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

Delineating a threshold migration rate for demographic independence is
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 different 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.