Complex food web models have been extensively used to identify mechanisms promoting food web stability over the last decade. These size structured models based on the allometric scaling of biological rates were mostly focused on population dynamics and largely ignored fundamental processes of ecosystem functioning such as nutrient cycling. Nutrients are excreted as mineral nutrient or detritus that respectively sustain the green food web based on photosynthesis and the brown food web based on detritus decomposition. Our objective is to add nutrient recycling in food web modelling to test the consequences of such an important process for ecosystem stability. As the green food web produces the biomass fuelling the brown food web and the brown food web recycles the nutrient taken up by the green food web, these two coupled food webs are thus mutualistic. However, they can be competing because decomposers also take up mineral nutrients depending on the stoichiometry of the detritus. In addition, consumers can eat species from both food webs and create a top-down coupling between the green and the brown food webs. Here, we assess the stability of the food web in different conditions of interactions between the green and the brown food webs along a nutrient enrichment gradient.
Results/Conclusions
We found that nutrient cycling acts as additional source of mineral nutrients as recycled nutrients represent more than 50% of the total input to the mineral pool. The detritus stock increases rapidly along a nutrient enrichment gradient as the biomass produced by the food web increases. The increased availability of detritus makes the brown food web responsible of most of the biomass production at low nutrient inputs, promoting species persistence. At higher nutrient inputs, primary production increases more than decomposers production and the food web mainly relies on the green food web. This complementary biomass production along an enrichment gradient leads to a higher species persistence in food webs where consumers can eat prey from both green and brown food webs. In addition, nutrient cycling can lead to a paradox of enrichment as it increases basal resources availability. As the detritus stock increases rapidly along the enrichment gradient, decomposers persistence strongly decreases because of the paradox of enrichment while primary producers are less affected. Our model merging population dynamics models and ecosystem functioning aspects provides a promising framework to better understand the response of ecosystems to environmental changes.