Figure 1.
Predictions of the tri-trophic interactions (TTI) hypothesis for the interactive effects of natural enemies, host-plant quality and diet breadth on herbivores.
Three well-studied hypotheses – the physiological efficiency (PE), enemy free space (EFS) hypotheses, and slow-growth/high-mortality (SGHM) – each address unique, pairwise combinations of these factors. The physiological efficiency (PE) hypothesis predicts specialists should outperform generalists on shared host plants (e.g. a>b), and that generalists should be more sensitive to variation in host-plant quality than specialists (e.g. a–c<b–d). The Enemy Free Space (EFS) hypothesis predicts natural enemies should have a stronger effect on dietary specialists than generalists (e.g. a–e<b–f). The Slow-Growth/High-Mortality (SGHM) hypothesis predicts low host-plant quality enhances the effects of natural enemies (e.g. b–f<d–h). The TTI hypothesis offers novel predictions for the three-way interaction among these factors: Dietary specialists (as compared to generalists) are predicted to escape natural enemies and be competitively dominant due to faster growth rates, and such differences should be greater on low quality (as compared to high quality) host plants. Such non-additive dynamics imply that predictions for the PE, EFS, and SGHM hypotheses are contingent upon the third, discounted factor. Natural enemies should mediate the predictions of the PE hypothesis, such that the differential effects of host-plant quality on specialists and generalists is greater in the presence of natural enemies (e–g≪f–h) than in the absence of natural enemies (a–c<b–d). Host-plant quality should mediate the predictions of the EFS hypothesis, such that the differential effects of natural enemies on specialist and generalist herbivores is greater on low-quality host plants (c–g≪d–h) than on high-quality host plants (a–e<b–f). Herbivore diet breadth should mediate the predictions of the SGHM hypothesis, such that SGHM dynamics are stronger for dietary generalist (b–d≪f–h) than specialist herbivores (a–c<e–g).
Table 1.
Descriptions of three long-standing hypotheses for plant-herbivore and herbivore-predator interactions and their relation to the tri-trophic interactions hypothesis.
Figure 2.
The dietary specialist herbivore Uroleucon macolai Blanchard (top) and the generalist Aphis gossypii Glover (bottom) feeding upon Baccharis salicifolia (Ruiz & Pav.) Pers.
Also shown are the predatory ladybird larvae (Coleoptera: Coccinelidae) (top) and the ant Linepithema humile (Mayr). Photo credits Kailen Mooney.
Figure 3.
Treatment means from the bi-trophic experiment.
Effects of intra- vs. inter-specific competition and genetically based variation in host plant Baccharis salicifolia quality on the fecundity of a generalist (Aphis gossypii) and a specialist (Uroleucon macolai) herbivore. The effects of genetically based variation in host plant quality are tested by comparing herbivore fecundity between male and female plants (panels A, B) and among 14 plant genotypes (panels C, D). For effects of plant sex, means (± 1SE) are shown for each competition treatment. For effects of plant genotype, overall means (± 1SE) are shown for each competition treatment, and means for individual genotypes (error bars omitted for clarity) are indicated with solid (male) or dashed (female) lines. See Table 2 for statistics.
Table 2.
Test statistics for the bi-trophic experiment.
Figure 4.
Treatment means from the tri-trophic experiment.
Effects of natural enemies and Baccharis salicifolia sex on the fecundity of a generalist, ant-tended herbivore (Aphis gossypii) and a specialist, untended herbivore (Uroleucon macolai). Least square means (±95% CI) for per capita daily growth are shown for each treatment controlling for initial aphid population size. See Table 3 for statistics.
Table 3.
Test statistics for the tri-trophic experiment.