Lion (Panthera leo) populations on small reserves (<1000 km²) require more intensive management than those in larger conservation areas. ‘Surplus’ animals are a major management concern, resulting from smaller inter-birth intervals, younger ages of first reproduction and higher survival rates. In the 1990s and early 2000s, this surplus was translocated to newly formed reserves. More recently, the proliferation of new reserves has slowed, often forcing managers to cull surplus animals. The Lion Management Forum has proposed several interventions to reduce the number of surplus lions by mimicking natural processes. These include the use of chemical contraception to increase the age of first reproduction and/or inter-birth intervals to match those found in the Kruger National Park (NP) lions; or ‘tube-tying’ to reduce litter size to match that found in the Serengeti NP. We developed a population simulation model, GrowLS, to evaluate the efficacy of these interventions. GrowLS predicts that alterations to inter-birth intervals or litter size would achieve the largest reduction in surplus animals. Alterations to the age of first reproduction in lionesses had almost no effect. A combined approach would result in an undesirable near-zero population growth. Additional features of GrowLS are its ability to predict population size and the number of breeding lionesses and cubs per year. GrowLS has applications in the management of other species for which the required input parameters are known. GrowLS will serve as an invaluable tool for managers wanting to control population size by manipulating female reproductive rates.