Are invertebrate herbivores of freshwater macrophytes scarce in tropical wetlands?

In this study we assessed the abundance and composition (taxa and feeding guilds) of invertebrate communities occurring on 11 aquatic plant species in Neotropical and Afrotropical wetlands. We tested the hypothesis that invertebrate herbivores form only a minor component of the invertebrate assemblages associated with freshwater macrophytes. The waterbodies experienced differing intensities of disturbance pressure (grazing, trampling, sediment disturbance) from large mammalian herbivores. General analysis of macrophyte samples collected during 2012–2013 from Zambia and Argentina showed that in Afrotropical wetlands invertebrate herbivores were significantly less abundant than non-herbivorous invertebrate taxa. In the Neotropical wetlands, the abundance of herbivorous and non-herbivorous invertebrate taxa did not differ significantly. Analysis per macrophyte species showed that invertebrate herbivores occurred in higher or similar abundance compared to non-herbivores in 83 % of macrophyte species in the Neotropics, but only in one macrophyte species in the Afrotropics. Taxa and feeding guilds of invertebrate herbivores varied between macrophyte species and ecozones, but were dominated by semiaquatic insects. TWINSPAN analysis of the plant samples hosting invertebrate morphospecies (n = 91) produced a sample-classification of four sample groups, with substantial differences in invertebrate morphospecies associated with each group of plant samples. The results show that invertebrate herbivores, mainly insects, are both numerically important and likely to form an important component of nutrient-cycling processes involving macrophyte biomass in warm-water wetlands. Whether the differences observed between the two ecozones are related to differences in large-herbivore presence, or to other inter-ecozone biogeographical differences in habitat conditions, or both, remains an issue for further research.