ENVIRONMENTAL HETEROGENEITY AND SEEDLING ESTABLISHMENT - ANT PLANT-MICROBE INTERACTIONS

1. Mounds produced by animals are important sources of disturbance in the sand shinnery oak community of western Texas. As a first step in t he examination of the importance of animal-generated disturbances on t he structure of the plant community we ask three questions in this stu dy. (a) Are there differences in the abiotic or biotic characteristics of patch disturbances produced by three different species of ants? (b ) Do seedling growth and survival vary when grown on soil from differe nt species of ant mounds? (c) What is the influence of two components of the biotic environment, mycorrhizal fungi and non-mycorrhizal micro bes, on seedling survival, growth rates, tissue nutrient content and p atterns of biomass allocation? 2. Both the biotic and abiotic characte ristics of ant mound soil differed from that of undisturbed soil and f rom each other. Mound soil typically had higher nutrient content than undisturbed soil and harvester ant mounds had higher nutrient content than the other species. Thus, ant mounds can generate environmental he terogeneity in this habitat. 3. Seedlings grown on unsterilized soil f rom harvester ant mounds were larger than seedlings grown on soil from the other two mounds, yet had lower tissue nutrient content and highe r mortality rates. Seedlings grown in harvester ant soil were larger w hen the soil was sterilized suggesting that mycorrhizal fungi were a n et energy drain in the higher nutrient content soil. Alternatively, se edlings grown without mycorrhizas in the other two mound soils were no t different in size and had lower tissue nutrient concentrations than seedlings grown with mycorrhizas. 4. Thus, it appears that the fungi w ere beneficial in the lower nutrient content soil. In the more nutrien t-rich harvester ant soil seedlings grown on sterilized soil with adde d non-mycorrhizal microbes were larger than seedlings grown in the abs ence of microbes.