Abstract

In their recent contribution, Wetzel et al. [Wetzel et al. (2016) Variability in plant nutrients reduces insect herbivore performance. Nature 539: 425-427] predict that variance in the plant nutrient level reduces herbivore performance via the nonlinear averaging effect (named Jensen’s effect by the authors) while variance in the defense level does not. We argue that the study likely underestimates the potential of plant defenses’ variance to cause Jensen’s effects for two reasons. First, this conclusion is based on the finding that the average Jensen’s effect of various defense traits on various herbivores is zero which does not imply that the Jensen’s effect of specific defense traits on specific herbivores is null, just that the effects balance each other globally. Second, the study neglects the nonlinearity effects that may arise from the synergy between nutritive and defense traits or between co-occurring defenses on herbivore performance. Covariance between interacting plant defense traits, or between plant nutritive and defense traits, can affect performance differently than would nutritive or single plant defense variance alone. Overlooking the interactive effects of plant traits and the traits’ covariance could impair the assessment of the true role of plant trait variability on herbivore populations in natural settings.

Highlights

  • Using an analogy of the Hedge’s d metric (Hedges and Olkin 1985), Wetzel et al (Wetzel et al 2016), calculate the average of the normalized Jensen’s effect (Ruel and Ayres 1999) which is based on the difference between the average performance in the absence and the presence of plant trait variance, i.e. F(x) − F(x)

  • They inferred this metric from a large number of curves fitted to published datasets relating insect herbivore performance to different levels of various defense and nutrition traits. They found that the average Jensen’s effect size is significantly negative for plant nutritive traits but is nearly zero for plant defense traits. The latter motivated the authors to conclude that the relationships between insect herbivore performance and plant defense levels are linear on average and that plant defense variance should not affect herbivore performance via nonlinear averaging contrary to the overall negative effects of variance in plant nutritive traits

  • We argue that this study may have underestimated the potential of plant defense trait variance to generate Jensen’s effects

Read more

Summary

Introduction

Wetzel et al (Wetzel et al 2016) predict that variance in plant nutrient level reduces herbivore performance via nonlinear averaging effects while variance in defense level does not. The conclusion is based on the average shape of the relationship between herbivore performance and plant defenses, i.e. F(x) − F(x) .

Results
Conclusion
Full Text
Published version (Free)

Talk to us

Join us for a 30 min session where you can share your feedback and ask us any queries you have

Schedule a call