Consumer species richness generally increases community-wide resource consumption. However, less is known about the impacts of species relative abundance (evenness), and pathogens are rarely considered as consumers in biodiversity studies. We manipulated both species richness and evenness among a community of soil-dwelling insect pathogens, and measured resulting mortality rates among two hosts (the moth Galleria mellonella and the beetle Leptinotarsa decemlineata). Our pathogen species pool included 3 pathogenic nematodes (Steinernema carpocapsae, Steinernema feltiae, and Heterorhabditis megidis) and a fungus (Beauveria bassiana). We created communities comprised of 0, 1, 2, 3, or 4 pathogen species from this species pool, manipulating species richness within a substitutive (replacement series) design. We completed several trials testing the effects of species richness on moth mortality in two different soil types, followed by trials where we compared moth mortality when facing even versus uneven pathogen communities, across species richness levels, in one soil type. Finally, we compared beetle mortality when facing single pathogen species versus diverse (4 species) pathogen communities.
Results/Conclusions
Moth mortality increased with pathogen species richness in soils of both types. Because the relationship with species richness did not differ across soil type, experiments examining the combined effects of consumer richness and evenness were conducted using only one type of soil. Moth mortality did not significantly increase with greater evenness within the pathogen community (even: 75% mortality; uneven: 70% mortality). However, species richness effects were robust: mortality increased with greater species richness in both even and uneven communities. The pathogens B. bassiana and S. carpocapsae were the most effective pathogens; species identity effects may have limited the importance of community evenness. Beetle mortality was on average greater when exposed to a diverse pathogen community (67% mortality) than single pathogen species (36% mortality). Thus, we found consistent evidence for strong effects of pathogen species richness, but not evenness, on host mortality rates.
