the second and third years of grazing. Pre-grazing effects were compromised by the high annual (more than double) precipitation.

Highlights

Selected species of ants recruit rapidly to a rich food source and may reduce the abundance of other species at that source [1] [2]
We examined the relationship between seasonal livestock grazing and the abundance of two ant species, Dorymyrmex insana and Forelius pruniosus, on three types of plants dominated by black grama (Bouteloua eriopoda)
There were no significant effects of grazing on the abundance of either of the ant species sampled from G. sarothrae canopies

Summary

Introduction

Selected species of ants recruit rapidly to a rich food source and may reduce the abundance of other species at that source [1] [2]. Two species of the dolichoderine ant, Dorymyrmex insana and Forelius pruinosus, are the most abundant species in most plant communities in the northern Chihuahuan Desert [4]. Both species are widely distributed in the southern Chihuahuan Desert and south into Mexico [5]. Both Dorymyrmex and Forelius are absent or occur at low colony abundance in the Great Basin. In Nash et al.’s (2004) [8] study in the Mojave Desert, with winter rainfall, both D. insana and F. pruinosus were abundant and affected by livestock grazing, especially at the most heavily grazed sites. The ant species similarity index decreased from 68% to 53% with distance from a water source [8]

Methods

Results

Discussion

Conclusion