Is herbivory greater in tropical latitudes?

Background/Question/Methods

Herbivory is commonly thought to increase towards tropical latitudes, although few studies exist to support this idea.  Our objective was to quantify herbivory rates from tropical to temperate latitudes in order to evaluate if herbivory is indeed higher in tropical latitudes. Using a common garden design, we examined the rate of herbivory and the density and diversity of herbivores on two cucurbitaceae species. The study was conducted across a 36° latitudinal gradient from Panama to Wisconsin. We used potted Cucurbita pepo and Cucumis sativus plants and controlled soil moisture and nutrient availability.  We censused the herbivores on the plants daily for six weeks and compared leaf area loss and leaf damage across the latitudinal gradient

Results/Conclusions

We found that herbivory rates were not higher in the tropical latitudes and that the herbivory pattern is more complicated than a gradient would imply. Herbivore abundance was the variable most related to leaf area loss on C. pepo, while herbivore abundance and location together best described the leaf area loss on C. sativus. In the latter case 78.4% greater leaf consumption/plant occurred at high compared to low latitudes. These results indicate that at least for some plant species, herbivory is positively correlated with latitude, with higher amounts of leaf consumption during the growing season in temperate latitudes.

We found that herbivore diversity followed a similar pattern with greater herbivore diversity at higher latitudes on both plant species. These results were primarily driven by generalist chewing herbivores, as the number of specialist chewing herbivores was not significantly different across locations for C. pepo and higher in lower latitudes for C. sativus. This data contradicts the common assertion that herbivory rates are higher at lower latitudes and indicates that a more thorough evaluation of these patterns should be conducted.