How to get the Ne/Nc ratio

A taxon-specific ratio can be obtained in one of several ways:

1- from recent reviews of the literature that have compiled average values for groups such as mammals, bony fish, annual plants, trees, etc. (see Hoban et al 2020), 2- from formulas that take into account a species’ biological characteristics (especially the male-female sex ratio and the variance in offspring production), or 3- from published literature on the species or even populations that are the focus of study. For instance, the ratio in large-bodied mammals and in some trees is often closer to 0.3 (thus the minimum Nc would be 1500).

These are all valid ways of obtaining the ratio.

When the Ne/Nc ratio CAN’T be used

There will be cases when the standard Ne/Nc approximation should not be used:

• If the population is known to have undergone a severe bottleneck which is followed by a population expansion, data need to be carefully interpreted, as census size does not reflect demographic changes in the past.

• In some species clonal variants exist, which will then present no individuals and Ne cannot be inferred correctly.

• Additional examples could be ex-situ populations with often large changes of historic populations and a mixture of origin.

For example, Agave plants can reproduce both sexually and asexually (clones). In the wild, mature adult individuals are easily distinguishable from their clones, which grow below the original plant. However, as several species of Agave are used to produce mezcal (traditional Mexican alcoholic beverage) it is common for farmers to increase the population using clones, either in plantations or in managed natural ecosystems. As a result, the Nc increases, but the Ne does not. Therefore, the Ne:Nc ratio can not be applied when this form of management exists.

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