Migration matters in conservation and management: Exploring the 10% rule for demographic independence via simulation

Delineating a threshold migration rate for demographic independence important for understanding connectivity among fragmented populations and defining management units for conservation and harvest regulation. In turn, defining management units is an essential step in sustainable management to avoid unintentional depletion of resources managed for conservation or harvest. The 10% rule of demographic connectivity is a rule of thumb that delineates the threshold of demographic independence when the behavior of two populations shifts from synchronous at >10% to independent at <10%. However, the accuracy of the 10% rule to real-world scenarios and application to natural resource management is unknown. We evaluated the 10% rule using simulation for two life history types: Pacific cod, Gadus macrocephalus, a gadid with relatively fast growth, and blackspotted rockfish, Sebastes melanostictus, a long-lived rockfish species. Results were obtained by simulating a real-world tool for evaluating demographic connectivity, positive correlation in estimated population sizes. We assessed the effect of migration on demographic connectivity on otherwise independent populations under one- and two-way migration, and with various population sizes and life history parameters. Sensitivity testing showed that positive correlation in population size does not occur in roughly a quarter of simulations, regardless of the migration rate. When positive correlation in population size does occur, mean migration rates over all simulations were between 5% and 10%: 0.089 (8.9%) for blackspotted rockfish and 0.058 (5.8%) for Pacific cod. However, the range of migration resulting in demographic connectivity was large, 0.02-0.44 for blackspotted rockfish and 0.02-0.40 for Pacific cod.