Mutualisms, interactions between two species that benefit both of them, have long captured the public imagination. Their influence transcends levels of biological organization from cells to populations, communities, and ecosystems. Mutualistic symbioses were crucial to the origin of eukaryotic cells, and perhaps to the invasion of land. Mutualisms occur in every terrestrial and aquatic habitat; indeed, ecologists now believe that almost every species on Earth is involved directly or indirectly in one or more of these interactions. Mutualisms are essential to the reproduction and survival of...