The role of biodiversity in the functioning of urban ecosystems is an emerging topic in ecology. Two negative pressures for urban plant communities are insect pests and disease. Research on grassland ecosystems has shown that plant species richness can affect insect herbivory and disease prevalence, but whether these same mechanisms apply in evolutionarily-novel urban plant-herbivore-pathogen assemblages is unknown. We tested the influence of urban garden plant species richness on herbivory by the Japanese Beetle (Popillia japonica), and disease prevalence by the Septoria leaf spot (Septoria lycopersici). We conducted our experiment at two research gardens in St. Paul, MN, which each had 32 4m2 plots. Seven crops were grown at each garden site. We used a 2x2 factorial design, manipulating plant species richness and pest management. Species richness was manipulated by half of the plots being in low diversity (2 species per plot) and half being in high diversity (7 species). The management treatment was applied to represent the varied response styles of people to pests and disease in urban gardens. In half of the plots, the beetles and infected leaves were removed weekly. Our response variables were yield, beetle abundance, beetle herbivory, and infection by the Septoria leaf spot disease.
Results/Conclusions
Contrary to results observed in native grassland ecosystems, yields were significantly higher for most crops (collard greens, kale, eggplant, and zucchini) in low-diversity treatments, possibly due to interspecies competition in the high-diversity plots. The pest management treatment resulted in higher yields for tomatoes (which were highly susceptible to Septoria infection), but not for other crops. Beetle herbivory and abundance were not explained by the experimental treatments. No significant interactions were observed between crop diversity and pest management. One possible explanation for the differential effects of management on Septoria compared to beetle herbivory is that the dispersal ability of beetles, but not Septoria, was high relative to the spatial scale of this manipulation. Our results suggest that the effects of diversity and top-down control in novel urban species assemblages may be different compared to those in intact native ecosystems.