Ecology, 92(8), 2011, pp. 1605–1615 Ó 2011 by the Ecological Society of America Genotypic richness and phenotypic dissimilarity enhance population performance JACINTHA ELLERS, 1 STEFANIE ROG,CISKA BRAAM, AND MATTY P. BERG Department of Ecological Sciences, VU University Amsterdam, The Netherlands Abstract. Increases in biodiversity can result from an increase in species richness, as well as from a higher genetic diversity within species. Intraspecific genetic diversity, measured as the number of genotypes, can enhance plant primary productivity and have cascading effects at higher trophic levels, such as an increase in herbivore and predator richness. The positive effects of genotypic mixtures are not only determined by additive effects, but also by interactions among genotypes, such as facilitation or inhibition. However, so far there has been no effort to predict the extent of such effects. In this study, we address the question of whether the magnitude of the effect of genotype number on population performance can be explained by the extent of dissimilarity in key traits among genotypes in a mixture. We examine the relative contribution of genotype number and phenotypic dissimilarity among genotypes to population performance of the soil arthropod, Orchesella cincta. Nearly homogeneous genotypes were created from inbred isofemale lines. Phenotypic dissimilarity among genotypes was assessed in terms of three life-history traits that are associated with population growth rate, i.e., egg size, egg development time, and juvenile growth rate. A microcosm experiment with genotype mixtures consisting of one, two, four, and eight genotypes, showed that genotypic richness strongly increased population size and biomass production and was associated with greater net diversity effects. Most importantly, there was a positive log-linear relationship between phenotypic dissimilarity in a mixture and the net diversity effects for juvenile population size and total biomass. In other words, the degree of phenotypic dissimilarity among genotypes determined the magnitude of the genotypic richness effect, although this relationship leveled off at higher values of phenotypic dissimilarity. Although the exact mechanisms responsible for these effects are currently unknown, similar advantages of trait dissimilarity have been found among species. Hence, to better understand population performance, genotype number and phenotypic dissimilarity should be considered collectively. Key words: biomass production; egg development time; egg size; genetic diversity; juvenile growth rate; net diversity effect; Orchesella cincta; phenotypic trait; population size. INTRODUCTION The relationship between biodiversity and community and ecosystem functioning is a central issue in ecology, because biodiversity can affect primary productivity, carbon storage, nutrient acquisition, decomposition rate, and ecosystem stability (Hector et al. 1999, Mulder et al. 2001, Ha¨ttenschwiler et al. 2005). Increases in biodiversity can result from an increase in species richness and functional diversity, as well as from a higher genetic diversity within species. The role of intraspecific genetic diversity in biodiversity has received growing attention over the past years because of the emerging synthesis of evolutionary biology and com- munity ecology (Johnson and Stinchcombe 2007). In parallel to the community effects of species richness, a large body of experimental work has now shown several community effects of increasing genetic diversity within species. First, a positive relationship was found between the number of genotypes and net plant primary productivity (Reusch et al. 2005, Crutsinger et al. 2006, Kotowska et al. 2010; but see Vellend et al. 2010), which is key to supporting a larger number and diversity of herbivores and predators. Second, the number of genotypes also has indirect community effects, such as an increase in coexistence of competing plant species (Booth and Grime 2003), arthropod richness (Crutsinger et al. 2006, Johnson et al. 2006), colonization success (Gamfeldt et al. 2005, Crawford and Whitney 2010), and resistance to invasion (De Meester et al. 2007). Hence, in addition to the well- known evolutionary impacts, these studies have docu- mented the ecological impacts of intraspecific genetic diversity (reviewed in Hughes et al. 2008). More detailed studies have proposed that the positive effect of genotypic mixtures is not only determined by properties and frequencies of constituent genotypes (additive effects) but also by interactions among genotypes (nonadditive effects; Reusch et al. 2005, Manuscript received 29 October 2010; revised 8 March 2011; accepted 9 March 2011. Corresponding Editor: N. J. Sanders. 1 E-mail: jacintha.ellers@falw.vu.nl 1605