The N_e/N ratio in applied conservation

Abstract

Recent developments within the IUCN and the Convention on Biological Diversity have affirmed the increasingly key role that effective population size (Ne) and the effective size: census size ratio (Ne/N) play in applied conservation and management of global biodiversity. This paper reviews and synthesizes information regarding the definition of Ne and demographic and genetic methods for estimating effective size, census size, and their ratio. Emphasis is on single‐generation estimates of contemporary Ne/N, which are the most informative for practical applications. It is crucial to clearly define which individuals are included in the census size (N). Defining N as the number of adults alive at a given time facilitates comparisons across species. For a wide range of applications and experimental designs, inbreeding Ne is simpler to calculate and interpret than variance Ne. Effects of skewed sex ratio are generally modest, so most reductions to Ne/N arise from overdispersed (greater‐than‐Poisson) variance in offspring number (). Even when fecundity changes with age, overdispersed within‐age variance generally contributes most to overall , and both random and deterministic (mediated by selection) factors can be important. Most species are age‐structured, so it is important to distinguish between effective size per generation (Ne) and the effective number of breeders in one season or year (Nb). Both Ne and Nb are important for applied conservation and management. For iteroparous species, a key metric is variance in lifetime reproductive success (), which can be affected by a variety of additional factors, including variation in longevity, skip or intermittent breeding, and persistent individual differences in reproductive success. Additional factors that can be important for some species are also discussed, including mating systems, population structure, sex reversal, reproductive compensation, captive propagation, and delayed maturity.

Strong dependence of demographic stochasticity on a small effective number of breeders in iteroparous species

“…The N b is defined by known pedigree [10,11] or life-history information [12,13] and is estimated from a single cohort sample using pedigree or life-history data or linkage disequilibrium (LD) information from genetic analyses [14]. N b (or N b /N) reflects the reproductive variance among parents due to parental and nonparental factors [15][16][17]. A small N b /N indicates overdispersed reproduction, which is an additional component of demographic stochasticity.…”

Pitfalls and windfalls of detecting demographic declines using population genetics in long‐lived species

“…N e represents the number of individuals in an idealized Wright–Fisher population with the same genetic behavior (e.g., same rate of genetic drift, or same amount of genetic diversity) as the focal empirical population (Charlesworth, 2009 ; Waples, 2022 ). Because of many factors, including changes in population size and the magnitude of heterogeneity in reproductive success, N e is usually smaller than N c for a given empirical population (Fisher, 1923 ; Frankham, 1995 ; Nunney, 1993 ; Turner et al., 2006 ; Waples, 2024 ; Wright, 1931 ). Current methods of detecting demographic trends from genetic data are quite accurate when inferring ancient demographic trends (>~100 generations in the past) but have low power to capture trends associated with recent events (within the last ~100 generations) without large amounts of sequencing data and/or individuals sampled (Antao et al., 2011 ; Beichman et al., 2018 ; Clark et al., 2023 ; Reid & Pinsky, 2022 ).…”

Estimation of effective number of breeders and effective population size in an abundant and heavily exploited marine teleost

“…The two genetic effective size parameters, N b (effective number of breeders) and N e (effective population size), are applicable in a fisheries management context (Hare et al, 2011). The parameter N b refers to the effective number of breeders in a single reproductive cycle, which provides important insight into eco-evolutionary processes taking place during reproduction (Waples, 1989(Waples, , 2024. This parameter is largely shaped by the number and size of families contributing to the sampled cohort, which is influenced by factors like adult density, mate choice, individual variation in fecundity and reproductive success, and habitat quality and quantity (Whiteley et al, 2015).…”