Cornelissen, T. and Stiling, P. 2005. Perfect is best: low leaf fluctuating asymmetry reduces herbivory by leaf miners. Oecologia 142: 46-56.
Fluctuating asymmetry (FA) is the terminology used to represent small, random variations from perfect symmetry in biological entities. These "deviations from perfect symmetry," in the words of Cornelissen and Stiling, "actually represent developmental instability, and the ability to develop symmetrical traits may be related to the ability to produce defensive chemicals (Moller, 1995)."
What was done
In April of 2002, the authors evaluated patterns of asymmetry in 40 leaves from each of 30 trees of each of two species of oak - sand live oak (Quercus geminata) and turkey oak (Q. laevis) - at the University of South Florida Botanical Garden in Tampa, Florida, USA, well before any herbivores had begun to attack the trees that growing season. Thereafter, patterns of leaf asymmetry, leaf quality and herbivory were examined for 30 individual trees of each of the two oak species from March to October of the same year.
What was learned
The "before and after" measurements clearly indicated that differential herbivory patterns neither caused nor affected patterns of leaf FA. However, they revealed, in the words of the authors, that "herbivores may use asymmetry as a cue to plant quality and suitable oviposition sites," as plants with a higher percentage of asymmetric leaves were attacked more frequently by various leaf miners, as were leaves on the same plant that were more asymmetric. One of the reasons for these choices may have been, as Cornelissen and Stiling report, that "asymmetric leaves of both plant species exhibited better nutritional quality for herbivores than symmetric leaves," with asymmetric leaves possessing "significantly lower concentrations of tannins [-22% for Q. geminata and -36% for Q. laevis] and higher nitrogen content [+8% for both species]."
What it means
The reason we report these results is that in an earlier study of Q. geminata and Q. myrtifolia, Cornelissen et al. (2003) demonstrated that "asymmetric leaves were less frequent in elevated CO2, and, when encountered, they were less asymmetric than leaves growing under ambient CO2," as described in our Editorial of 19 May 2004. Consequently, when Cornelissen and Stiling declare in the title of their paper that "perfect is best: low leaf fluctuating asymmetry reduces herbivory by leaf minors," one can also conclude that high CO2 "is best," as it leads to "low leaf fluctuating asymmetry" and thereby "reduces herbivory by leaf miners."
Cornelissen, T., Stiling, P. and Drake, B. 2003. Elevated CO2 decreases leaf fluctuating asymmetry and herbivory by leaf miners on two oak species. Global Change Biology 10: 27-36.
Moller, A.P. 1995. Leaf-mining insects and fluctuating asymmetry in Ulmus glabra leaves. Journal of Animal Ecology 64: 697-707.Reviewed 2 February 2005