In many urban areas watersheds have been sublimated to underground sewer systems (blue). Animal habitat has been marginalized to residual and irregular areas of remaining dense brush and trees (green). Human residential areas of single-family homes tend to provide small animal habitats in the centers of blocks (dark gray). The business district represents the greatest control of the ecology, having fully straightened the waterways, and removed all animal habitat (light gray). Railways follow the ancient waterways, trespassing on the regularity of the urban grid and providing connected pockets of animal habitat (red).