Combined effects of soil heterogeneity, herbivory and detritivory on growth of the clonal plant Hydrocotyle vulgaris

Fine-scale spatial heterogeneity of soil nutrients often increases growth of plants, especially clonal species. Herbivory could change this positive effect of soil heterogeneity. Furthermore, detritivory may alter soil heterogeneity and indirectly modify herbivory. However, little is known about the interactive effects of soil nutrient heterogeneity, herbivory, and detritivory on plant growth. We conducted a greenhouse experiment in which we grew the creeping, perennial herb Hydrocotyle vulgaris in two soil treatments (homogeneous and heterogeneous) with or without the aphid Myzus persicae and with or without the earthworm Eisenia fetida. H. vulgaris grew more in heterogeneous than in homogeneous soil, even though total nutrient availability was held constant, and less with than without aphids. The presence of aphids decreased plant growth less in both the high- and the low-nutrient patches in the heterogeneous soil treatment than in the medium-nutrient soil in the homogeneous soil treatment. Earthworms did not affect plant growth but survived better in soil with higher nutrients and in the presence of aphids. Herbivory can increase the positive effects of soil heterogeneity on clonal plants. However, results do not suggest that detritivory modifies effects of soil heterogeneity on plant growth.