THE IMPACT OF FUNGI ON SOIL SEEDS - IMPLICATIONS FOR PLANTS AND GRANIVORES IN A SEMIARID SHRUB-STEPPE

Fungi are ubiquitous in soil and may affect seed survivorship directly by decomposition or pathogenesis, or indirectly through the effects o f fungi on seed preference of granivores. We studied the role of fungi in the persistence of soil seeds in a shrub-steppe ecosystem using tw o experimental approaches. In the first, we examined survivorship of 9 00 seeds of five shrub-steppe species in nylon bags over a 10-mo perio d and determined viable seed losses to germination, decomposition, and pathogens. The proportion of seeds representing a viable seed bank de creased from 84.4% in the May pre-experimental conditions to 4.4% in M arch. Decomposition and attack by fungi were responsible for the great est decrease from fall to winter (0% in August, 32.0% in November, 35. 2% in March) while germination accounted for the greatest decrease fro m winter to spring (18.5% in August, 21.9% in November, 55.0% in March ). Small seeds were more likely to be lost to decomposition and pathog ens, whereas larger seeds had greater losses to germination. A subset of 80 experimental seeds was cultured for seed-infecting fungi. We fou nd fungi in virtually all seeds; the most common fungal isolates were cosmopolitan soil fungi, such as species of Penicillium and Mucor. The se species are known producers of extracellular enzymes and mycotoxins . Two sterile (nonfruiting) forms of fungi were also frequently isolat ed, as were two pathogenic species of Fusarium. In a second experiment we placed seeds in fungal cultures obtained from soil seeds and prese nted moldy seeds along with control seeds to a shrub-steppe granivore, the western harvester ant (Pogonomyrmex occidentalis). Ants harvested control seeds at nearly twice the rate of moldy seeds, suggesting tha t they avoided seeds infected with fungi. In this way, fungi may indir ectly affect seed mortality due to predation by granivores, in additio n to the direct effects fungi can have on seed survivorship. We sugges t that fungi may play an important role in soil seed dynamics, but one that is poorly understood.