We present a Monte Carlo simulation, MANAGEDPOP, to project the loss of allelic diversity in a population with overlapping generations supported (or invaded) by a prodigious subpopulation. Input parameters allow the user to account for complex life histories and critical management practices, such as the frequency at which supportive breeding stocks are replaced. The simulation could also be used to examine the threat of species or population level extinction via hybridization. MANAGEDPOP merges theoretical formulations on the effective size of supported populations and of populations with overlapping generations using easily measured life history traits.