Several studies in boreal and temperate forests, semiarid woodlands and grassland ecosystems have demonstrated the positive relationship between herbivory and decomposition rates and have considered leaf palatability and litter quality as the main drivers of such a relationship. In tropical rainforests however, no strong association has been found between herbivory and decomposition rates. The questions of how herbivores influence plants to produce high or low quality litter, and how this might feedback on plant nutrition still remain obscure, especially because in tropical ecosystems herbivore specialization is known to be low. We tackled this issue by testing the hypothesis that herbivory on canopy leaves facilitates subsequent decomposition of these mechanically damaged leaves on the forest floor by increased physical accessibility of detritivores and decomposers. First, we compared the decomposition rate of 15%-eaten and non-eaten leaves of a Ficus cuatrecasana tree using coarse and fine fauna exclusion mesh-bags in a 260 days field-experiment in a Western Ecuador lowland tropical rainforest. Second, we manipulated percentage of damage on cellulose filter paper disks and followed their decomposition rates during 58 days.
Results/Conclusions
As expected, we found significant differences between mesh treatments within eaten and non-eaten leaves (P < 0.0001, paired t-tests). Mesh exclusion of macro- and meso fauna highlighted the role of detritivore arthropods as initial litter shredders, confirming their critical role in the decomposer food web by facilitating decomposition processes. However, ANCOVA analyses showed no significant association between herbivory and decomposition. Our experiment using cellulose filter-papers showed that their decomposition was not influenced by the percentage of damage. The probability of 1–99% decomposition was the same for all the cellulose disks treatments (Anderson-Darling P-value > 0.05) suggesting that decomposition rates fitted significantly to randomness so that all the treatments had the same probability of being consumed. Our results indicate that canopy herbivores do not mechanically facilitate the activity of detritivores and decomposers. In conclusion, our study provides no evidence of a functional link between herbivore damage and subsequent decomposition of Ficus cuatrecasana leaves.
