Rainfall reduction increases insect herbivory in tropical herb communities

Questions Climate change is dramatically altering rainfall patterns and species distribution around the world. However, little is known about how rainfall reduction may affect plant-herbivore interactions that are crucial to the of input energy and nutrients into terrestrial ecosystems. Following Coley's growth rate hypothesis, we assessed whether rainfall reduction in a seasonally dry tropical forest decreases community-level herbivory owing to water shortage in drier areas. Location Catimbau National Park, Northeast Brazil. Methods We sampled 16 herb communities along a rainfall gradient (564-917 mm). Using digital photographs, we monitored for 3 weeks the frequency, magnitude and type (feeding guild) of insect damage on young, fully expanded, short-lived leaves (lifespan <1 month). We related herbivory levels to local rainfall and examined potential confounding effects of herb abundance, richness and floristic composition on such relationships. Results We monitored 290 leaves from 202 plants belonging to 28 species. About one-third of the leaves and six species escaped from herbivory. Leaf-chewing insects were the most frequent herbivores, attacking 17 species, 44% +/- 15% of the leaves and removing the same amount of foliar tissue as all the other insects combined removed (sap-sucking, rasping, leaf-mining and 'unknown'). Contrary to expectations, foliar damage accumulated more in drier areas following the increase in the frequency of leaf-chewing damage. In addition, frequency and magnitude of herbivory did not vary with herb abundance and richness and were weakly influenced by floristic composition. Conclusions Our findings suggest that rainfall underlies community-level herbivory by mechanisms not related to plant availability, identity and diversity. Based on recent evidence that aridity favours leaf-chewing proliferation in the study region, we hypothesize that rainfall reduction may weaken the top-down control of herbivores, increasing folivory pressure on herbs. If prolonged droughts become more frequent in the coming decades, plant-herbivore interactions may be altered permanently.