Decomposing demographic contributions to the effective population size with moose as a case study

Levels of random genetic drift are influenced by demographic factors, such as mating system, sex ratio and age structure. The effective population size (Ne) is a useful measure for quantifying genetic drift. Evaluating relative contributions of different demographic factors to Ne is therefore important to identify what makes a population vulnerable to loss of genetic variation. Until recently, models for estimating Ne have required many simplifying assumptions, making them unsuitable for this task. Here, using data from a small, harvested moose population, we demonstrate the use of a stochastic demographic framework allowing for fluctuations in both population size and age distribution to estimate and decompose the total demographic variance and hence the ratio of effective to total population size (Ne/N) into components originating from sex, age, survival and reproduction. We not only show which components contribute most to Ne/N currently, but also which components have the greatest potential for changing Ne/N. In this relatively long-lived polygynous system we show that Ne/N is most sensitive to the demographic variance of older males, and that both reproductive autocorrelations (i.e., a tendency for the same individuals to be successful several years in a row) and covariance between survival and reproduction contribute to decreasing Ne/N (increasing genetic drift). These conditions are common in nature and can be caused by common hunting strategies. Thus, the framework presented here has great potential to increase our understanding of the demographic processes that contribute to genetic drift and viability of populations, and to inform management decisions.
Methods
Since 1992, all new calves that survived the annual hunt during autumn have been radio/ GPS collared and measured annually during winter (except in 2003 and 2008). In addition, female moose have been observed around potential parturition dates and recorded with/without calves. Sex, age and tissue samples have been collected

from nearly all moose harvested on the island.

With the collected tissue samples, Haanes et al. (2013) constructed a nearly complete genetic pedigree of the population based on 22 microsatellite loci. This parent assignment enabled the number of offspring to be genetically determined for both sexes. The data include individual histories of 207 females and 240 males from 1984 to 2011, where 111 females and 137 males were alive at the start of their first potential mating season (1.5 years old).