1. The long-term ecological impact of pathogens on group-living, large mammal populations is largely unknown. We evaluated the impact of a pathogenic bacterium, Streptococcus equi ruminatorum, and other key ecological factors on the dynamics of the spotted hyena Crocuta crocuta population in the Ngorongoro Crater, Tanzania.
2. We compared key demographic parameters during two years when external signs of bacterial infection were prevalent (‘outbreak’) and periods of five years before and after the outbreak when such signs were absent or rare. We also tested for density dependence and calculated the basic reproductive rate R0 of the bacterium.
3. During the five pre-outbreak years, the mean annual hyena mortality rate was 0·088, and annual population growth was relatively high (13·6%). During the outbreak, mortality increased by 78% to a rate of 0·156, resulting in an annual population decline of 4·3%. After the outbreak, population size increased moderately (5·1%) during the first three post-outbreak years before resuming a growth similar to pre-outbreak levels (13·9%). We found no evidence that these demographic changes were driven by density dependence or other ecological factors.
4. Most hyenas showed signs of infection when prey abundance in their territory was low. During the outbreak, mortality increased among adult males and yearlings, but not among adult females – the socially dominant group members. These results suggest that infection and mortality were modulated by factors linked to low social status and poor nutrition. During the outbreak, we estimated R0 for the bacterium to be 2·7, indicating relatively fast transmission.
5. Our results suggest that the short-term ‘top–down’ impact of S. equi ruminatorum during the outbreak was driven by ‘bottom–up’ effects on nutritionally disadvantaged age–sex classes, whereas the longer-term post-outbreak reduction in population growth was caused by poor survival of juveniles during the outbreak and subsequent poor recruitment of breeding females. These results suggest synergistic effects of ‘bottom–up’ and ‘top–down’ processes on host population dynamics.