Concurrent habitat and life history influences on effective/census population size ratios in stream-dwelling trout

Lower effective sizes (N_e) than census sizes (N) are routinely documented in natural populations, but knowledge of howmultiple factors interact to lower N_e/N ratios is often limited.We showhowcombined habitat and life-history influences drive a 2.4-to 6.1-fold difference in N_e/N ratios between two pristine brook trout (Salvelinus fontinalis) populations occupying streams separated by only 750 m. Local habitat features, particularly drainage area and stream depth, govern trout biomass produced in each stream. They also generate higher trout densities in the shallower stream by favoring smaller body size and earlier age-at-maturity. The combination of higher densities and reduced breeding site availability in the shallower stream likely leads to more competition among breeding trout, which results in greater variance in individual reproductive success and a greater reduction in N_e relative to N. A similar disparity between juvenile or adult densities and breeding habitat availability is reported for other species and hence may also result in divergent N_e/N ratios elsewhere. These divergent N_e/N ratios between adjacent populations are also an instructive reminder for species conservation programs that genetic and demographic parameters may differ dramatically within species.