Rodent competition and fire alter patterns of mound and disk formation of western harvester ants.

Consumers exert top-down controls on dryland ecosystem function, but recent increases in fire activity may alter consumer communities in post-fire environments. 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 that significantly influence plant community structure 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 in each experimental plot. Fire increased ant mound density by 126% compared to unburned plots. Rodent presence decreased 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 ant disk density were greater in burned than in unburned plots. The results suggest that booms in rodent populations are likely to have suppressive effects on ant mound and disk formation in native shrublands but that harvester ants may be released from rodent competition with the emergence of invasive grass-fire cycles.