Post-dispersal seed predation in relation to selection on seed size in Dithyrea californica

Question: We asked whether a desert ant population and a community of heteromyid rodents select Dithyrea californica seeds in relation to their size, frequency of seed sizes, and absolute density and thus have the capacity to influence natural selection on seed size. Hypothesis: Following the predictions of optimal diet theory, we hypothesized that seed predators will prefer to prey upon larger seeds, that seed selection should be frequency independent, and that only variation in the absolute density of the preferred seeds will influence predator behaviour by lowering their preference for larger seeds. Organisms: A colony of the desert harvester ant Pogonomyrmex rugosus and a community of heteromyid rodents (Dipodomys merriami and Dipodomys desertii). Site of experiments: Reserva de la Biósfera El Pinacate y Gran Desierto de Altar and Hermosillo, Sonora, México. Methods: We conducted two preference experiments in which we offered D. californica seeds that varied in size, relative frequency of sizes, and absolute density of seeds to a population of the harvester ant and to the community of heteromyid rodents of the Sonoran Desert. Results: We found that ants had a preference for larger seeds whereas rodents had an equal preference for large and small seeds, despite variation in seed frequency and density. In accordance with optimal diet theory, seed predation was frequency independent. Contrary to our predictions, neither changes in absolute density of larger seeds nor of smaller seeds had a significant influence on predator behaviour. Conclusion: Our results suggest that only Pogonomyrmex rugosus influenced selection on seed size, which it does by preferring larger seeds and counteracting the competitive effect of larger seeds in other life stages. Selection via seed predation conflicts with post-germination selection on seed size, when encountered by ants but not rodents.