Herbivory variation - latitude or climate?

Ecologists have repeatedly reported that lower latitudes are associated with increases in herbivore abundance and damage. However, recent studies have confirmed that this pattern is not as common as previously thought. In our paper, "Latitudinal variation in herbivory: influences of climatic drivers, herbivore identity and natural enemies", we tried to identify some potential biotic (herbivore feeding guild, natural enemies of herbivores) and abiotic (climate) factors that may explain the lack of consistency in latitude-herbivory relationships. To do this, we surveyed herbivory by a seed-eating caterpillar and leaf chewing insects, as well as attack by parasitic wasps associated to the former, across 30 populations of the tropical herb Ruellia nudiflora (Acanthaceae).

These populations were distributed along a 5° latitudinal gradient extending from northern Yucatan (Mexico) to southern Belize, almost covering the entire climatic gradient for this species. We found that plant populations from lower latitudes showed higher rates of leaf herbivory but a lower proportion of attacked fruits by the seed herbivore. Interestingly, while latitudinal variation in seed herbivory was driven predominantly by temperature differences, latitudinal variation in leaf herbivory was not explained by climatic correlates of latitude. Parasitism associated to the seed herbivore was not associated to either latitude or climatic factors. Our findings show that tests of latitude-herbivory relationships must account for climatic effects and herbivore traits (e.g. feeding guild), and also that the effects of latitude and climate on consumers may vary across trophic levels, with parasitoids in this case being less responsive than herbivores.

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