Inter-specific competitive stress does not affect the magnitude of inbreeding depression

Hypothesis: Stressful inter-specific competition enhances inbreeding depression. Organisms: Creeping spearwort (Ranunculus reptans L.) and its common competitor, the creeping bentgrass (Agrostis stolonifera L.). Field site: Outdoor common garden experiment at the University of Potsdam. Methods: We collected plants of 12 natural populations of R. reptans differing in mean parental inbreeding coefficient (0.01–0.26). We performed within-population crosses for two generations and kept the offspring in a common environment. In the second generation, we raised plants in the presence or absence of the grass competitor A. stolonifera. After 2 months, we assessed survival and growth of two replicate plants per seed family and treatment. To compare our results with those of previous investigations, we compiled a list of studies on inbreeding and competition. Conclusions: Both population mean inbreeding and inter-specific competition led to a significant reduction in plant performance (survival and growth) and were therefore sources of stress. However, inbreeding and competition did not interact in their effects on plant performance or developmental instability (estimated by within-family coefficient of variation in performance). Inter-specific competition did not enhance inbreeding depression, which is in line with the results of most previous studies.