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

Abstract

Lower effective sizes (N(e)) than census sizes (N) are routinely documented in natural populations, but knowledge of how multiple factors interact to lower N(e)/N ratios is often limited. We show how combined 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.

Keywords: Abundance; effective population size; habitat; life history; trout.

Figure 1

The geographic locations of Bob's Cove and Whale Cove rivers on Cape Race, Newfoundland, and in relation to the rest of North America. Figure 1.

Figure 2

The means and standard deviations of estimated log probabilities for Whale Cove (A) and Bob's Cove (B) rivers after 10 replicate trials in STRUCTURE allowing for admixture and correlated allele frequencies and using a burn-in period of 100,000 replicates and 500,000 MCMC reactions.