Ignitions, fuels, topography, and climate interact through time to create temporal and spatial differences in the frequency of fire, which, in turn, affects ecosystem structure and function. In many ecosystems non-human ignitions are overwhelmed by anthropogenic ignitions. Human population density, culture, and topographic factors are quantitatively related to fire regimes and the long-term pattern in fire frequency and species composition. These factors can be quantitatively related and used to reconstruct and predict the frequency of fire in ecosystems and to identify changing factors involved in anthropogenic fire regimes. Quantitative fire histories from oak-pine sites in Arkansas, Indiana, Missouri, and Ontario are used to examine patterns of interaction in fuels, ignitions, and topography over a period of 300 years. Fire regimes and fire frequencies are associated with the abundance of many species of reptiles, birds, fungi, and plants. Human popoulation density and topographic roughness are master variables in understanding temporal and spatial differences in fire regimes and their effects on ecosystems.