Inbreeding and demography interact to impact population recovery from bottlenecks

Biodiversity loss driven by climate change and human activities poses a critical global challenge. Population restoration and reintroduction programs are essential for mitigating this threat, yet their outcomes are often unpredictable due to poorly understood success factors. The conservation program of the crested ibis (Nipponia nippon) marks a successful example where the population rose from seven survivors to over 9,000 in the past four decades. To learn whether this successful restoration was due to chance or largely repeatable, we developed an individual-based model that simulates the restoration process by incorporating life-history parameters from empirical data. Our simulation results closely mirror empirical findings, including the time taken to reach the current population size and population-level inbreeding coefficients. We further analyzed the model to compare the effectiveness of two reintroduction strategies and analyzed how inbreeding depression interacts with demography to influence the chance of recovery from bottlenecks. The reintroduction simulations reveal that the ‘firework’ approach (one-source translocations) outperforms the ‘stepping-stone’ (serial translocations) approach in restoration effectiveness. Our simulations over broad demographic parameters demonstrate that the net effect of inbreeding varies with species-specific demography, and highlight the importance of considering this interaction when interpreting conservation outcomes and designing future reintroduction programs.