We test whether high moose density results in smaller moose, slower growth rates, lower reproductive rates, and more variable year-to-year population size by comparing demographic characteristics of 15 Canadian moose Alces alces populations that spanned a range of population density (0.08–4.5 moose/ km²). Density negatively affected growth rate, reproductive rates and recruitment. We argue that primary productivity, measured as percent forest cover, and natural predation link density to reproduction in moose. Populations that lived in greater forest cover and experienced greater natural predation were associated with more predictable year-to-year variation in population size. In contrast, moose populations living in areas of low forest cover and low natural predation experienced greater density independent food limitation and greater unpredictability in population size. Thus, moose populations living in areas of low primary productivity and low natural predation show less persistence and require greater conservation efforts.