Herbivore damage increases methane emission from emergent aquatic macrophytes

Wetlands are the largest natural methane (CH4) source and the vegetated littoral areas are the major contributors for CH4 release from sediment to the atmosphere. Although the effects of herbivores on biomass removal, growth and reproduction of emergent macrophytes have been well documented, their effect on plant-mediated CH4 fluxes, especially by insects, remains unknown. We performed a mesocosm experiment in which we simulated the damage caused by herbivorous insects and manipulated the density of damaged culms of Eleocharis equisetoides (4 levels - 0, 20, 50 and 100%) measuring the corresponding CH4 emission, concentration and potential production in the sediment. We hypothesized that an increased percentage of culms with simulated herbivory would be associated with increased CH4 fluxes from sediment toward the atmosphere. Simulated herbivory positively affected CH4 emissions, but only under high herbivory pressure. The average CH4 flux from mesocosms with 50% and 100% damaged culms was 3.5 higher than those with intact or low levels of damage. These results indicate that physical damage on macrophytes affects gas transport within the plants. A field survey in our studied system revealed that plant biomass consumed by herbivores is relatively low. This result highlight that insects may have a disproportional effect on CH4 emissions, i.e., a very small damage (low biomass removal), when performed in many culms (50% and 100% of damaged culms treatments), may substantially increase CH4 fluxes. In summary, our findings bring a new perspective to the influence of herbivory on CH4 and carbon cycling, especially regarding the role insects might play.