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Lecture 22 ICA 5 ENERGY FLOW IN THE ECOSYSTEM. 1. What are two over-arching themes of ecosystem ecology? A. flow of energy B. cycling of matter 2. To what two thermodynamic laws must energy transformations conform? Explain. A. Law 1: Energy is neither created or destroyed; it is conserved and must be accounted for (= ‘balance the books’). B. Law 2: Energy transfer from one chemical bond to another is ‘inefficient’; energy is lost as heat as a result (=’heat tax’). 3. 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. 4. What does it mean to say: ”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. 5. 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. 6. Figure 1. How + why does energy available decrease at each higher level in food chains? 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. 7. Figure 2. Is carrying capacity of the earth affected by whether humans are vegetarians or omnivores? Yes Explain. More humans can be feed at lower trophic levels because less transformation of chemical bonds is done than if the energy was transferred to higher trophic levels; hence more energy is available directly for human consumption. 8. Figure 3. Which pyramid can be inverted and why? Energy: Never; because energy is lost at each transfer in food chain; can’t have greater at higher than lower trophic levels Biomass: Yes: when turnover is high among phytoplankton, no large amount of biomass is present at any one time, but large amounts of food are available to support a larger biomass of zooplankton. Numbers: Yes; if size of organism is smaller at higher trophic levels, then greater numbers can occur at higher than lower levels (e.g. parasites on a predator). 9. What happens at higher trophic levels to toxins absorbed by plants? They become ‘biomagnified’ = accumulate to higher and higher concentrations. 10. 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. 11. T or F? Photosynthesis occurs by day and respiration by night in plants. PS = day; respiration = night + day Figure 4. 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 reproduction 12. Figure 5. 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 13. Figure 6. If measuring CO 2, what is NPP? Explain. NPP = 12 mg CO2/100cm2 leaf/hr NPP is CO2 input – CO2 output; difference = NPP During full sun both Gross photosynthesis and respiration are occurring; their difference = NPP What is rate of respiration? Explain. Respiration = 1.5 mg CO2/100cm2/hr. Only respiration is occurring at night so a direct measure of CO2 output is a direct measure of What is GPP? Explain. GPP = 13.5 mg CO2/100cm2/hr = NPP + Respiration 14. How is Photosynthetic Efficiency measured? What are its units? It’s value? % of light energy from sun converted to NPP = 1-2% How is Net Production Efficiency measured? What are its units? % of GPP that remains after respiration = (NPP/GG)x100. What is one explanation why it is so much lower in tropical than temperate ecosystems? Respiration occurs year-round, night and day at a higher temperature in the tropics; it stops in winter and slows at night more in the temperate zone. 15. Which abiotic factors limit NPP? light; temperature, nutrients, precipitation; CO2 Figure 7. Explain the shape of the relationship of NPP vs. temperature. NPP increases linearly with an increase in temperature. Enzymatic reactions increase with increasing temperature. Figure 7. 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. Figure 8. 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 those dependent on rubisco) begin to limit photosynthetic rate. 16. What determines whether NPP increases with >[CO2]? Is photosynthesis rate limited by concentration of CO2 (e.g. by photorespiration in C3 plants)? 17. Figure 9 + 10. How does the community affect NPP? With a greater number of species and a greater number of functional types present, NPP (measured as plant biomass) increases up to some point and then levels off. 18. What limits NPP of freshwater lakes? P; with low N/P, blue-green algae increase NPP because they are nitrogen fixers; with high N/P, green algal ‘blooms’ occur. open ocean? near shore = N; open ocean: silica and Fe 19. 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 12. 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. 20. Figure 13. Summarize major global patterns of NPP. Would you base such comparisons on NPP/area or % of total NPP? Based on NPP/area, highest NPP occurs on land in wetter and warmer biomes and in water in swamps, marshes, algal beds and coral reefs; it is very low in the open ocean. When take land area of each biome into account, NPP is highest in open ocean and tropical rain forests. It declines in other areas generally according to their rank in land area. Which type of comparison to make depends on the question being raised. Global-level questions require knowledge as % of total NPP. 21. Define secondary production. Energy fixed in carbon compounds by animals. Figure 14. What explains pyramid shape of energy for the trophic levels? Energy transfer to each higher level is limited because of heat loss at each lower level during chemical transformations. 22. On what factors does amount of energy transfer depend? size of the ‘heat tax’; it is lower in aquatic than terrestrial systems. 23. 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 15. 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 5-20%; it is higher in aquatic than terrestrial systems. 24. Fig. 16. What is NPP of Primary Producers? 1114-234 = 880 Prim. consumers? = 148-44 = 104 Sec. consumers? = 31-18 = 13 What is Ecological Efficiency between level 1 and 2? = (104/880)100 = 11.8% between 2 and 3? = (13/104)100 = 12.5% What ultimately happens to: 1) energy: The energy not lost as heat (via respiration) in these 3 trophic levels is stored in the unconsumed biomass. This energy may flow to decomposers that extract it in respiration. The remaining energy remains in non-decomposed matter. 2) biomass not consumed in this lake? The unconsumed biomass will likely sink to the lake bottom where it will either be decomposed, and thus continue immediately in the BGC cycle, or stored in the sediments on the lake bottom. 25. Figure 17.How many lengths are there in short? 3 long 5-7 food chains? What are hypotheses that explain the limit on length of food chain? Explain them. A. Energetics. Energy limits the number of levels because of NPP available, amount of energy needed by consumers, and ecological efficiency of transfer. B. Dynamic stability. Longer chains are less stable because fluctuations at lower trophic levels are magnified at higher levels and top predators go extinct. 26. Figure 18. Do aquatic or terrestrial ecosystems have more trophic levels? Aquatic Which factor contributes most to variation in food chain length among these ecosystems? Ecological efficiency. 27. Figure 19. What processes result in transfer of energy within a trophic level? Assimilation, respiration, excretion What are processes that lower energy transfer during secondary production? Respiration; excretion; 28. How is Exploitation Efficiency quantified: = (ingestion by trophic level n / production of trophic level n-1) X100 What are typical values for herbivores? 20% carnivores? 30% 29. What is assimilated energy = ingestion - egestion How is Assimilation Efficiency quantified? = (assimilation/ingestion) X 100 What primary factor affects this value? food quality; amount of non-digestible matter 30. How is Secondary Production quantified? = Assimilation – respiration - excretion 31. How is Net Production Efficiency quantified? = (2nd production / assimilation) X100 Why does it depend on metabolic activity? Metabolic activity depends on rate of respiration. Therefore, NPE depends on metabolic rate. Why do the most active animals have the lowest values? They have the highest metabolic (and respiration) rates. Contrast values for warm- (<1% birds; < 6% mammals) vs. cold-blooded animals (15-75%). QUANTIFICATION OF ENERGY FLOW WITHIN PLANTS Photosynthetic Efficiency = NPP/energy from sun in same time interval Gross Primary Production (GPP) = Net Primary Production (NPP) + Respiration Net Production Efficiency = NPP/GPP BETWEEN TROPHIC LEVELS Ecological Efficiency = Net production of level n/net production of level n-1 WITHIN ANIMAL TROPHIC LEVEL Exploitation Efficiency = Ingestion by trophic level n/production of trophic level n-1 Assimilated energy = Ingestion – Egestion Assimilation Efficiency = Assimilation / Ingestion Secondary Production = Assimilated Energy - Respiration - Excretion Net Production Efficiency = Secondary Production/Assimilation