Measuring the effect of environmental stress on inbreeding depression alone obscures the relative importance of inbreeding–stress interactions on overall fitness in Callosobruchus maculatus

Environmental stress can have a profound effect on inbreeding depression. Quantifying this effect is of particular importance in threatened populations, which are often simultaneously subject to both inbreeding and environmental stress. But while the prevalence of inbreeding–stress interactions is well known, the importance and broader applicability of such interactions in conservation are not clearly understood. We used seed beetles, Callosobruchus maculatus, as a model system to quantify how environmental stressors (here host quality and temperature stress) interact with inbreeding as measured by changes in the magnitude of inbreeding depression, δ, as well as the relative importance of inbreeding–stress interactions to overall fitness. We found that while both environmental stressors caused substantial inbreeding–stress interactions as measured by change in δ, the relative importance of these interactions to overall survival was modest. This suggests that assessing inbreeding–stress interactions within the framework of δ alone may give an inaccurate representation of the relevance of interactions to population persistence. Furthermore, we found that the effect of environmental stress on fitness, but not inbreeding depression, varied strongly among populations. These results suggest that the outcomes of inbreeding–stress interactions are not easily generalized, an important consideration in conservation settings.