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

We estimated local and metapopulation effective sizes ((N) over cap (e) and meta-(N) over cap (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) over cap (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) over cap (e) ((N) over cap eOSMP) between species, consistent with a hierarchy of adult population sizes ((N) over cap S-e.salarS-e.fontinalisS-e.fontinalisS-e.alpinus). Using another method based on a measure of linkage disequilibrium (LDNE: (N) over cap eLDNE), we found more finite (N) over cap (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) over cap (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) over cap (e), suggesting that migration may be at equilibrium between genetically similar sources. Second, we hypothesized that meta-(N) over cap (e) might be significantly smaller than the sum of local (N) over cap (e)s (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) over cap (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) over cap (e)s and meta-(N) over cap (e)s and their relevance in ecology, evolution and conservation.