Explaining variation in human population density constitutes a basic research problem in human ecology and archaeological science. To contribute to this basic research problem, we build a graphic model and conduct a global analysis of the effects of ecological variables, controlling for technological differences, on human population density. Our results indicate that human population density displays a consistent relationship with ecological variables across productive technologies. Human population density peaks in high productivity environments with moderate levels of species richness and moderate to low levels of pathogens among hunter-gatherer, subsistence agricultural, and industrial societies. Population density is lower in low productivity environments as well as environments with high productivity, high species richness and high pathogen loads. The productive technologies associated with agriculture and industrial production impact human population density more than ecological variables, and shifts toward agriculture or industrial production may weaken the effect of net primary productivity on population density. These results illustrate the nature of the relationship between population density and ecological variables, which are partly driven by climate, and the results provide a basis for hypotheses that researchers can use to analyze the potential effects of climate change on material record estimates of human paleo-population density.