Assessing the evidence for latitudinal gradients in plant defence and herbivory

P>1. The idea that biotic interactions, including herbivory, predation and competition are more intense at lower latitudes is widely accepted and underpins several dominant theories on the latitudinal gradient in biodiversity. Current theory also predicts that the intense biotic interactions at low latitudes will select plants for greater defence against herbivores. We reviewed the literature to provide an assessment of the evidence for and against the hypothesis that herbivory is more intense at lower latitudes, and that plants from low latitudes are better defended than are plants from high latitudes. 2. Only 37% of the 38 latitudinal comparisons of herbivory showed higher herbivory at lower latitudes, and the average effect size in a meta-analysis was not significantly different from zero. Thus, the available data do not support the idea that herbivory is generally more intense in the tropics. 3. Only nine of 56 comparisons found higher chemical defences at lower latitudes, and a meta-analysis showed that overall, chemical defences were significantly higher in plants from higher latitudes. This result is counter to the predictions of much of the literature. 4. A meta-analysis showed no significant effect of latitude on physical defence. 5. A review of the literature on feeding trials and common garden experiments showed that herbivores tend to prefer tissue from high latitudes. This trend could stem from differences in overall defence that were not captured by the metrics used in the literature, but could also result from differences in nutritional quality. 6. The empirical data do not support the widespread view that herbivory is generally more intense at lower latitudes, or that plants from low latitudes are generally better defended than are plants from higher latitudes. These results are counter to the prevailing thought on this topic, and suggest that this field may be ripe for the development of new theory.