Herbivory is important in freshwater systems

Posted on March 27, 2017

Until the 1990s it was believed that, in the main, freshwater and marine plants were connected to aquatic food webs only indirectly, through the breakdown of decaying vegetation by decomposers. However, over the last two decades there’s been a growing realisation that the activities of aquatic herbivores in consuming plants and channeling the derived nutrients into food webs play a key role in shaping benthic ecosystems. Recent meta-analyses indicate that, on average, herbivores remove around 44-48% of freshwater plant biomass, compared with 40-44% in marine systems and only 4-8% on land – so grazing pressures are 5-10 times greater in aquatic environments than in terrestrial ecosystems. This ecological reassessment has wide-ranging implications for our interpretation of fundamental processes such as primary production, biogeochemical cycling, plant dispersal, habitat provision and regime shifts between clear and turbid water. Freshwater plants have low carbon : nitrogen ratios (median values of 12-16, compared with 24-28 and 25-30 for marine and terrestrial plants), and their resulting high nitrogen concentration makes them attractive to grazers. This is particularly true of submerged plants, which need to invest relatively little in structural carbon because they are supported by water. The activities and impacts of herbivores are predicted to increase with global environmental change, and there’s a need for better ways to manage undesirable effects of grazers on aquatic plants.

Reference: Bakkera, E.S. et al. 2016. Herbivory on freshwater and marine macrophytes: a review and perspective. Aquatic Botany 135, 18–36. http://www.sciencedirect.com/science/article/pii/S0304377016300419