The Production and Utilization of Seeds in Plains Grassland of Southeastern Arizona

In the plains grassland in southeastern Arizona there are two peaks of seed production. Following the winter rains, seeds are produced which are subject to relatively high predation by ants and rodents, and following the summer rains seeds are produced which are subject to relatively high predation from sparrows and rodents but not ants. The differences in seed selection by ants and sparrows have apparently been important in determining the evolution of seed morphology and phenology. Seeds produced after the winter rains are smooth in outline and difficult for ants to carry. Seeds produced after the summer rains have conspicuous awns, hairs, or other projections and usually require husking by sparrows. The bill structure of sparrows is such that they can more easily eat the species of seeds which become available after the winter rains, but this is the time when sparrows switch their diets from seeds to insects. The mandibular structure of Pogonomyrmex ants makes it difficult for them to harvest the seeds of smooth outline as are produced after the winter rains. The seeds which ants can most easily transport back to their nests are those species produced only after the summer rains and which ripen just as autumn cold forces the ants underground. Thus, the plants appear to have adapted the morphology of their seeds and their reproductive phenology to minimize predation. Interclass competition for seeds by ants, sparrows, and rodents is minimized by a complex combination of factors involving seed selection, differences in seasonal activity, and the bimodal nature of seed production. What interclass competition that might exist is probably manifest only in years of low seed production.