Fire and plant invasion, but not rodents, alter ant community abundance and diversity in a semi‐arid desert

Abstract

Human activities are increasing the size, frequency, and severity of disturbance across earth's ecosystems including deserts. Ants are important drivers of ecosystem function and are good bioindicators of ecosystem sensitivity to disturbance and change. Rodents also play an important role in ecosystem response to disturbance and often compete with ants for resources. The purpose of our study was to test the main and interactive effects of fire, rodent activity, and time on ant forager abundance, species richness, and diversity, as well as changes in ant mound density and disk area in the Great Basin Desert. We experimentally applied burn and rodent exclusion treatments and used pitfall traps to collect ants each month from April through October from 2014 to 2016. Over the three‐year period, burned areas had lower richness and diversity than unburned areas. Rodent exclusion had minimal effects on the ant community, and there was not a significant rodent exclusion interaction with fire. Treatment effects varied by month and year. The western harvester ant, Pogonomyrmex occidentalis, was the most abundant ant species, comprising about 70% of the total ants captured. Shifts in ant diversity following fire were driven by positive responses of harvester ants to burned habitat conditions. In contrast, all other ant species when analyzed together had lower forager abundance in burned plots, which drove lower ant diversity in burned plots. Ant forager abundance, richness, and diversity increased each year of the study in all plots; however, richness and diversity remained lower in burned areas than in unburned areas each year. Structural equation modeling shows that the effects of fire on ant community diversity are partially mediated through the plant community. While rodents affected the plant community, those effects do not seem to transfer over to the ant community. Pogonomyrmex occidentalis mound density was higher in burned areas, but disk area was smaller. Our results suggest that fire has adverse effects on ant community diversity. This could have long‐lasting effects on ecosystem function in the face of a changing fire regime in deserts of North America caused by invasive annual plants.

Invertebrate community response to fire and rodent activity in the Mojave and Great Basin Deserts

“…Ostoja et al (2009) reported lower ant diversity in cheatgrass-dominated plots compared with sagebrush intact plots in the Great Basin which is a typical vegetation conversion after fire as seen in our plots. These results are consistent with our previous research at the Great Basin site, where ant species diversity was reduced in burned areas but ant abundance was unaffected (Day, Bishop, & St. Clair, 2018). In that study, abundance of most species was reduced in burned plots, but the abundances of some dominant ant species increased, which kept overall ant abundance in burned areas similar to those in unburned areas.…”

“…In our study, fire had very little effect on ant abundance; however, fire reduced ant species diversity at both sites (Tables 5 and 6; Figures 1 and 2). Ostoja et al abundance was unaffected (Day, Bishop, & St. Clair, 2018). In that study, abundance of most species was reduced in burned plots, but the abundances of some dominant ant species increased, which kept overall ant abundance in burned areas similar to those in unburned areas.…”

“…This increase in Pogonomyrmex abundance may be the result of shrub removal, allowing for increased colony densities (Day et al, 2018;Sneva, 1979), or it may also be caused by increased seed resources from increases in annual plant cover.…”

Contrasting effects of fire and rodent competition on western harvester ant activity and disk formation

“…Ants have been shown to limit the dispersion and burrowing activities of rodents (Panteleeva et al 2016), and both species can limit each other's access to seeds through exploitative competition (Brown et al 1979;Valone et al 1994). However, the idea that rodents and ants vigorously compete for seed resources has been disputed; rodents prefer larger seeds, while ants prefer smaller seeds (Connolly et al 2014;Day et al 2018;Martyn et al 2022). Rodent activity can increase the abundance of smaller seed-producing species that ants tend to prefer (Brown et al 1979;Johnson 2001;Bishop et al 2020).…”

“…However, there was little evidence in the Great Basin Desert that rodent exclusion signi cantly affected ant abundance (Day et al 2018) (Reed et al 2004) due to altered habitat conditions (Holbrook et al 2016). Fire affects ant activity and the plant community processes they mediate (Day et al 2018). Ant response to re varies by species, with some increasing in abundance while others decrease (Ostoja et al 2009 The Great Basin Desert is a semi-arid ecosystem occupied by rodents and western harvester ants, where both taxa play critical ecological roles (Uhey and Hofstetter 2022).…”