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

Abstract

Adult census population size (N) and effective number of breeders (Nb ) 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 Nb 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 Nb (3-567) and identified major environmental variables affecting the two parameters. N or habitat size alone explained 47-57% of the variance in Nb , and Nb was strongly correlated with effective population size. The ratio Nb /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 Nb . We found no evidence for a consistent, directional difference between variability in Nb and/or Nb /N among small and large populations; however, small populations had more varying temporal variability in Nb /N ratios than large populations. Finally, Nb and Nb /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 Nb 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 Nb vs. N dynamics.

Limited variability in upper thermal tolerance among pure and hybrid populations of a cold-water fish

“…Previous studies have found that although these CR brook trout populations differ nearly 50-fold in census size N and 10-fold in effective number of breeders N b (Bernos and Fraser, 2016), there is no evidence for differences in (i) quantitative genetic variation and trait differentiation in relationship to population size, nor (ii) phenotypic plasticity in relationship to population size (Wood and Fraser, 2015; Wood et al ., 2015). Therefore, our study provides further evidence that population size may not be tightly related to the ability of a population to respond to environmental change, and that thermal tolerance, in particular (physiologically and behaviourally), may be highly conserved even in such a plastic species as S. fontinalis .…”

“…Another expected outcome might be that hybridizing between populations above and below the minimal viable population threshold would benefit smaller populations disproportionally. In our hybridized crosses, one large population (FW, see above) was hybridized with two smaller populations (STBC, see above; WC: mean N b = 31.36 and mean N  = 783.09; Bernos and Fraser, 2016) with little effect on any thermal tolerance trait, whether measured by mean or variance.…”

“…First, the small size of CR streams (ranging in length from 0.27 to 8.10 km) allows for thorough sampling and accurate estimates of population size. Second, CR populations range greatly in size, with N  = 780–5120 and N b = 27–200 (where N is the census population size and N b the effective number of individuals breeding in one spawning season; an analogue of effective population size that is positively related to genetic diversity of a population; Bernos and Fraser, 2016). Third, consistent with theory, small CR populations have less neutral genetic variation than large CR populations (Fraser et al ., 2014), yet genetic variation underlying quantitative traits does not vary with population size (Wood et al ., 2015); such apparent discrepancies make this population system an intriguing one for investigating what genetic metrics best predict responses to environmental change.…”

Latitudinal biodiversity gradients at three levels: Linking species richness, population richness and genetic diversity

“…While genetic drift has not received much empirical support as an explanation for the species richness gradient, it could be more relevant for intraspecific diversity. As patch size or a species’ range size is correlated with population size (Bernos & Fraser, ; Currie et al, ), it is a reasonable assumption that species with small ranges have, on average, smaller population sizes. This would lead to increased levels of genetic drift and an increased possibility of inbreeding, and hence lower GenPerSpp.…”

Population variation in density‐dependent growth, mortality and their trade‐off in a stream fish

“…For this purpose, a field experiment was conducted in which the density of young‐of‐the‐year (YOY) brook trout ( Salvelinus fontinalis ) was manipulated in isolated sections of small streams of Cape Race, Newfoundland, Canada. Our experiment was replicated both spatially (three sites per population) and temporally (three consecutive years) across three neighbouring populations that are known to differ in life history and stream characteristics (Bernos & Fraser, ; Hutchings, ). In our streams, food abundance is low, predation is negligible and growth rate of brook trout decreases with increasing water temperature, whereas mortality is exacerbated in acidic conditions (Hutchings, ; Yates, ).…”