Accurate estimation of effective population size is important in attem
pts to conserve small populations of animals or plants. We review the
genetic and ecological methods that have been used to estimate effecti
ve population size in the past and suggest that, while genetic methods
may often be appropriate for the estimation of N. and its monitoring
ecological methods have the advantage of providing data that can help
predict the effect of a changed environment on N(e). Estimation of N(e
) is particularly complex in populations with overlapping generations,
and we summarize previous empirical estimates of N(e) that used ecolo
gical methods in such populations. Since it is often difficult to asse
ss what parameters and assumptions have been used in previous calculat
ions, we suggest a method that provides a good estimate of N(e), makes
clear what assumptions are involved, and yet requires a minimum of in
formation. The method is used to analyze data from 14 studies. In 36%
(5) of these studies, our estimate is in excellent agreement with the
original, and yet we use significantly less information; in 21% (3) th
e original estimate is markedly lower, in 43% (6) it is markedly highe
r. Reasons for the discrepancies are suggested. Two of the underestima
tes involve a failure in the original to account for a long maturation
time, and four of the overestimates involve problems in the original
with the correction for overlapping generations.