Lecture 22 ICA 4 ENERGY FLOW IN THE ECOSYSTEM1. Figure 1. Is your food (and C) footprint affected by whether you eat as a vegetarian or omnivore? Explain. Less energy (and C) is lost at lower trophic levels because less transformation of chemical bonds is done than if the energy were transferred to higher trophic levels; hence more energy is available directly for human consumption when humans are vegetarians.2. Why does amount of energy transfer decrease after each chemical transformation? Inefficiency on transfer of energy; energy is lost as heat, thus causing the decrease.3. Explain: ”Energy flows, while matter cycles in ecosystems? Energy moves (flows) up the food chain… energy is lost as heat at each biochemical transfer. Energy is never recycled, so input (flow) of energy from sun is necessary to sustain life. In contrast, matter moves through food chain, organisms die, are decomposed, organic matter is transformed to inorganic forms, is returned to abiotic pools from which it is recycled and used once again by plants and passed up the food chain. Matter is re-used.4. What is the last transfer in a ‘plant-based’ food chain? From predator to top predator What happens at the last transfer in a ‘decomposer-based’ food chain? Last remaining chemical bond energy is lost as heat. INTRA-TROPHIC LEVEL ENERGY (PLANT-BASED)5. What is Primary Production (PP)? What element does it summarize? What are its units? Rate of converting light energy to chemical bond energy in carbohydrates via photosynthesis. Element = C; unit = g C / m2/ time period Why is its rate important for the ecosystem? It determines limit of chemical bond energy available to all other organisms in ecosystem.6. T or F? Plants photosynthesize; animals respire. F. Plants photosynthesis; all living cells (plant or animal) respire. T or F? Photosynthesis occurs by day and respiration by night in plants. F. PS = day; respiration = night + day Figure 2. What is the relationship among Gross PP, Net PP, and Respiration? Gross PP = Net PP + Respiration Imagine two plants with the same GPP, but plant 1 has twice as high a respiration rate as plant two. Contrast their NPP. Plant 2 will have > NPP than Plant 1. Which processes would be affected by their difference in NPP? growth, maintenance, and reproduction7. Figure 3. Is GPP measured directly or indirectly? Indirectly. Describe how it is estimated. Measure CO2 uptake and CO2 output during light (day). Uptake – output = NPP. Measure CO2 output during dark (night) = respiration. Add NPP + respiration = GPP. 8. Figure 4. Explain this ‘light response curve’. As light intensity increases, productivity increases up to a point, after which it levels off. ‘Compensation point’ =light intensity at which PS = respiration. ‘Saturation point’ = light intensity above which no further increase in PS occurs.9. Figure 5B. Explain the shape of the relationship of NPP vs. temperature. NPP increases linearly with an increase in temperature. Enzymatic reactions increase with increasing temperature. 10. Figure 5C. Explain the shape of the relationship of NPP vs. precipitation. NPP increases up to about 3000 mm rain/yr and then slowly declines. High rainfall comes from many cloudy days with light limitation. High rainfall also brings low transpiration, low nutrient uptake, and water-logged soils with less nutrient uptake. ‘Water use efficiency’ (WUE) = g NPP per kg water transpired 11. Figure 6. Why is nitrogen important for an ecosystem? It is a part of every rubisco molecule; the chief molecule responsible for fixing C from the atmosphere during photosynthesis. Explain the shape of the relationship of NPP vs. [N]. NPP increases with a decreasing slope and then levels off with greater N. Other biochemical steps (other than those dependent on rubisco) begin to limit photosynthetic rate. ‘Nutrient use efficiency’ (NUE) = g production per g N assimilated 12. What determines whether NPP increases with >[CO2]? Is photosynthesis rate limited by concentration of CO2 (e.g. by photorespiration in C3 plants)? Explain how ‘vegetation acts as a C sink. Via photosynthesis plants take CO2 out of the atmosphere and fix it as C-compounds; they sequester C.13. Figure 7. Why is productivity higher near the coast than in the open ocean? Near the coast, Sediments with nutrients are stirred and brought up to higher water levels with enough light for photosynthesis. The open ocean does not get a replenished supply of nutrients that settle to the bottom.14. Design an experiment to test whether Fe limits NPP in the open ocean. Add iron sulfate to an area of ocean; compare with control area with no addition. Measure amount of chlorophyll a (a surrogate for NPP). Figure 8. Describe the results of one such experiment. Chlorphyll a increased at surface levels with added Fe, but did not increase at 30 m depth with added Fe or in control areas with ambient Fe. What is the conclusion? Fe limits NPP at surface levels in the open ocean.15. Figure 9. Would you base comparisons on NPP/area or % of total NPP? It depends on the question being raised. Global-level question require knowledge as % of total NPP. INTER-TROPHIC LEVEL ENERGY TRANSFERS16. Figure 10. Why does energy available decrease at each higher level in food chains? Why does an energy ‘pyramid’ develop? Energy decreases sharply at each higher trophic level because the lower trophic level uses much of the energy for its own functions/structures, and much is lost as heat during these transformations. Energy transfer to each higher level isless and less because of loss at each lower level during chemical transformations. 17. Describe the traditional ‘law’ of energy transfer. 10% of production at a given trophic level is available to transfer to the next higher level. Figure 11. How is Ecological (food chain) Efficiency quantified? = (net production of torphic level n / net production of trophic level n-1) X 10 How should the “10% Law” be modified? It ranges from