Large scale latitudinal changes in herbivore diversity and herbivore pressure on two widely distributed neotropical Piper species

Background/Question/Methods

It has long been hypothesized that herbivore pressure in the tropics is both qualitatively and quantitatively different from that in temperate regions. To test the hypothesis that herbviore pressure is both higher and caused by a more diverse array of herbivore species at low latitudes, we sampled two Piper (Piperaceae) species, P. aduncum and P. aequale, at five locations, eight populations per location, between Mexico and Bolivia. In sampling the same plant species across this latitudinal range, we were able to control for the effects of plant phylogeny, a complicating factor that has plagued previous studies. In each population we counted the number of lepidopteran herbivores, and measured amount of leaf damage caused by specialist versus generalist insect herbivores. We also sampled leaves for secondary chemistry and mechanical defenses. We predicted more specialist herbivore species, more species overall, greater damage, and a greater diversity and concentration of secondary compounds (piperamides).

Results/Conclusions

We found that the number of herbivore species and the number of herbivore individuals per leaf area increased 2-3 times for each plant species from high latitude towards the equator. Contrary to our expectation, however, this increase in number of herbivore species was due to the addition of generalist not specialist species. The total number of species attacking at the equator was 3 and 7 times greater at the equator for P. aequale and P. aduncum, respectively. Finally, and again contrary to expectation, the increase in herbivore numbers did not translate into differences in damage along the latitudinal gradient. The results going south to the equator from Mexico mirrored those going north to the equator from Bolivia. Thus, herbivore pressure varies by latitude in the identity and quantity of herbivores but not in the amount of damage they produce. Because the identity of the herbivores is changing, and the balance between generalist and specialist-caused herbivory changes, then it is likely that the nature of the selective force is changing with latitude. We predict that forthcoming analyses of leaf chemistry will reveal more kinds of compounds and in higher concentration at the equator, thus explaining the lack of change in damage across latitudes.