We thank members of the AACR research group for their help with data analysis and their thoughtful insights on the manuscript. We also thank Paula Lemons, Karen Pelletreau, Kate Semsar, and two anonymous reviewers for their feedback on this article. This work was funded through grants from the National Science Foundation (NSF; DUE 1022653, 1323022, 1322851, 1347578, and 1347626). Any opinions, findings, and conclusions or recommendations expressed in this article are those of the authors and do not necessarily reflect the views of the NSF.