Surviving flames and foes: Ant responses to wildfire and predation in sagebrush steppe

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Camdon Kay, Biological Sciences, Idaho State University, Pocatello, ID and Joshua Grinath, Department of Biological Sciences, Idaho State University, Pocatello, ID

Background/Question/Methods
Disturbances are key structuring factors in ecological communities. While organismal responses to single types of disturbances are well understood, responses to multiple types of disturbances are less clear. Here, we evaluated ant survival in relation to two common types of disturbance affecting ants in sagebrush steppe: wildfire and predation by bears. Thatch ants, Formica obscuripes, and harvester ants, Pogonomyrmex salinus, are abundant in sagebrush steppe, and we expected that thatch ant colonies would be less likely to survive wildfire than harvester ants because their nests are more susceptible to fire. Moreover, unlike harvester ants, thatch ants are common prey of bears (Ursus americanus), which dig into the aboveground portions of nests during predation. We asked whether disturbance by bears exacerbates the mortality of thatch ant colonies following wildfire. In early July 2020, we simulated bear damage to thatch ant nests across three sites in southeast Idaho, where harvester ants co-occurred. A wildfire partially burned one site in late July, after which we monitored ant colonies within and immediately outside this burned area. We assessed ant colony survival ten days and two months after the wildfire, and will survey nests in June 2021 to evaluate delayed responses.
Results/Conclusions
Ten days after the wildfire, 94.1% of thatch ant nests remained active, while 100% of harvester ant nests were active, within the burned area. Two months post-burn, thatch ant survival was no less at the burned site compared to the unburned sites. Simulated bear damage to thatch ant nests did not result in greater mortality within the burned site or across sites. At all sites, thatch ants had a higher abundance of nests, but lower survival, compared with harvester ants. Though thatch ant colony survival was high, they are likely more vulnerable to these disturbances than harvester ants due to differences in nesting traits. Thatch ant colony mortality was associated with smaller nest volumes, suggesting that the activity of larger nests is more stable through time than smaller nests. A survey this upcoming June will yield additional results to evaluate our questions. Altogether, the results thus far indicate that short-term thatch ant survival is resistant to the independent and combined effects of disturbances by wildfire and predators. Further work is needed to understand the consequences of co-occuring types of disturbances within ecosystems.