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

In North American deserts, the spread of invasive annual grasses is resulting in larger and more frequent fires that threaten the biological resilience and stability of desert ecosystems. Native consumers, including ants and rodents, likely have critical roles in defining post-fire plant community assembly and resilience to biological invasions. This study aimed to understand how western harvester ants ( Pogonomyrmex occidentalis ) that form mounds and large vegetation-free disks in the Great Basin Desert respond to fire and rodent community abundance. We tested this by installing treatment plots that excluded or allowed rodents and were burned or unburned in a full factorial design. We measured ant disk and mound size and density, along with a direct measure of ant activity in each experimental plot. Fire increased ant mound density by 126% compared to unburned plots. Rodent presence decreased ant activity by 39%, mound density by 59%, mound diameter by 13%, and mound height by 166%. We also show an interaction where the adverse effects of rodents on ants were greater in burned than in unburned plots. These results suggest that more frequent fires are likely to benefit ants, and fluctuations in rodent populations are likely to affect ant activity and disks and alter their responses to fire.