Spatiotemporal relationship between adult census size and genetic population size across a wide population size gradient

Adult census population size ( N ) and effective number of breeders ( N b ) are highly relevant for designing effective conservation strategies. Both parameters are often challenging to quantify, however, making it of interest to determine whether one parameter can be generalized from the other. Yet, the spatiotemporal relationship between N and N b has not been well characterized empirically in many taxa. We analysed this relationship for 5–7 consecutive years in twelve brook trout populations varying greatly in N (49‐10032) and N b (3‐567) and identified major environmental variables affecting the two parameters. N or habitat size alone explained 47–57% of the variance in N b , and N b was strongly correlated with effective population size. The ratio N b / N ranged from 0.01 to 0.45 and increased at small N or following an annual decrease in N , suggesting density‐dependent constraints on N b . We found no evidence for a consistent, directional difference between variability in N b and/or N b / N among small and large populations; however, small populations had more varying temporal variability in N b / N ratios than large populations. Finally, N b and N b / N were 2.5‐ and 2.3‐fold more variable among populations than temporally within populations. Our results demonstrate a clear linkage between demographic and evolutionary parameters, suggesting that N b could be used to approximate N (or vice versa) in natural populations. Nevertheless, using one variable to infer the other to monitor trends within populations is less recommended, perhaps even less so in small populations given their less predictable N b vs. N dynamics.