Many species of the phytoplankton family Scenedesmaceae are able to develop multicellular colonies in the presence of freshwater zooplankton, and to go back to the unicellular morph when no grazers are around. The proportion of colonial morphs appears to be related to the actual grazing presure of zooplankton. Colony size in Scenedesmus obliquus appears to increase the food handling time of the herbivorous rotifer Brachionus calyciflorus, and thus has a defensive advantage. On the other hand, the disadvantage of colony formation is that it increases the sedimentation rate of these algae and that colonies have higher maintenance costs. Simulation models of plankton food chains consisting of either undefended, constitutively defended or inducible defended algae, herbivorous rotifers and carnivorous rotifers have been developed. Analysis of these models revealed that in food chains consisting of species with inducible defences, bottom-up control and the average biomass of the different trophic levels increase gradually in response to increasing carrying capacity. This was in sharp contrast to the 'classical' step-wise fashion of biomass increase with resource enrichment, and shifts between bottom-up and top-down control with increasing food chain length, which were observed in the food chains with undefended and constitutively defended species. Experiments done on the dynamics of bi- and tritrophic food chains with undefended and inducible defended Scenedesmaceae, herbivorous rotifers (B. calyciflorus) and carnivorous rotifers (Asplanchna brightwelli) revealed strong population fluctuations in food chains with undefended algae, which did not occur when algae had inducible defences. Thus, inducible defences increased the stability of the system by preventing strong population fluctuations. Furthermore, inducible defences promoted species persistence by preventing stochastic extinctions due to these strong fluctuations. In summary, inducible defences add a new perspective on the structure and behaviour of the more complex communities that we observe in nature.
|Award date||17 Feb 2005|
|Place of Publication||Nijmegen|
|Publication status||Published - 17 Feb 2005|