Exotic invasive plants are a well-known threat to biodiversity. Their success alters both biotic and abiotic processes with large repercussions on native flora and fauna. In the Great Basin, the exotic invasive annual cheatgrass (Bromus tectorum) is converting millions of acres of sagebrush habitat to cheatgrass-dominated communities by increasing the frequency and size of wildfire. Rodents are effective modifiers of habitat and their presence can suppress cheatgrass. The purpose of our study was to measure the effects of fire and rodent presence or absence on biodiversity, using ants as an indicator. Ants are known to be good indicators of system change due to their high abundance, diversity, rapid responses to subtle changes in the environment, and their functional importance. Our experimental site was split into five blocks with four plots in each block, with each plot receiving a combination of two treatments: fire (burned or unburned), and rodent activity (rodent access or exclusion), in a full factorial design. Ants were collected once a month from April through October over 3 years using pitfall traps, and were sorted and identified to species. We analyzed abundance, species richness, and Shannon’s diversity.
Rodent abundance had little to no effect on ant abundance, richness, or diversity. The burn treatment created a 40% increase in ant abundance, a 33% decrease in species richness and a 60% decrease in Shannon’s diversity over the unburned treatment. Ant abundance was highest in June; richness and diversity were highest in May, June, and July. The overall increase in ant abundance was largely driven by a single species, Pogonomyrmex occidentalis, which comprised 70% of all of the ants collected. P. occidentalis abundance increased in burned plots, whereas most other species decreased in abundance, this produced high total abundance, but low richness and diversity. We conclude that fire reduces ant diversity in a sagebrush community, which is a good indicator of potential diversity losses in other invertebrate taxa. These losses may persist or become permanent with a changing fire regime caused by cheatgrass invasion.