Genus Communities as a Systematic Tool in the Study of New World Botrychium (Ophioglossaceae)

Summary Problems in the taxonomy of Botrychium (Ophioglossaceae) are outlined and discussed. Species interpretation has been controversial because many taxa are little known, poorly collected, highly variable, and subtly differentiated. A method of study based upon the tendency of the species to co‐exist with each other in the same habitats is discussed, and examples from our field work are given. The “Genus Community Method” and the related “Mutual Associates Method” allow critical comparisons to be made. Cohabitation of two or more species without successful interbreeding demonstrates biological discreteness and confirms that the character differences are most likely genetically fixed. A summary of our knowledge of what pairs of species have been found to co‐exist is given for subg. Sceptridium and subg. Botrychium. For the former, the major associations occur in eastern North America and tropical America north of Panama, including the Antilles. For the latter, the major associations occur in western North America. As many as seven species may grow together in one habitat. We conclude that the genus may not be as difficult taxonomically as previously believed. By discovering and analyzing genus communities of Botrychium, we can show which taxa are consistently and persistently distinct.