Sustainable wildlife management is becoming increasingly difficult in the face of expanding anthropogenic threats. Mitigation planning requires a solid mechanistic understanding of population drivers. Measuring population processes is especially challenging for species where demographic factors operate at large spatial or temporal scales. Key population processes are sometimes driven by male dynamics, but these drivers are often overlooked because of the scale over which they operate. African lions (Panthera leo) provide an ideal case study for investigations of the factors governing male dynamics and the influence on population stability or sustainability. Lions display sexually selected infanticide, and resident males must defend their offspring from nomads that may have dispersed over long distances. Thus, 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 nomadic male lions in the Serengeti National Park, Tanzania. We recorded all nomadic and resident male coalitions observed within the study area from 1974-2012. We used general linear models to examine factors affecting male immigration and their impacts on the resident population. We apply this mechanistic understanding to principles underpinning conservation planning.
From 1974-2012, a total of 471 coalitions (796 males) entered the study area, with a median annual immigration rate of 12.4 coalitions. The best model of nomad immigration included the explanatory factors: immigration year and SOI in the year prior to immigration. SOI in the previous year reduced immigration in the current year. We observed a significant decline in male immigration into the study area, which resulted in lowered levels of male replacement in the study population, reduced infanticide, and increased cub survival; the number of nomadic males entering the study population each year significantly affected cub survival and male mating access. Success rates of nomadic males increased with age and coalition size. The decline in incoming males likely resulted from increased anthropogenic pressures in surrounding areas. Conversely, we observed an increase in the core population over the study period; 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 population in Africa, this compensatory mechanism may become overwhelmed.