Inbreeding depression in benign and stressful environments

Understanding the consequences of inbreeding has important implications for a wide variety of topics in population biology. Although it is often stated in the literature that the deleterious effects of inbreeding (inbreeding depression) are expected to be more pronounced under stressful than benign conditions, this issue remains unresolved and controversial. We review the current literature on the relationship between the magnitude of inbreeding depression and environmental stress and calculate haploid lethal equivalents expressed under relatively benign and stressful conditions based on data from 34 studies. Inbreeding depression increases under stress in 76% of cases, although this increase is only significant in 48% of the studies considered. Estimates of lethal equivalents are significantly greater under stressful (mean = 1.45, median = 1.02) than relatively benign (mean = 0.85, median = 0.61) conditions. This amounts to an approximately 69% increase in inbreeding depression in a stressful vs a benign environment. However, we find strong lineage effects to be ubiquitous among studies that examine inbreeding depression in multiple environments, and a prevalence of conditionally expressed deleterious effects within lineages that are uncorrelated across environments. These results have important implications for both evolutionary and conservation biology.