Inbreeding Depression and Competition in the square-stemmed monkey-flower (Mimulus ringens)

Abstract

Environmental stress often arises from biotic interactions and is thought to increase the magnitude of inbreeding depression. However, empirical studies have yielded mixed results. For many plants, invasive species may be an important source of stress through competition, but it is unclear whether interspecific competition alters inbreeding depression, and whether invasive and native competitors differ in their effects. Here, we report the results of the first study to test whether the magnitude of inbreeding depression is altered by competition with an invasive species. Selfed and outcrossed individuals of the native wetland plant Mimulus ringens were grown in three competitive treatments: intraspecific competition and interspecific competition with either the invasive Lythrum salicaria or its native congener L. alatum. Inbreeding depression did not simply increase with stress; it was least in the presence of L. salicaria, the greatest stressor. For biomass and stolon production, inbreeding depression was greatest in the presence of L. alatum, the intermediate stressor. These results suggest that variation in interspecific competition may alter inbreeding depression, and that invasive species have the potential to disrupt the selective regimes that maintain mating systems. Furthermore, stress may affect inbreeding depression in a nonlinear fashion, with the greatest inbreeding depression found at intermediate stress levels.