Ecologists no longer debate whether top-down or bottom-up factors are responsible for suppressing herbivore populations. A more nuanced approach now focuses on asking how variation in abiotic factors and community composition affects the relative importance of top-down and bottom-up control across space. Across latitudes in food webs associated with Iva frutescens in Atlantic Coast salt marshes, aphid densities are similar, but grasshoppers, beetles, gall-making midges and mites, and an omnivorous crab are several-fold more abundant at low- versus high-latitudes. We used a combination of predation trials and a mesocosm experiment to test whether latitudinal variation in plant quality (higher at high latitudes), consumption by omnivores (present only at low latitudes), consumption by mesopredators (present at all latitudes) or herbivore ontogeny could explain geographic variation in field patterns of herbivore density.
Results/Conclusions
In our experiments, the omnivorous crab exerted strong top-down control on beetles, ladybugs exerted strong top-down control on aphids, and both consumers benefited plants through trophic cascades. In contrast, plant origin (high versus low latitude) had very small effects on consumers. Beetle density was greatest and aphid density was reduced when mesocosms were stocked with adult beetles, suggesting competitive interactions between the two herbivores. We conclude that, for the Iva frutescens food web, latitudinal variation in plant quality is less important than latitudinal variation in the presence of top consumers in mediating food web structure, and we hypothesize that climate plays a strong role in structuring high-latitude food webs by limiting the number of herbivore generations per growing season and by causing high overwintering mortality.