Environmental Heterogeneity and Seedling Establishment: Ant-Plant-Microbe Interactions

Abstract

1. Mounds produced by animals are important sources of disturbance in the sand shinnery oak community of western Texas. As a first step in the examination of the importance of animal-generated disturbances on the structure of the plant community we ask three questions in this study. (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 different species of ant mounds? (c) What is the influence of two components of the biotic environment, mycorrhizal fungi and non-mycorrhizal microbes, on seedling survival, growth rates, tissue nutrient content and patterns of biomass allocation? 2. Both the biotic and abiotic characteristics of ant mound soil differed from that of undisturbed soil and from 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 heterogeneity in this habitat. 3. Seedlings grown on unsterilized soil from harvester ant mounds were larger than seedlings grown on soil from the other two mounds, yet had lower tissue nutrient content and higher mortality rates. Seedlings grown in harvester ant soil were larger when the soil was sterilized suggesting that mycorrhizal fungi were a net energy drain in the higher nutrient content soil. Alternatively, seedlings grown without mycorrhizas in the other two mound soils were not different in size and had lower tissue nutrient concentrations than seedlings grown with mycorrhizas. 4. Thus, it appears that the fungi were beneficial in the lower nutrient content soil. In the more nutrient-rich harvester ant soil seedlings grown on sterilized soil with added non-mycorrhizal microbes were larger than seedlings grown in the absence of microbes.