Geographic information systems are often used to quantify landscape patterns. However, most current approaches for quantifying patterns at landscape scales use categorical maps and fail to recognize heterogeneity between and within patches of habitat. We develop a method to calculate fragmentation of habitat that explicitly handles gradients of patch quality and varying impedance to movement among habitat patches. Our approach characterizes patches based on functional properties of organisms including home range area and dispersal distance. Using this approach we identify "keystone" patches, areas that are particularly important to maintaining viability of populations using the landscape.