Graduate Defense: Michaela Grossklaus

Thesis Information

Title: Seed Predation By Harvester Ants On Grasses And Native Forbs: Implications For Management In A Changing Landscape

Program: Master of Science in Biology

Advisor: Dr. Ian Robertson, Biological Sciences

Committee Members: Dr. David Pilliod, Ecology, and Dr. Trevor Caughlin, Biological Sciences

Abstract

Vegetation restoration efforts in the sagebrush steppe to date have yielded mixed results. Although many factors contribute to variation in restoration outcomes, seed predation by harvester ants may be an important yet overlooked source of seed loss. Using a selection of grass and forb seeds commonly seeded in local restoration projects, we conducted a field experiment to evaluate whether seed species and spatial arrangement (i.e., distance of seed patches from ant colonies and from each other) affected patterns of seed consumption by Owyhee harvester ants, Pogonomyrmex salinus. To further examine the ecological factors that may affect foraging by harvester ants, we mapped the matrix of vegetation surrounding each harvester ant colony in our experiment and assessed the influences of cheatgrass, Bromus tectorum, cover and Sandberg bluegrass, Poa secunda, cover on seed removal. To provide context for ant seed preferences, we evaluated differences in metrics of seed handling and nutrient content among the seed species in our mix. We found that seeds placed closer to ant colonies were generally more vulnerable to predation than those placed farther away, but the overall effects of spatial arrangement were more nuanced and differed by seed species. Cheatgrass cover and bluegrass cover both had small protective effects on seeds, but the strengths of their effects differed, suggesting that harvester ant foraging behaviors may be shaped by local vegetation cover. We found that harvester ants exhibited preferences among the seed species, but our seed handling and nutrition data yielded no clear explanations for their preferences. These results indicate that the spatial arrangement of seeds around harvester ant colonies affects the likelihood of removal, that some seed species are at greater risk of consumption than others, and that the matrix of vegetation surrounding harvester ant colonies may affect harvester ant foraging behaviors. Taken together, this information may provide context for the implementation and success of future seed introduction efforts.