CANCELLED - Mycorrhizal interactions and competition with invasive mustard affect arthropod communities on native Deinandra fasciculata

Background/Question/Methods

The structure of arthropod communities on plants may be directly affected by plant-traits, but may also be indirectly affected by microbial associates such as mycorrhizal fungi. Plant neighbors in turn have the potential to further impact arthropod communities if they disrupt interactions with mycorrhizal fungi. We sought to understand how arthropod communities on the California native plant Deinandra fasciculata are affected by plant interactions with mycorrhizal fungi (AMF) in coastal sage scrub communities of Southern California. Additionally, D. fasciculata co-occurs with the exotic plant Brassica nigra which is known to produce allelochemicals that lower mycorrhizal colonization in competitors. Consequently, we also investigated whether the belowground effects of B. nigra mediate aboveground-belowground linkages in this system. In a common garden experiment, we grew D. fasciculata individuals from seed in treatments with and without fungicide and with and without B. nigra in a two-by-two factorial design. Plants were grown from January to August 2010. At peak insect and plant biomass, we destructively sampled the entire insect community, harvested D. fasciculata aboveground biomass and collected root samples to determine percent colonization of Mycorrhizal Fungi (AMF).

Results/Conclusions

Treatment with fungicide lowered D. fasciculata biomass and mycorrhizal colonization, and indirectly affected the associated arthropod community. The indirect effect of mycorrhizae varied among insect feeding guild, with sap-feeding insects being the most strongly influenced. B. nigra lowered arthropod diversity and increased evenness and abundance, in part due to a large increase in thrips density. Additionally, B. nigra had a negative effect on D. fasciculata biomass. These effects of B. nigra did not however, appear to be mediated by effects on mycorrhizal interactions, as B. nigra did not lower the percent colonization by AMF in D. fasciculata roots. In summary, we determined that both mycorrhizal symbionts and plant competitors have strong effects on plant performance and arthropod community composition, but these effects appear to operate through separate, complementary mechanisms.