Competitor relatedness, indirect soil effects and plant co-existence

How species co-exist is a central question in ecology. Niche differentiation can prevent two species from excluding each other by competition. However, this interaction can vary in space because of internal factors such as intraspecific genetic variation, or external ones such as the presence of a third species. While these effects have been repeatedly observed, their joint effect on the outcome of competition has rarely been considered. We experimentally investigated how a dominant ecosystem engineer (the shrub Thymus vulgaris) affects interactions between two co-occurring and closely related annuals (Medicago minima and M. rigidula). We ask first if the outcome of their interaction depends on the genetic identity of the competitors, and in particular if intraspecific interaction differs between genetically related versus unrelated conspecifics. Second, we ask if co-existence of the two species is modified by the presence of thyme plants altering local soil conditions. We grew genotype...