Unequal sex ratio

$$N_e = {4N_fN_m \over {N_f + N_m}}$$

If there is only one reproductive male, the greatest the effective size can ever be is 4. Management implication - Unequal sex ratios can dramatically increase the susceptibility of a population to random genetic changes.

Example: one reproductive male mates with 10 reproductive females:

$$N_e = \frac{4(10)(1)}{(10 + 1)} = \frac{40}{11} \approx 3.6$$

Another example: ten reproductive males and 100 reproductive females:

$$N_e = \frac{4(100)(10)}{(100+10)} = \frac{4000}{110} \approx 36.3$$