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

Abstract

We provided a theoretical basis for integrating the effective number of breeders (Nb) into the stochastic dynamics of the number of adults (N) for iteroparous species based on individual relationships between adult females and their offspring. Recruitment variation was partitioned into i) parental, ii) nonparental, and iii) environmental processes (including density-dependent effects). The first two processes, summarized byNb, magnified the contribution of the last process to recruitment variation. Assuming that there were no sex differences and an equal sex ratio, the relationship betweenNband the factor amplifying recruitment variation whenNb/N<0.1 was approximated to provide quantitative guidance for using onlyNb. For example,Nb<42 would have a non-negligible impact on population stochasticity in terms of amplifying the environmental impact by over 5%. The advantage of assessing recruitment variation usingNbalone is that only genetic information on samples from the same cohort is sufficient. Thus, the proposed method is applicable in data-limited situations (e.g. endangered species), where there is typically insufficient information on life-history parameters or current age/size structure. The study findings reveal the usefulness ofNbin the field of conservation biology and wildlife management.