Presented Papers Main paper: Elser et al. (2007) Global analysis of nitrogen and phosphorus limitation of primary producers in freshwater, marine and terrestrial ecosystems Ecology Letters 10: 1135 1142 * Comment: Davidson and Howarth (2007) Nutrients in Synergy Nature 449: Aim of the study Find out wheter patterns of autotroph nutrient limitation differ across ecosystems (N and P considered). Actual Paradigms: Marine and terrestrial ecosystems are N limited Lakes are P limited -> ARE THEY TRUE? *INSERTION: a) Liebigs Law of the Minimum: Once the demand of N is met, it does not have to be increased further. b) Synergy: The whole is greater than its sum. Methods: General Meta-Analysis: Field studies about N and P limitation were compiled and analyzed. Methods: Finding studies Searching Databases (using ISI Web of Science) with a combination of key words AND Using the studies analyzed in recently published syntheses Methods: Choosing studies Conditions: Independant manipulations of N and P (less bias) Mean community level biomass (with few exeptions: no single species) -> MORE THAN THOUSAND STUDIES CHOSEN: (*around 300 publications) 653 freshwater experiments 243 marine experiments 173 terrestrial experiments Area: Global distribution of study sites for freshwater (yellow), marine (blue), and terrestrial (red) nutrient enrichment experiments. Methods: Measurements Ln transformed Response Ratio (here biomass): (most frequently used in meta-analyses) ln (Response Ratio) = ln (Effect / Control) Effect = Value of the response variable in the enrichment treatment Contol = Value in the control treatment Results & Discussion: Comparison of three ecosystems: No differences in P limitation N higher limitation in marine ecosystem coastal P pollution Paradigms not true Results & Discussion: Comparison of three ecosystems: combined N and P strongly increased biomass production in every ecosystem Synergistic effect Results & Discussion: Comparison of subhabitats: Every subhatitat with synergistic effects Terrestrial:Soil age: P limitation in tropical forests N limitation in tundras Freshwater: P limitation in lake for algae Marine:N limitationin coral reef for algae and phytoplankton Results: Confounding effects? (App. S2) Applied nutrient enrichement? -> No significant correlation between applied nutrients and Response Ratio. N / P availability in study site? ->comparable in the studies Duration of experiment ->Did not show a clear tendency Strength and Weakness of the study Many studies analyzed, all ecosystems over the world Many confounding effects considered Well documented Doses of N and P addition not addressed* Carefully done meta-analysis