Plant mediation of tritrophic interactions

Background/Question/Methods

Trophic cascades are the best examples of the importance of indirect interactions as determinants of community structure and ecosystem function, and there has been great interest in elucidating the sources of variation both within and among ecosystems. It is recognized that plant traits (e.g. stoichiometry, antiherbivore defense) can mediate trophic cascade strength from the bottom up.  Nevertheless, it is as yet unclear what role intraspecific variation in heritable plant traits plays in mediating carnivore-herbivore interactions, or the potential consequences of such variation for the evolution of plant defense.  We investigated the interactions between natural enemies (parasitoids, carnivores) and two aphid herbivores occurring on males and females of the dioecious perennial, woody shrub Baccharis salicifolia. The two herbivores addressed in this study vary dramatically in their diet breadth, with Aphis gossypii feeding on an exceptionally broad range of host plants and Uroleocon macolai feeding only on Baccharis.  This study thus tested for sex-based genetic variation in B. salicifolia for resistance to herbivores of differing diet breadths, and whether plant genotype mediates enemy-herbivore interactions.

Results/Conclusions

In the absence of predators, the two herbivores differed in their performance, with the dietary specialist exhibiting higher fecundity than the generalist.  The herbivores also varied in their response to plant sex; where the dietary generalist demonstrated higher performance on males than females, the specialist was insensitive to sex-based variation in plant quality.  With respect to natural enemies, the densities of both herbivores were suppressed to a similar extent.  For generalist herbivores, the effects of natural enemies were mediated by plant sex, with enemies more strongly suppressing generalists on female (where fecundity was low) than male plants (where fecundity was high).  In contrast, for specialist herbivores, the effects of natural enemies were consistent on male and female plants.  These results thus suggest that genetically based variation in plant quality can mediate top-down suppression of herbivores, but that these dynamics are more important for generalist than specialist herbivores.