Comparing the Effects of Granivorous Rodents on Persistence of Indian Ricegrass (oryzopsis Hymenoides) Seeds in Mixed and Monospecific Seed Patches

—In desert environments seeds are often heterogeneously distributed in small patches that vary in number of seed species and in seed density. Because seed harvest by rodents is often density dependent (a larger proportion of seeds is removed from high-density seed patches than from low-density patches), the proportion of residual or post-harvest seeds should be greater in low-density patches. In addition, seed preference can affect harvest. We tested whether the residual proportion of a highly preferred seed (Indian ricegrass, Oryzopsis hymenoides) was less when in a seed patch with a 2nd species (mixed-species patch) than when in a monospecific seed patch. We predicted that the increased overall seed density due to the presence of 2 species in a patch would result in a lower residual proportion of ricegrass seeds in the mixed-species seed patches than in the monospecific patches. As predicted, the residual proportions of Indian ricegrass seeds were less each time ricegrass was paired with one of 6 other species in mixed-species patches. Similarly, the residual proportion of each of those 6 species was less when paired with ricegrass than when in a monospecific patch. We speculate on the potential implications of these results for the population dynamics of plant species and the physical structure of plant communities.