The cumulative impacts of developing up to 24 coal tracts through four leasing alternatives in northwest Colorado and southwest and south-central Wyoming are analyzed. The four leasing alternatives propose offering for lease 101, 495, 759, or 991 million tons of coal recoverable by surface, subsurface, or in situ gasification mining methods. The significant, and unquantifiable but potentially significant, impacts to human biological resources are analyzed by considering the impacts that existing and future growth and development in the region could have (baseline), together with the impacts of the four leasing alternatives. Impacts increase in number and magnitude as the amount of coal to be developed increases through the alternatives. However, the number and magnitude of impacts can be influenced by which of the 24 tracts are included in a particular alternative, except for the maximum leasing alternative, which includes all tracts to be leased.