Interspecific differences in milkweeds alter predator density and the strength of trophic cascades

Trophic cascades occur when predators benefit plants by consuming herbivores, but the overall strength of a trophic cascade depends upon the way species interactions propagate through a system. For example, plant resistance to, or tolerance of, herbivores reduces the potential magnitude of a trophic cascade. At the same time, plants can also affect predator foraging or consumption in ways that either increase or decrease the strength of trophic cascades. In this study, we investigated the effects of plant variation on cascade strength by manipulating predator access to aphid populations on two species of milkweed: the slower-growing, putatively more-defended Asclepias syriaca and the faster-growing, putatively less-defended Asclepias incarnata. Predatory insects increased plant growth and survival for both species, but the strength of these trophic cascades was greater on A. incarnata, which supported more aphid growth early in the season than did A. syriaca. More predators were observed per aphid on A. incarnata, and cage treatments generated significant patterns consistent with predator aggregation on A. incarnata, but not A. syriaca. Although predators strongly affected aphids, this effect did not differ consistently between milkweed species. Plant tolerance to herbivory may therefore be the primary driver of the difference in trophic cascade strength observed. Importantly, we observed that the timing of predator exclusion affected plant growth and survival differently, indicating that measures of “cascade strength” may change with phenology and plant physiological responses. Together, our results suggest a mechanism by which differences in resource allocation patterns could explain differences in growth, phenology, and cascade strength between species.