Red Queen communities.

Abstract

SummaryWe extend the Red Queen hypothesis, a mechanism proposed to maintain ge ne tic diversity at the population level, to explain diversity at the community level. The Red Queen hypothesis assumes that pathogens become specialized on common host genotypes and reduce their fi tness, thereby favoring ge ne tic mechanisms generating rare genotypes. Here we develop the idea that pathogens favor diversity in communities by an analogous mechanism of frequency- dependent selection against common species. Empirical evidence from a variety of studies suggests that host-specifi c pathogens are more likely to attack and reduce population sizes of common species relative to rare species. Pathogens could therefore counteract competitive exclusion and maintain species diversity in com-munities. We also postulate an indirect relationship between pathogens and ecosystem function because fi eld and laboratory studies suggest a link between species diversity and improved functioning of ecosystems.IntroductionThe interaction between pathogens and plants has traditionally been the purview of agricultural plant pathologists, but because crop species are typically grown in monocultures, there has been little consideration of the role of pathogens in multispecies plant communities. However, ecol-ogists have become increasingly appreciative of the potential role of pathogens in natural communities and ecosystems (Burdon 1982; Holah and Alexander 1999; Jarosz and Davelos 1995; Mitchell et al. 2002; van der Kamp 1991; Van der Putten et al. 1993). Over the past several de-cades a series of related hypotheses and concepts on frequency, density, and distance dependence of disease and their effects on plant diversity have been developed (Bever 1994; Burdon and Chilvers 1982; Connell 1971; Gilbert et al. 1994; Hansen and Goheen 2000; Janzen 1970; Van der Putten et al. 1993). A pathogen that causes the local extinction of its host species will reduce diversity by reducing species richness, but more