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The Influence of Age Structure and Fecundity on Effective Population Size

Leonard Nunney


Simple formulae are developed which define the effective size (N$_{\text{e}}$) of populations with overlapping generations, and their use is illustrated using data from a squirrel population. Two mating systems are considered, the random union of gametes and monogamy, in combination with age-independent fecundity. In the simplest case of age-independent (type 2) survivorship in a population of N adults, N$_{\text{e}}$ = N/(2-T$^{-1}$) where T is the generation time. As T increases, N$_{\text{e}}$ declines asymptotically to N/2. A generalization of this result (N$_{\text{e}}$ = N/[1 + k$^{-1}$-T$^{-1}$], where k influences survivorship) shows that given type 1 survivorship (k > 1) this decline in N$_{\text{e}}$ is less severe. A biased sex ratio results in N$_{\text{e}}$ differing between the two mating systems; however, in both systems, a sex ratio bias resulting from survival differences has much less influence on N$_{\text{e}}$ than a sex ratio bias resulting from recruitment differences. Low fecundity can increase N$_{\text{e}}$, but realistic levels of variation among breeding individuals (Poisson or greater) negate the effect. The effect on N$_{\text{e}}$ of variation resulting from the presence of non-breeders is also considered.

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