Population demographics influence genetic responses to fragmentation: A demogenetic assessment of the ‘one migrant per generation’ rule of thumb

Fragmented landscapes reduce gene flow and impair long term population viability. Stream networks are particularly susceptible to fragmentation because dispersal is constrained to linear upstream and downstream movements. Despite these potential effects, infrequent migrations can maintain genetic diversity and as few as one migrant per generation (OMPG) is commonly suggested as sufficient gene flow to minimize losses in genetic diversity. However, demography varies by taxa, space and time, making such a generalized rule of thumb unlikely to be applicable across a diverse array of fragmentation scenarios and species. We utilized a demogenetic model to evaluate the OMPG rule and simulate the influence of population demographics on the rate of genetic changes following fragmentation in a headwater meta-population of brook trout (Salvelinus fontinalis). A single migrant per generation increased allelic diversity by an average of 15% and decreased genetic differentiation by 31% following 40years of simulations compared to complete isolation, however OMPG was not sufficient to prevent significant changes in within- or between-population genetic metrics in all but the largest population scenario (N=500). Less than 10 individuals were typically required to achieve no changes in both genetic metrics, yet this pattern was dependent on the source populations and will be context-specific given the population sub-structuring in a given stream network. Sensitivity analyses indicated the parameter controlling the proportion of mature females spawning annually was the most influential on population genetic responses in isolated populations, suggesting that when fewer females contribute to each generation the population is more likely to experience rapid changes in allelic frequency through genetic drift. This finding supports the use of metrics such as effective population size and the number of effective breeders in predicting population stability and viability following fragmentation. Variability in population dynamic processes and associated responses to fragmentation suggest that generalized rule of thumbs for management should be used with caution. Particularly when violations of the underlying theoretical assumptions exist, consideration of demographic processes (i.e. vital rates, species specific life history strategies and dispersal) and genetic structuring will allow for more appropriate conservation recommendations.