LETTER Genetic diversity of Daphnia magna populations enhances resistance to parasites Florian Altermatt 1,2 * and Dieter Ebert 1,2 1 Zoologisches Institut, Universita ¨ t Basel, Vesalgasse 1, CH-4051 Basel, Switzerland 2 Tva ¨ rminne Zoological Station, SF-10900 Hanko, Finland *Correspondence: E-mail: florian.altermatt@unibas.ch Abstract The diversity-disease hypothesis states that decreased genetic diversity in host populations increases the incidence of diseases caused by pathogens (= monoculture effect) and eventually influences ecosystem functioning. The monoculture effect is well- known from crop studies and may be partially specific to the artificial situation in agriculture. The effect received little attention in animal populations of different diversities. Compared with plants, animals are mobile and exhibiting social interactions. We followed the spread of a microsporidian parasite in semi-natural outdoor Daphnia magna populations of low and high genetic diversity. We used randomly selected, naturally occurring host genotypes. Host populations of low diversity were initially monoclonal, while the host populations of high diversity started with 10 genotypes per replicate. We found that the parasite spread significantly better in host populations of low diversity compared with host populations of high diversity, independent of parasite diversity. The difference was visible over a 3-year period. Host genotypic diversity did not affect host population density. Our experiment demonstrated a monoculture effect in independently replicated semi-natural zooplankton populations, indicating that the monoculture effect may be relevant beyond agriculture. Keywords Daphnia magna, density, epidemic, epidemiology, genotypic diversity, metapopulation, microsporidium, Octosporea bayeri, prevalence. Ecology Letters (2008) 11: 918–928 INTRODUCTION The diversity of species or genotypes influences the functioning and dynamics of natural ecosystems (Loreau et al. 2001). Several studies found a positive correlation between a higher diversity of species or functional groups and productivity and density within plant communities (Tilman et al. 1996; Hector et al. 1999; Engelhardt & Ritchie 2001). A higher genotypic diversity within species produces the same effect (Hughes & Stachowicz 2004; Reusch et al. 2005). Furthermore, a high diversity increases resilience, and more diverse ecosystems resist better to negative influences and recover faster after disturbances (Reusch et al. 2005). Parasites pose such a negative influence (Anderson & May 1979; Poulin 1998), and are therefore an important factor to consider when studying effects of host genetic diversity. Parasites are of general importance in all natural systems, and epidemics can lead to population extinctions (Anderson & May 1979; Weisser 2000; Ebert 2005; Pounds et al. 2006; Rauch & Weisser 2006). Parasites and parasitoids can reduce host density, host growth or productivity and may thus affect ecosystem functioning (Anderson & May 1979; Hudson et al. 1998; Ebert 2005; Pounds et al. 2006). However, the diversity of a system itself may influence parasites as well (Elton 1958; van der Plank 1963; Browning & Frey 1969; Leonard 1969; Garrett & Mundt 1999; Keesing et al. 2006). The question of how host genetic diversity influences parasite spread is the topic of this study. The interest of genotypic or species diversity affecting the spread of parasites originated in agricultural research (Elton 1958; Leonard 1969). The rapid and devastating spread of diseases in agricultural monocultures is a well-known phenomenon (e.g. potato blight or rice blast Zhu et al. 2000; Pilet et al. 2006). The diversity-disease hypothesis states that decreased genetic diversity of hosts increases the incidence of diseases caused by specialist pathogens, a phenomenon called the monoculture effect (Elton 1958; van der Plank 1963; Browning & Frey 1969; Leonard 1969; Ecology Letters, (2008) 11: 918–928 doi: 10.1111/j.1461-0248.2008.01203.x Ó 2008 Blackwell Publishing Ltd/CNRS