The magnitude of inbreeding depression is often thought to increase with increasingly stressful environmental conditions. Recent work in plant systems has supported this hypothesis for some ecological interactions, such as herbivory, but the relationship between inbreeding depression and competitive stress remains unclear. We investigated the effects of interspecific competition with the invasive purple loosestrife (Lythrum salicaria) on inbreeding depression in Mimulus ringens. We grew hand-crossed M. ringens individuals in pots under intraspecific competition, interspecific competition with the native L. alatum, and interspecific competition with the invasive L. salicaria, and measured both aboveground and belowground performance.

Results/Conclusions  

The magnitude of competitive stress, as measured by performance of M. ringens, was least under intraspecific competition and greatest under competition with L. salicaria. However, the magnitude of inbreeding depression was least in M. ringens competing against L. salicaria. This pattern was consistent for both aboveground and belowground performance measures, including plant biomass, fecundity and rhizome production. Our results are not consistent with the common result that inbreeding depression increases with increasing environmental stress, suggesting that this relationship may be more complex in the context of competition. Our results also suggest that novel stressors, such as invasive species, may alter the magnitude of inbreeding depression in native plant populations, potentially affecting their population growth, maintenance of mating systems, and many other important population properties.