Résumé
How species coexist 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 whether the outcome of their interaction depends on the genetic identity of the competitors, and in particular whether intraspecific interaction differs between genetically related versus unrelated conspecifics. Secondly, we ask whether coexistence of the two species is modified by the presence of thyme plants altering local soil conditions. We grew genotypes collected in nature of both annual species in inter- and intraspecific competition experiments using soil collected directly underneath thyme plant (thyme soil) and soil collected away from thyme plants (no-thyme soil). We found that thyme soil affected interactions in a way that shifted the species' rank. Crucial for this result was the division of intraspecific competition between genetically related and unrelated individuals. Intraspecific competition was significantly less detrimental to kin than to unrelated conspecifics. In highly structured plant populations, such effects could favour local dominance of a patch by one species. However, this kin effect occurred for each species only in its preferred soil (thyme vs. no thyme), promoting a stabilizing coexistence mechanism at the landscape level.Synthesis. This study shows that the importance of intraspecific competition relative to interspecific competition can be highly dependent on the genotype identity of intraspecific competitors and of the local environment in which the interaction occurs. The local environment can itself be modified by the presence of a third species in the community. These results emphasize how, in order to understand the overall problem of species coexistence, it can be insightful to divide this into smaller local-scale problems of coexistence.