New Evidence Suggests No Sex Bias in Herbivory or Plant Defense.

AbstractDioecious plants can exhibit sexual dimorphism across a suite of plant traits, including susceptibility to herbivory and secondary chemistry. One hypothesis is that because of greater costs of reproduction in females, males should grow faster and invest less in defense, resulting in male-biased herbivory. Indeed, a series of articles and a prominent meta-analysis have established male-biased herbivory as a rule. However, more recent reviews have raised questions about how general the pattern is, citing the low breadth of taxon sampling. The literature on this topic has not been formally quantified by meta-analysis in over 15 years. Here, we report the results of a meta-analysis of studies that measured sex bias in either herbivory and/or secondary defense in 71 dioecious plant species. We added 58 observations of herbivory and 41 of secondary chemistry to the original. We control for nonindependence of effects from the same study and taxonomic group to address critiques of earlier studies. For secondary chemistry, we found no support for any consistent difference between male and female plants. For herbivory, results are directionally similar to earlier reports, although not statistically significant once we accounted for taxonomic group and study. We also found that earlier studies reported stronger male bias than more recent studies. We discuss our results in light of the decline effect, where the magnitude of a described effect declines as the number of observations increases, and consider whether the data sets exhibit signs of evidence of the type(s) of biases that can result in declining effect sizes over time.