Grassland Composition Affects Season Shifts in Seed Preference by Pogonomyrmex barbatus (Hymenoptera: Myrmicinae) in the Edwards Plateau, Texas

Abstract

The effects of season and community composition as generated by livestock herbivory of differing intensity on seed species preference by Pogonomyrmex barbatus (F. Smith) were studied in a semiarid savanna on the Edward’s Plateau, TX. Seasonal differences in nutrient requirements of the colony could lead to differential preferences for seeds harvested in spring and fall. Field cafeteria studies were conducted to test the hypothesis that late successional species, with their high nutrient content, would be chosen regardless of grazing intensity or season. Commercial seeds of known nutrient content were used to test the hypothesis that high protein levels would be chosen in spring and high carbohydrate levels in the fall. Naturally occurring seeds were differentially harvested and some were preferred regardless of relative availability. Total seed harvest in cafeteria experiments was higher in spring than in fall. Commercial seeds were harvested equally among treatments within a season; thus, nutrient selection was indistinguishable. Preference for native species was significantly different in both seasons but was influenced by a significant interaction with grazing treatments. Bouteloua curtipendula, a late successional mid-grass, was harvested significantly more in the spring than the fall and at higher rates in the heavily grazed treatment, rejecting the hypothesis that they would be chosen regardless of treatment or season. Seed preference for late successional grasses within heavily grazed communities may slow succession after grazing. During disturbance recovery, late successional species may be reduced by forager preference and rates of spring harvest.