The effects of plant genotype and soil fertility on tritrophic interactions have usually been investigated separately and the mechanisms underlying their effects are not always well understood. We studied the interactions among Ruellia nudiflora, a pre-dispersal seed predator, and its parasitoids using plant genotypes grown under contrasting soil conditions. By doing so, we examined if plant genotype and soil fertility, in controlling the abundance of a basal resource (fruit number), had effects of similar magnitude cascading up the food chain. Furthermore, we evaluated if these bottom-up forces acted independently or interactively and if their effects occurred through changes in abundance across trophic levels (density-mediated) or changes in the function of species interactions (interaction modification). We established a common garden with 14 plant genotypes, fertilized half of the plants per genotype, and recorded fruit number, seed predator number, and parasitoid number and identity per plant.
Results/Conclusions
Results indicated that the magnitude of plant genotype effects was greater than that of fertilization. Moreover, we did not find plant genetic variation for fertilization effects on fruit, seed predator or parasitoid abundance, showing that these bottom-up forces acted independently on plant resources and higher trophic levels. Both bottom-up forces were transmitted via density-mediated effects where fertilization increased fruit number, and in turn seed predator and parasitoid abundance, while genotypes producing more fruits had greater seed predator and parasitoid abundances. Nonetheless, only fertilization modified the function of seed predator-parasitoid interactions by causing a decrease in the number of parasitoids per seed predator relative to unfertilized plants. Finally, we found that plant genotype (but not fertilization) influenced parasitoid community structure. Together, these findings show that R. nudiflora genotype and soil fertility effects cascaded up the food chain by influencing the availability of resources for consumers, but that their effects acted independently and were transmitted via different mechanisms.