The practice of supporting weak, wild populations through release of i
ndividuals bred in captivity is becoming an increasingly important con
servation measure. A frequently recommended form of such breeding-rele
ase activity refers to supportive breeding: a fraction of the target p
opulation is brought into captivity for reproduction, and the resultin
g progeny are released to mix with the wild segment of The Population.
We derived an expression for the variance effective size of a populat
ion managed through supportive breeding and discuss its relationship t
o previously published equations that are based on the assumption of r
andom mating. We show that the effect of supportive breeding may be qu
ite different on the inbreeding and the variance effective sizes. Wher
eas supportive breeding always results in a reduction of the inbreedin
g effective number the variance effective number may either decrease i
ncrease, or remain unchanged. We discuss these observations in relatio
n to conservation management and suggest some general guidelines for s
upportive breeding situations. Our recommendations include making a di
stinction between inbreeding and variance effective numbers; taking pa
rticular care when dealing with organisms with high reproductive poten
tial; assuring that the amount of drift be no larger than it would be
without supportive breeding; and focusing primarily on the variance ef
fective size of a population-that is, on the effective number directly
related to the rate of loss of gene diversity.