Ecological stoichiometry and the paradox of enrichment: A new approach to a classical problem

1. Ecological stoichiometry and the paradox of enrichment: A new approach to a classical problem Presentation of postdoctoral project Jannicke Moe (Div. of Zoology, Dep. of Biology, University of Oslo, Norway) Also involved: Nils Chr. Stenseth (Div. of Zoology) Dag O. Hessen (Div. of Limnology) Ole Christian Lingjærde (Dep. of Informatics)

2. Image 1 Image 2 Processed image Daphnia individuals can be measured by image analysis Information from image analysis: • no. of individuals • size of individuals • condition of individuals (width:length) • dead individuals (Færøvig, Hessen & Andersen 2002)

3. Experimental setup: chemostats • 2 L bottles containing algae + Daphnia • Continuous input of nutrient medium • Gradient of input phosphorous concentration

4. Data collection • Daphnia populations: • number of individuals • size of individuals ( age / stage) • concentrations of P, C and N • Algal populations: • number algal cells • volume of algal cells • concentrations of P, C and N • Nutrient medium • concentrations of P, C and N

5. INTRODUCTION Lotka-Volterra models may not be suitablefor all consumer-resource systems Predator-prey systems: • Resource similar to consumer • Energy limiting factor • Lotka-Volterra-based models suitable Herbivore-plant systems: • Resource different from consumer • Nutrients additional limiting factor • Lotka-Volterra-based models less suitable?

6. Consumer quantity Z (Daphnia carbon biomass) Resource quantity C (algal carbon biomass) Resource quality Q (algal P content) Recycling of P P (phosphorous in environment) Phosphorous influx PL BACKGROUND A stoichiometric model: The Daphnia-algae-phosphorpus system

7. BACKGROUND A stoichiometric model: The Daphnia-algae-phosphorpus system Z = biomass of Daphnia (mg C L-1) C = biomass of algae (mg C L-1) P = mass of phosphorous (mg P L-1)

8. Stable eqilibrium Unstable equilibrium Algal isocline Daphnia isocline Daphnia (mgC/L) Algae (mg C/L) Algae (mg C/L) BACKGROUND Model predictions: effect of P enrichment on dynamics high P influx Low P influx

9. Algae Daphnia Medium P High P Plankton biomass Time Plankton biomass Plankton biomass Time Time EXPERIMENTS Aim of experiments: Different type of population dynamics along P gradient Low P

10. Problem with stoichiometric model: ignores demography The stoichiometric model does not distinguish between populations with ... • equal biomass • different number of individuals • equal biomass • different size structure Real population Stoic. model

11. What type of model is optimal for analysing the Daphnia-algae system? Population Physiological Stoichiometric models models models Limiting factors: energy only energy only energy + nutrients Currency: no. of ind. ind. biomass total biomass Density dependence: + - + Demograpic structures: + - - An Individual-based population model could consider • limitation by energy + nutrients • no. of individuals + biomass • individuals condition (width:length) • density dependence • demographic structures (size / stage) • demographic stochasticity

12. IBPM of the Daphnia system - some challenges • Individuals cannot be "recognised" - can data still be used for IBPM? What kind of assumptions must be made? • How can discrete models (IMPB) be combined with continuous models (stoichiometric)? • Will an IBPM that includes stoichiometry get too complicated?