Inbreeding depression in the wild

Despite its practical application in conservation biology and evolutionary theory, the cost of inbreeding in natural populations of plants and animals remains to a large degree unknown. In this review we have gathered estimates of inbreeding depression (δ) from the literature for wild species monitored in the field. We have also corrected estimates of δ by dividing by F (coefficient of inbreeding), to take into account the influence that the variation in F will have on δ. Our data set includes seven bird species, nine mammal species, four species of poikilotherms (snakes, fish and snails) and 15 plant species. In total we obtained 169 estimates of inbreeding depression for 137 traits; 81 of those estimates included estimates of F. We compared our mammalian data (limited to those traits related to juvenile mortality) to the estimates for captive zoo species published by Ralls et al. (1988) to determine if, as predicted from the literature, natural estimates of inbreeding depression are higher than captive estimates. The mean δ ± SE (significantly different from zero and not corrected for F) for homeotherms was 0.509 ± 0.081; for poikilotherms, 0.201 ± 0.039; and for plants, 0.331 ± 0.038. Levels of inbreeding depression this high in magnitude will be biologically important under natural conditions. When we limited our data set to mortality traits for mammals and corrected for F=0.25 (as is the case for the Ralls et al. data set), we found a significant difference between the two data sets; wild estimates had a substantially higher mean cost of inbreeding at F = 0.25: 2.155 (captive species: 0.314). Of the 169 estimates of δ, 90 were significantly different from zero, indicating that inbred wild species measured under natural conditions frequently exhibit moderate to high levels of inbreeding depression in fitness traits.