Interspecific variation in plants alters the strength of trophic cascades

Background/Question/Methods

Plant traits may alter the magnitude of trophic cascades in a variety of ways.  Both plant resistance and plant tolerance to herbivory limit the potential for strong trophic cascades. Furthermore, plant traits may affect predator foraging or consumption rates in ways that could either increase or decrease the strength of trophic cascades.  To assess the impact of these various pathways on species-level cascades, we manipulated predator access to aphid populations on two different species of milkweed, the slower-growing and putatively more-defended Asclepias syriaca and the faster-growing, putatively less-defended Asclepias incarnata.  We assessed resistance to herbivores as the density of aphids on plants when predators were excluded, and tolerance as the effect of aphid density on plant growth.  To evaluate the effect of plant species on predation, we calculated per-aphid predator densities on each plant.  We measured the strength of trophic cascades by comparing the growth and survival of plants exposed to aphids in the presence and absence of aphid predators.

Results/Conclusions

We find that predator exclusion generates trophic cascades that persist across years, particularly on A. incarnata.  Cage treatments effectively excluded predators, resulting in increased aphid density and patterns consistent with predator aggregation when aphids are abundant.  Although per-plant aphid densities in the absence of predators were higher on the faster growing A. incarnata, aphids accumulate at a faster per-leaf rate on A. syriaca, suggesting that direct resistance to the specialist aphid does not explain variation in cascade strength.  However, per-leaf aphid density more strongly reduced the growth of A. incarnata compared to A. syriaca.  Furthermore, per-aphid predator densities were higher on A. incarnata.  Together, these results show that lower tolerance and higher predation may both contribute to the strong trophic cascades observed for A. incarnata in comparison with A. syriaca.