Date of this Version
Ecology, 91(3), 2010, pp. 902–914.
We empirically assess the relationship between population growth rate (λ, a parameter central to ecology) and effective population size (Ne, a key parameter in evolutionary biology). Recent theoretical and numerical studies indicate that in semelparous species with variable age at maturity (such as Pacific salmon, many monocarpic plants, and various other species), differences in mean reproductive success among individuals reproducing in different years leads to variation in λ, and this in turn can reduce Ne. However, this phenomenon has received little empirical evaluation. We examined time series of abundance data for 56 populations of chinook salmon (Onchorhynchus tshawytscha) from the northwestern United States and compared Ne (calculated from demographic data) with the total number of spawners each generation (NT). Important results include: (1) The mean multigenerational ratio Ñe/ÑT was 0.64 (median = 0.67), indicating that annual variation in λ reduces effective population size in chinook salmon by an average of ~ 35%. These reductions are independent of, and in addition to, factors that reduce Ne within individual cohorts (uneven sex ratio and greater-than-random variance in reproductive success). (2) The coefficient of variation of λ was the most important factor associated with reductions in Ne, explaining up to two-thirds of the variance in Ñe/ÑT. (3) Within individual generations, Ne was lower when there was a negative correlation between annual Ni and λ, i.e., when relatively few breeders produced relatively high numbers of offspring. Our results thus highlight an important and little-studied eco-evolutionary trade-off: density-dependent compensation has generally favorable ecological consequences (promoting stability and long-term viability) but incurs an evolutionary cost (reducing Ne because a few individuals make a disproportionate genetic contribution). (4) For chinook salmon, ˆNeH (an estimator based on the harmonic mean number of breeders per year) is generally a good proxy for true Ne and requires much less data to calculate.