Key population processes are sometimes driven by male dynamics, but these drivers are often overlooked because of the scale over which they operate. Lions (Panthera leo) provide an ideal case study for investigating factors governing male dynamics and their influence on population sustainability. Lions display sexually selected infanticide, and resident males must defend their offspring from nomads that may have dispersed over long distances; factors affecting male-male competition over large spatial scales can have population wide consequences. We report here on the first systematic analysis of long-term individual-based data of male lions in the Serengeti National Park, Tanzania. From 1974-2012, we observed 471 coalitions (796 males) in our study area. We investigate factors affecting male immigration and the impacts on the resident population. The yearly number of nomadic males entering the study population affected cub survival and mating access. Success rates of nomadic males gaining tenure with a pride increased with age and coalition size. We observed a significant decline in male immigration, which resulted in lowered levels of male replacement in the study population, reduced infanticide, and greater cub survival. The decline in incoming males likely resulted from increased anthropogenic pressures in surrounding areas. Conversely, the core study population was largely buffered from anthropogenic threats and likely served as a source to neighboring sinks. Reduced infanticide in the core population might have compensated for rising lion mortalities in surrounding areas, but as human-wildlife conflicts intensify with the rapidly growing human population, compensatory mechanisms may become overwhelmed.