Fungal pathogens as selective forces in plant populations and communities *

Abstract Pathogens are potent selective forces whose importance in shaping the size and structure of individual plant populations and whole communities has been underestimated. Even in situations where host and pathogen have been associated over long periods of time, pathogens regularly affect host fitness by reducing fecundity and increasing mortality either directly or indirectly through reductions in competitive ability. The genetic consequences of such disease‐induced reductions in fitness are profound. On a broad geographic scale, race‐specific resistance generally occurs more frequently in regions characterized by environments favourable for disease development. Within such areas, however, the distribution of resistant plant genotypes is often very patchy. This probably reflects the importance of extinction and colonization events in the continuing co‐evolutionary dynamics of host‐pathogen associations. At a demographic level, pathogen‐induced reductions in host fitness may lead to changes in the size of populations. In turn, this may lead to changes in the relative diversity of whole communities. Documentation of this scale of interaction is poor, but the devastating consequences of the introduction of pathogens into alien environments provides a salutary reminder of their power to change plant communities radically.