Professor University of California, Davis, United States
Background/Question/Methods
Spatial variation in the presence and abundance of species is a ubiquitous pattern across natural landscapes, with consequences for ecological processes across scales. Variation in soil characteristics is a key source of such heterogeneity, with strong effects on the traits and species composition of plant communities that, in turn, form the base of terrestrial food webs. Here, we ask whether soil type also shapes distinct identities and strengths of key biotic interactions. Linking classic theories of resource availability with contemporary hypotheses of tri-trophic interactions, we measure strength of top-town effects imposed by two guilds of natural enemies (parasitoid wasps and flies; birds) on a diverse and abundant community of herbivorous insects (caterpillars), distributed across a natural mosaic of harsh (serpentine) and benign (non-serpentine) soils. We also measure host plant resistance, and patterns of herbivore abundance and diversity across soil patches. We hypothesized that host plants on serpentine should be lower quality to herbivores due to greater investment in defense and/or lower nutrition, resulting in slower growth and/or lower densities of caterpillars. This could, in turn, result in greater attack rates on serpentine caterpillars if enemies benefit from their prolonged growth; or lower rates of attack if enemy attack is density-dependent.
Results/Conclusions
We find that soil type mediates strength of top-down effects on caterpillars – and that enemy guilds act in different ways. Specifically, bird predation was higher on non-serpentine soil, while parasitism rates were greater on serpentine soil. We also find an important role for caterpillar diet breadth in mediating strength of top-down effects, consistent with tri-trophic interaction theory: parasitism was greater, and soil effects stronger, for dietary specialist caterpillar species compared to generalists. We also find that serpentine host plants have negative effects on caterpillar growth and survival in the lab, and that caterpillar abundances and diversities are lower on serpentine host plants in the field. Putting these findings together, we conclude that bird predation counters the positive bottom-up effect of improved plant quality on caterpillars in non-serpentine soils, while parasitoids reinforce negative effects of low plant quality on caterpillars in serpentine soil. Thus, soil type has cascading effects on the architecture of multi-trophic species interactions, and may be key to understanding how bottom-up and top-down forces jointly shape abundance and composition of insect herbivore communities across heterogeneous landscapes.
