PS 46-24
Does herbivore damage increases methane emission by aquatic macrophytes?
Wetlands are the largest natural source of atmospheric methane (CH4). Emergent aquatic macrophytes are the greatest contributors of CH4 flux, acting as a conduit between sediment and atmosphere. Recent studies have shown that CH4 emissions can be enhanced if plants culms are damaged or clipped. In tropical costal lagoons, these plants are consumed and damaged by orthopterans, although they are responsible for a small fraction of biomass loss. Despite the overall well documented effect of herbivores on aquatic macrophytes, their effects on plant-mediated CH4 fluxes has been poorly investigated and a direct comparison of herbivory effects on plant biomass loss and CH4 emissions has never been addressed. We performed a mesocosm experiment where we manipulated the density of damaged culms (4 levels – 0, 20, 50 and 100% of damaged culms), using the aquatic macrophyte Eleocharis equisetoides (Elliott.) Torr, and measured the corresponding CH4 flux. We hypothesized that increased percentage of plants with herbivory-simulated damage would promote a linear increase on CH4 flux rates. Additionally, we evaluated the proportion of damaged plants in natural fields to address the relative effects of herbivory on CH4 emissions and actual biomass removal.
Results/Conclusions
Simulated herbivory had a strong effect on CH4 emissions. Mesocosms fluxes exhibited a linear increased in response to the percentage of damaged culms (R²=0.933, P=0.034). Average CH4 flux rates were 0.06, 0.02, 0.26, 0.51 mg. m-2. h-1, respectively for 0, 20, 50 and 100% treatment levels. Mesocosms with 100% of damaged plants had CH4 flux 9 times higher than mesocosms with all intact plants (0%). Field evaluations of actual macrophyte biomass loss, due to herbivory, revealed that only 3.57% of plant biomass was consumed. This estimation derives from 15.2% of damaged culms in E. equisetoides stands. According to our results, this amount of damage would cause an approximately 15% proportional increase on CH4 upward fluxes by macrophyte stands, highlighting the disproportional effect that herbivores have on biomass loss and CH4 flux rates. Current estimations of CH4 emissions in the tropics neglect the role of herbivory on plant-mediated CH4 emissions. Our study showed that herbivory should be accounted in these estimations, and their magnitude cannot be depicted simply by herbivore biomass removal.