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The influence of inbreeding depression on the stability of a small insular population : modeling inbreeding-stress interactions Runyan, Simone Elizabeth


I investigated the potential for inbreeding to increase variability in the population trajectory of a small insular population. Increased stochastic variability in population size increases extinction risk, especially in small populations, and is therefore of interest in conservation biology. Inbreeding depression may vary with severity of environmental stress such weather extremes. I explored three main inbreeding-stress interaction scenarios: 'no interaction', a 'linear interaction', and a 'threshold interaction' in which the survival rates of inbred individuals were greatly reduced above a stress threshold. I varied inbreeding depression in annual survival rate from 0% to 80%. I also explored the influence of immigration rate on population variability and extinction rate. I investigated these scenarios using an individual-based population model that traced a pedigree and determined an inbreeding coefficient for each individual. The model was parameterized using data from an island population of Song Sparrows (Melospiza melodia), monitored closely since 1975. Given constant immigration, little increase in variability in the population trajectory occurred as inbreeding depression increased from 0% to 80% with 'no interaction' between stress and inbreeding depression, and only slightly more with a 'linear interaction'. However, imposing a 'threshold interaction' between stress and inbreeding increased extinction rate markedly and led to regular crashes averaging 83% ± 1 1% (mean ± SD) of population size at a mean interval of 11 ± 5 years. The immigration rate required to reduce the extinction rate to less than 5% increased with inbreeding depression, and with the level of the inbreeding-stress interaction. With inbreeding depression of 50%, an immigration rate of 1.25, or 5 immigrants in 4 years, was required given the 'threshold interaction'. By comparison, immigration rates of only 0.50 given a 'linear interaction', and 0.38 given 'no interaction', were required to reduce extinction rates to 5%. These results suggest that inbreeding-stress interactions have the potential to increase stochastic variability in a population trajectory and that higher levels of immigration may be required to reduce the risk of extinction if inbreeding depression is high or if inbreeding-stress interactions occur in nature.

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