Contemporary effective population and metapopulation size (Ne and meta‐Ne): comparison among three salmonids inhabiting a fragmented system and differing in gene flow and its asymmetries

Abstract

Abstract We estimated local and metapopulation effective sizes (N^e and meta‐N^e) for three coexisting salmonid species (Salmo salar, Salvelinus fontinalis, Salvelinus alpinus) inhabiting a freshwater system comprising seven interconnected lakes. First, we hypothesized that N^e might be inversely related to within‐species population divergence as reported in an earlier study (i.e., FST: S. salar> S. fontinalis> S. alpinus). Using the approximate Bayesian computation method implemented in ONeSAMP, we found significant differences in N^e (N^eOSMP) between species, consistent with a hierarchy of adult population sizes (N^eS.salar<N^eS.fontinalis<N^eS.alpinus). Using another method based on a measure of linkage disequilibrium (LDNE: N^eLDNE), we found more finite N^e values for S. salar than for the other two salmonids, in line with the results above that indicate that S. salar exhibits the lowest N^e among the three species. Considering subpopulations as open to migration (i.e., removing putative immigrants) led to only marginal and non‐significant changes in N^e, suggesting that migration may be at equilibrium between genetically similar sources. Second, we hypothesized that meta‐N^e might be significantly smaller than the sum of local N^es (null model) if gene flow is asymmetric, varies among subpopulations, and is driven by common landscape features such as waterfalls. One ‘bottom‐up’ or numerical approach that explicitly incorporates variable and asymmetric migration rates showed this very pattern, while a number of analytical models provided meta‐N^e estimates that were not significantly different from the null model or from each other. Our study of three species inhabiting a shared environment highlights the importance and utility of differentiating species‐specific and landscape effects, not only on dispersal but also in the demography of wild populations as assessed through local N^es and meta‐N^es and their relevance in ecology, evolution and conservation.

Keywords

• Effective population size
• gene flow
• metapopulation
• Salmo salar
• Salvelinus fontinalis
• Salvelinus alpinus