Desert Granivore Foraging and Its Impact on Seed Densities and Distributions

Abstract

Field experiments were conducted to determine the foraging behaviors of granivorous desert ants and rodents and to ascertain the impact of these foraging efforts on the density and distribution of seeds in the soil. Foraging experiments were performed which involved providing seeds within taxon—specific enclosures in scattered and clumped distribution on the soil surface and 1.5 cm below the surface. These experiments indicated that ants only foraged on the surface, whereas rodents were able to garner seeds from below the ground. Ants foraged °85% of the experimental seed distributions on the surface and were able to retrieve °45% of the seeds they detected during any 24—h experimental run. Rodents detected 100% of all distributions and got °96% of the seeds from all distributions except scattered/belowground, from which they gathered only 75% of the seeds. Three—year experiments using exclosures which excluded either ants, rodents, both taxa, or neither taxon, indicated that either taxon alone, or both taxon together, have a severe impact on the density of seeds in the soil. When neither is present, seed densities remain high. Furthermore, any combination of granivores greatly reduces the number of samples with either high numbers of seeds or high biomass of seeds, although the ants tend to leave a greater number of large clumps of seeds than the rodents. Clumping indices are also extremely high in the absence of granivores. In opposition to the apparently detrimental effect of seed use by the granivores, surface seed—caching by rodents appears to enhance seed germination.