The Coupling Between Grazing and Detritus Food Chains and the Strength of Trophic Cascades Across a Gradient of Nutrient Enrichment
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A minimal food web model was constructed comprising one grazing and one detritus food chain coupled by nutrient cycling and generalist carnivores to investigate how prey preference by carnivores may affect the strength of trophic cascades across a gradient of nutrient enrichment. The equilibrium or mean abundance of each food web component and the magnitude of the carnivore effect on lower trophic levels were calculated for different values of the prey preference and nutrient input parameters. Our model predicts that nutrient enrichment increases the mean abundances of carnivores, autotrophs and detritus, but the magnitude of this effect is dependent on the prey preference term. On the other hand, herbivores and detritivores are relatively unaffected by enrichment but are strongly affected by carnivore preference. Carnivores have a negative effect on herbivores and a positive effect on autotrophs and detritus, whereas the effect on detritivores can be both positive and negative. At high preference for herbivores, carnivores have a positive effect on detritivores, because the positive effect of increased detritus availability due to reduced herbivore grazing outweighs the negative effect of predation. At high preference for detritivores, the balance is changed in the other direction. We argue that in systems where authochtonous primary production is the major source of detritus, herbivores can control the rates of detritus production and have indirect effects on detritivores, which may feed back into effects on herbivores through their shared enemies. This positive feedback is probably one mechanism affecting the resilience of alternative stable states in shallow lakes.
Keywordsadaptive foraging, eutrophication, food webs, habitat coupling, nutrient cycling, shallow lakes, trophic dynamics
We would like to thank Kathryn Cottingham, Donald DeAngelis, Claire de Mazancourt, Jean Paul Metzger, Egbert van Nes and two anonymous reviewers for their valuable suggestions to improve the manuscript. Financial support to JLA was received from the Brazilian Postgraduate Agency (CAPES) and the Swedish Foundation for International Cooperation in Research and Higher Education (STINT). J.R. was financially supported by the Swedish Research Council for Forestry and Agriculture and the Nordic Academy for Graduate Training (NorFA). We also thank Wilhelm Granéli and STINT for making the trip of J.R. to Brazil possible.
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