I am indebted to many people who have helped this project become reality.  I would especially like to thank my wife Sharon and my boys, Isaac and Owen, who had to endure my long hours of absence (or at least absentmindedness) while in the field and in front of the computer, and who more recently have accompanied me in the field; my father for his unbridled enthusiasm for doing field science (wherever he happened to be); my mother for setting me on this course of discovery long ago with all of those amazing family trips to the national parks and monuments of the western U.S. and Canada; and my students for their curiosity and motivation.